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<front>
<journal-meta>
<journal-id journal-id-type="publisher-id">Front. Mar. Sci.</journal-id>
<journal-title>Frontiers in Marine Science</journal-title>
<abbrev-journal-title abbrev-type="pubmed">Front. Mar. Sci.</abbrev-journal-title>
<issn pub-type="epub">2296-7745</issn>
<publisher>
<publisher-name>Frontiers Media S.A.</publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.3389/fmars.2023.1162685</article-id>
<article-categories>
<subj-group subj-group-type="heading">
<subject>Marine Science</subject>
<subj-group>
<subject>Original Research</subject>
</subj-group>
</subj-group>
</article-categories>
<title-group>
<article-title>Drivers of diatom production and the legacy of eutrophication in two river plume regions of the northern Gulf of Mexico</article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author" corresp="yes">
<name>
<surname>Krause</surname>
<given-names>Jeffrey W.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="author-notes" rid="fn001">
<sup>*</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/185649"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Boyette</surname>
<given-names>Adam D.</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
<xref ref-type="author-notes" rid="fn003">
<sup>&#x2020;</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/2286226"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Marquez</surname>
<given-names>Israel A.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Pickering</surname>
<given-names>Rebecca A.</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="author-notes" rid="fn003">
<sup>&#x2020;</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1094739"/>
</contrib>
<contrib contrib-type="author">
<name>
<surname>Maiti</surname>
<given-names>Kanchan</given-names>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
<uri xlink:href="https://loop.frontiersin.org/people/1355425"/>
</contrib>
</contrib-group>
<aff id="aff1">
<sup>1</sup>
<institution>Dauphin Island Sea Lab</institution>, <addr-line>Dauphin Island, AL</addr-line>, <country>United States</country>
</aff>
<aff id="aff2">
<sup>2</sup>
<institution>School of Marine and Environmental Sciences, University of South Alabama</institution>, <addr-line>Mobile, AL</addr-line>, <country>United States</country>
</aff>
<aff id="aff3">
<sup>3</sup>
<institution>Division of Marine Science, School of Ocean Science and Engineering, University of Southern Mississippi</institution>, <addr-line>Stennis Space Center, MS</addr-line>, <country>United States</country>
</aff>
<aff id="aff4">
<sup>4</sup>
<institution>Department of Oceanography and Coastal Studies, Louisiana State University</institution>, <addr-line>Baton Rouge, LA</addr-line>, <country>United States</country>
</aff>
<author-notes>
<fn fn-type="edited-by">
<p>Edited by: Alex J. Poulton, Heriot-Watt University, United Kingdom</p>
</fn>
<fn fn-type="edited-by">
<p>Reviewed by: Wei Liu, Wenzhou Vocational College of Science and Technology, China; Arnaud Laurent, Dalhousie University, Canada</p>
</fn>
<fn fn-type="corresp" id="fn001">
<p>*Correspondence: Jeffrey W. Krause, <email xlink:href="mailto:jkrause@disl.edu">jkrause@disl.edu</email>
</p>
</fn>
<fn fn-type="present-address" id="fn003">
<p>&#x2020;Present addresses: Adam D. Boyette, Naval Oceanographic Office, Stennis Space Center, MS, United States; Rebecca A. Pickering, Department of Geology, Lund University, Lund, Sweden</p>
</fn>
</author-notes>
<pub-date pub-type="epub">
<day>05</day>
<month>07</month>
<year>2023</year>
</pub-date>
<pub-date pub-type="collection">
<year>2023</year>
</pub-date>
<volume>10</volume>
<elocation-id>1162685</elocation-id>
<history>
<date date-type="received">
<day>09</day>
<month>02</month>
<year>2023</year>
</date>
<date date-type="accepted">
<day>31</day>
<month>05</month>
<year>2023</year>
</date>
</history>
<permissions>
<copyright-statement>Copyright &#xa9; 2023 Krause, Boyette, Marquez, Pickering and Maiti</copyright-statement>
<copyright-year>2023</copyright-year>
<copyright-holder>Krause, Boyette, Marquez, Pickering and Maiti</copyright-holder>
<license xlink:href="http://creativecommons.org/licenses/by/4.0/">
<p>This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.</p>
</license>
</permissions>
<abstract>
<p>In the northern Gulf of Mexico (nGoM), the Louisiana Shelf (LS) and Mississippi Bight (MB) subregions are influenced by eutrophication to varying degrees. Despite recognition that dissolved silicon may regulate diatom productivity in the nGoM, there is only one published data set reporting biogenic silica (bSiO<sub>2</sub>) production rates for each subregion. We report that bSiO<sub>2</sub> production rates on the LS and MB are high and appear to be controlled by different nutrients among seasons. Despite exceptional upper trophic level biomass regionally, which suggests significant primary production by diatoms (as in other systems), gross euphotic-zone integrated bSiO<sub>2</sub> production rates are lower than major bSiO<sub>2</sub> producing regions (e.g. upwelling systems). However, when normalizing to the depth of the euphotic zone, the bSiO<sub>2</sub> production rates on the LS are like normalized rates in upwelling systems. We suggest local river-plume influenced hydrography concentrates diatom productivity within shallow euphotic zones, making production more accessible to higher trophic organisms. Comparison of rates between the LS and MB suggest that the fluvial nitrate within the LS stimulates bSiO<sub>2</sub> production above that in the MB, which has a smaller watershed and is less eutrophic (relatively). Beyond understanding the factors controlling regional bSiO<sub>2</sub> production, these data offer the most comprehensive Si-cycle baseline to date as the LS and MB will likely exchange freely in the mid to late century due to land subsidence of the Mississippi River delta and/or sea-level rise.</p>
</abstract>
<kwd-group>
<kwd>biogenic silica production</kwd>
<kwd>Gulf of Mexico</kwd>
<kwd>Louisiana Shelf</kwd>
<kwd>Mississippi Bight</kwd>
<kwd>eutrophication</kwd>
<kwd>diatom</kwd>
</kwd-group>
<contract-sponsor id="cn001">Division of Ocean Sciences<named-content content-type="fundref-id">10.13039/100000141</named-content>
</contract-sponsor>
<contract-sponsor id="cn002">Gulf of Mexico Research Initiative<named-content content-type="fundref-id">10.13039/100007240</named-content>
</contract-sponsor>
<contract-sponsor id="cn003">Gulf of Mexico Research Initiative<named-content content-type="fundref-id">10.13039/100007240</named-content>
</contract-sponsor>
<counts>
<fig-count count="6"/>
<table-count count="3"/>
<equation-count count="0"/>
<ref-count count="63"/>
<page-count count="16"/>
<word-count count="9821"/>
</counts>
<custom-meta-wrap>
<custom-meta>
<meta-name>section-in-acceptance</meta-name>
<meta-value>Marine Biogeochemistry</meta-value>
</custom-meta>
</custom-meta-wrap>
</article-meta>
</front>
<body>
<sec id="s1" sec-type="intro">
<title>Introduction</title>
<p>Diatoms play important roles in coastal biogeochemistry and often dominate the phytoplankton biomass and bloom productivity in these environments. Diatoms are unique in their obligate requirement for silicon, which is required in near unity to nitrogen under the many growth conditions among low- to high-latitude ecosystems (<xref ref-type="bibr" rid="B6">Brzezinski, 1985</xref>; <xref ref-type="bibr" rid="B36">Lomas et&#xa0;al., 2019</xref>). Widespread use of agricultural fertilizer has increased fluvial nitrogen loading in many watersheds and in some cases has resulted in a doubling of riverine nitrogen input to the coastal ocean (<xref ref-type="bibr" rid="B10">Cloern, 2001</xref>). In many eutrophied rivers for nitrogen, dissolved silicon (Si(OH)<sub>4</sub>) has declined or remained stable, thereby lowering the Si:N ratio below diatoms&#x2019; optimum (i.e. &lt;1.0) potentially favoring diatoms blooms to deplete Si(OH)<sub>4</sub> before NO<sub>3</sub>. The decline in Si(OH)<sub>4</sub> availability relative to nitrate and phosphate has been linked to shifts from diatom-dominated to flagellate-dominated ecosystems; this so called &#x2018;Si(OH)<sub>4</sub> Paradigm&#x2019; (<xref ref-type="bibr" rid="B50">Ragueneau et&#xa0;al., 2006</xref>) was first proposed for coastal systems by <xref ref-type="bibr" rid="B44">Officer and Ryther (1980)</xref> and has been evoked as a potential mechanism for changes in many systems (reviewed by <xref ref-type="bibr" rid="B10">Cloern (2001)</xref>). However, the linkage among significant Si:N perturbations and the changing role of diatoms is built largely on standing stock data (e.g. nutrients, phytoplankton abundance), rather than physiological rate data, as the latter data are absent in many systems. Furthermore, testing for an Si:N effect in these systems is challenging given the lack of baseline information prior to the nutrient perturbation.</p>
<p>In the Mississippi River (MR), which discharges onto the Louisiana Shelf (LS) in the northern Gulf of Mexico (nGoM), the annual average Si:N ratio declined from &gt;3 in 1960 to ~1 in 1980. The change was driven by both increasing nitrate and decreasing Si(OH)<sub>4</sub> concentrations and has remained around the latter Si:N value to present day (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>). The MR now exhibits strong seasonal oscillations between favorable (&gt;1.0, late summer/autumn) and unfavorable (&lt;1.0, spring) Si:N conditions for diatoms (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1C</bold>
</xref>). This unfavorable period corresponds to the annual peak in MR discharge and elevated primary production rates on the LS from nitrate loading (<xref ref-type="bibr" rid="B35">Lohrenz et&#xa0;al., 1997</xref>). The changes in the MR-derived nutrients in the late 20<sup>th</sup> century coincided with major ecosystem-wide changes in the nGoM, e.g. frequent and intense hypoxia events (<xref ref-type="bibr" rid="B49">Rabalais et&#xa0;al., 2007</xref>). On the LS, multiple species from the pennate diatom genera <italic>Pseudo-nitzschia</italic> numerically dominate the diatom assemblage during periods of high MR flow (<xref ref-type="bibr" rid="B2">Bargu et&#xa0;al., 2016</xref>). These results, combined with increasing <italic>Pseudo-nitzschia</italic> preserved in regional sediments during the late 20<sup>th</sup> century (<xref ref-type="bibr" rid="B45">Parsons et&#xa0;al., 2002</xref>) and the rise of more flagellate harmful algal species on the LS (<xref ref-type="bibr" rid="B13">Dortch et&#xa0;al., 1999</xref>), have led to the hypothesis that the Si:N changes in the MR watershed may have reduced the diatom contribution to food web processes in this system (<xref ref-type="bibr" rid="B59">Turner et&#xa0;al., 1998</xref>).</p>
<fig id="f1" position="float">
<label>Figure&#xa0;1</label>
<caption>
<p>
<bold>(A)</bold> Time series of Mississippi River silicate and nitrate at the United States Geological Survey Station 07373420 in St. Francisville, Louisiana, United States, from August 1954 through December 2021. <bold>(B, C)</bold> Comparison of the monthly average ( &#xb1; standard error) for each nutrient (left scale) and the silicate:nitrate (Si:N) molar ratio (right scale) among 10-year subsets (blue highlights in A) separated by 50 years: 1958 through 1967 <bold>(B)</bold> and 2008 through 2017 <bold>(C)</bold>.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g001.tif"/>
</fig>
<p>Biogenic silica (bSiO<sub>2</sub>) production rates can serve as a proxy for diatom growth as silicon is used to produce their bSiO<sub>2</sub> shell. While recent reports have shown the importance of cyanobacteria and Rhizaria in bSiO<sub>2</sub> cycling, e.g. (<xref ref-type="bibr" rid="B1">Baines et&#xa0;al., 2012</xref>; <xref ref-type="bibr" rid="B5">Biard et&#xa0;al., 2018</xref>), diatom biomass is significantly higher on the LS than these other groups. The only published bSiO<sub>2</sub> production data in this area was acquired on the LS in the early 1990s (<xref ref-type="bibr" rid="B42">Nelson and Dortch, 1996</xref>), approximately a decade after the major MR watershed shift in nutrients stabilized, and this prior work only examined processes (e.g. kinetics of Si(OH)<sub>4</sub> uptake by diatoms) at a single depth. There are currently no available euphotic-zone profile data to determine whether diatom Si(OH)<sub>4</sub> consumption on the shelf may exceed the delivery of Si(OH)<sub>4</sub> from the MR or benthic flux sources, which (if found) would support the hypothesis that Si may exert a strong control over regional diatom productivity, e.g. <xref ref-type="bibr" rid="B12">Dale et&#xa0;al. (2007)</xref>.</p>
<p>The Mississippi Bight (MB) is located to the east of the LS shelf and MR delta extending to Perdido Bay, Florida (<xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2</bold>
</xref>). The MB is a river-dominated system fed from the combined rivers of Mobile Bay (Alabama). Analysis of water oxygen isotopic signatures show that MR water did not dominate the freshwater loading to the MB during multiple years in mid 2010s, instead this was driven largely by the combined rivers from the northern domain, especially those discharging through Mobile Bay (<xref ref-type="bibr" rid="B52">Sanial et&#xa0;al., 2019</xref>). Mobile Bay has the fourth highest freshwater discharge rate in the United States (<xref ref-type="bibr" rid="B55">Stumpf et&#xa0;al., 1993</xref>) but nitrate concentrations rarely exceed 20 &#xb5;M (<xref ref-type="bibr" rid="B46">Pennock et&#xa0;al., 1999</xref>). The MB is also highly productive, being the central habitat range for a variety of economically important fish and invertebrate fauna and is the eastern extension of the so called &#x2018;Fertile Fisheries Crescent&#x2019; where, historically, ~70-80% of fisheries landings in the entire GoM occur (<xref ref-type="bibr" rid="B23">Gunter, 1963</xref>). Comparison of diatom processes within the water column of the LS and MB allows an assessment of similarities, i.e. both are river plume systems where the main distributaries are separated by ~200 km, and differences occurring due to the watershed sizes and the degree of nutrient loading and/or eutrophication.</p>
<fig id="f2" position="float">
<label>Figure&#xa0;2</label>
<caption>
<p>Station map for four cruises in the Louisiana Shelf (LS) and Mississippi Bight (MB). LS cruises were part of the Coastal Louisiana Silicon Cycling (CLASiC) project in August-September 2016 and May 2017 (<xref ref-type="bibr" rid="B47">Pickering et&#xa0;al., 2020</xref>, <xref ref-type="bibr" rid="B20">Ghaisas et&#xa0;al., 2021</xref>); MB cruises were part of the Consortium for Oil Spill Exposure Pathways in Coastal River-Dominated Ecosystems (CONCORDE) project in October-November 2015 and March-April 2016 (<xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B52">Sanial et&#xa0;al., 2019</xref>). Map made using Ocean Data View (<xref ref-type="bibr" rid="B54">Schlitzer, 2016</xref>).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g002.tif"/>
</fig>
<p>Hydrographic processes in both the MB and LS have significantly changed in the last century. Since the 1950s, the river discharge increased 31% and 15% in the MR and Mobile Bay river system, respectively (<xref ref-type="bibr" rid="B41">Milliman et&#xa0;al., 2008</xref>), and this trend is expected to increase in the 21<sup>st</sup> century (<xref ref-type="bibr" rid="B58">Tao et&#xa0;al., 2014</xref>). Land subsidence rates in southeastern Louisiana are rapid (<xref ref-type="bibr" rid="B27">Kolker et&#xa0;al., 2011</xref>) and there will be pelagic connectivity between the MB and LS domains as the MR delta becomes submerged. Thus, baseline data on diatom biogeochemical processes are required to ground truth models attempting to predict how the MB and LS pelagic ecosystems will alter as their connectivity increases.</p>
<p>The overarching goal of this study was to evaluate whether bSiO<sub>2</sub> cycling rates in the LS have significantly changed in the last 30 years and test the hypothesis that Si(OH)<sub>4</sub> availability controls diatom productivity. Here, we evaluated seasonal variability between systems and establish baseline rates and stocks. The nGoM is a model region for river-dominated systems which have undergone significant anthropogenic perturbations. The bSiO<sub>2</sub> cycling processes in this region may be relevant to many low-latitude river-dominated regions that have undergone, or are presently experiencing, similar stressors.</p>
</sec>
<sec id="s2">
<title>Methods</title>
<p>Four cruises in the LS and MB were completed as part of the Coastal Louisiana Silicon Cycle (CLASiC) and Consortium for Coastal River-Dominated Ecosystems (CONCORDE) projects, respectively. These programs were independent and had different goals (<xref ref-type="bibr" rid="B22">Greer et&#xa0;al., 2018</xref>; <xref ref-type="bibr" rid="B47">Pickering et&#xa0;al., 2020</xref>), but similar silicon-cycling rate and ancillary measurements were obtained. The CLASiC cruises were conducted aboard the R/V <italic>Pelican</italic> during summer (27 August &#x2013; 6 September 2016) and spring (4 &#x2013; 13 May 2017), corresponding to the low and high-flow periods of the MR discharge onto the LS, respectively. The two CONCORDE cruises were conducted aboard the R/V <italic>Point Sur</italic> during autumn (28 October &#x2013; 7 November 2015) and spring (29 March &#x2013; 11 April 2016), where the spring cruise corresponded to the annual maximum discharge onto the MB from the Mobile Bay riverine complex and other local rivers.</p>
<p>Water column stations were sampled in salinity zones (CLASiC) and geographic corridors (CONCORDE). Given the dominance of the MR in surface hydrography on the LS, CLASiC cruise stations were targeted to river-influence zones operationally defined by a strong (salinity &lt;25), moderate (salinity 25 &#x2013; 30), or low riverine signal (salinity &gt;30) in surface salinity. CONCORDE stations were aligned primarily among meridional transects in the west, middle, and east of the MB, with the autumn cruise sampling also in the southern zone near the eastward distributary of the MR and the spring cruise had some stations parallel to the barrier island coast of Alabama in the northern domain. All water column sampling was done using a 12-bottle rosette sampler equipped with 12 L Niskin bottles and a SeaBird SBE911 CTD with a Biospherical photosynthetically active radiation sensor and WET Labs C-Star transmissometer (instruments on both vessels). For CLASiC stations, CTD sampling was conducted ~1 hour after sunrise or in the late afternoon/early evening &lt;2 hours prior to sunset. Samples were either selected at pre-determined depths (standing stocks) or specified light levels (for incubations) normalized to the irradiance available just below the surface (i.e. I<sub>0</sub>). Specific light levels included 100% (just below surface), 50%, 20%, 5%, and 1% I<sub>0</sub>. During most CONCORDE stations, sampling was done throughout the night and only surface samples were taken for incubation (i.e. no light depths). Select vertical profiles were conducted, and these hydrocasts were taken in the morning to target the defined light depths as during CLASiC. As in prior studies, e.g. (<xref ref-type="bibr" rid="B37">Lomas et&#xa0;al., 2009</xref>; <xref ref-type="bibr" rid="B31">Krause et&#xa0;al., 2021</xref>), Niskin samples were pooled into 10 L carboys (covered with opaque bags) and then subsampled for various measurements; the carboy was gently mixed during sampling to keep the contents in suspension.</p>
<p>Subsamples were collected for nutrients and particulate matter. 50 mL of seawater was immediately filtered using a 0.2 &#xb5;m polycarbonate filter (CLASiC) or ~0.6 &#xb5;m glass-fiber filter (CONCORDE) for nutrient analyses. Analyses were done as described previously (<xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al., 2017</xref>; <xref ref-type="bibr" rid="B47">Pickering et&#xa0;al., 2020</xref>) but are briefly discussed here. For Si(OH)<sub>4</sub>, samples were analyzed using a manual colorimetric method at sea (CLASiC) or on frozen samples (CONCORDE), which were thawed at room temperature for sufficient time to allow Si polymers to break down prior to colorimetric analysis. Filtrate for all other nutrients was immediately frozen. Dissolved inorganic nitrogen (nitrate + nitrite, NO<sub>3</sub>+NO<sub>2</sub>; ammonium, NH<sub>4</sub>) and dissolved soluble reactive phosphorus (SRP) were analyzed using a Skalar autoanalyzer. During CONCORDE, ~500 mL samples were filtered onto 1.2 &#xb5;m polycarbonate filters for bSiO<sub>2</sub> and frozen. In the laboratory, samples were dried and run using a sodium hydroxide digestion (<xref ref-type="bibr" rid="B32">Krause et&#xa0;al., 2009</xref>). Samples were collected and stored similarly during CLASiC. However, given the quantity of lithogenic silica in the waters, time-course digestions in sodium carbonate were used. This method is like those used for quantifying bSiO<sub>2</sub> in sediments, e.g. <xref ref-type="bibr" rid="B47">Pickering et&#xa0;al. (2020)</xref>, and can better constrain uncertainty among individual bSiO<sub>2</sub> and lithogenic sample pools. Although it is less sensitive, there is sufficient bSiO<sub>2</sub> in this system such that detection and quantitation are not issues. Other studies in coastal systems with high and variable lithogenic silica have used modified sodium hydroxide digestions, e.g. <xref ref-type="bibr" rid="B51">Ragueneau et&#xa0;al. (2005)</xref>, however, we chose the time course to avoid using empirical ratio-based corrections for the lithogenic material.</p>
<p>Additional subsamples were collected to determine rates of bSiO<sub>2</sub> production (denoted as &#x3c1;). 125 mL or 250 mL polycarbonate bottles (depending on the station biomass) were filled to the brim and received 333&#xa0;Bq of <sup>32</sup>Si(OH)<sub>4</sub> (&gt;20 kBq &#xb5;g Si<sup>-1</sup>). Bottles were immediately capped, gently inverted to mix the tracer, and placed into mesh bags made of neutral density screening to simulate the percent of incident irradiance (%I<sub>0</sub>) at the depth of sample collection. The bottles were then incubated for 24 hours in a deck-board incubator cooled by continuously flowing surface seawater. Following incubations, samples were filtered onto 25&#xa0;mm diameter and 1.2 &#xb5;m pore size polycarbonate filters, which were then placed onto nylon disc planchettes to dry and covered with a mylar film (secured using a nylon ring). <sup>32</sup>Si activity was quantified using gas-flow proportional counting with a GM-25 multicounter (Ris&#xf8; National Laboratory, Technical University of Denmark) after aging the daughter isotope of <sup>32</sup>Si, <sup>32</sup>P, for seven half-lives (~4 months) to achieve secular equilibrium (<xref ref-type="bibr" rid="B30">Krause et&#xa0;al., 2011</xref>). Although some data from the middle MB corridor during autumn 2015 CONCORDE has been published (<xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al., 2017</xref>), the remaining data for this cruise, the spring 2016 CONCORDE cruise, and the two CLASiC cruises are original. Blanks were run on filtered seawater samples in which <sup>32</sup>Si was added, incubated, and filtered again. Due to the cost of <sup>32</sup>Si, only single replicates were done per sample point. Recent observations in the Bering and Chukchi Seas, both high diatom biomass systems like the nGoM, indicated that the coefficient of variation for replicate <sup>32</sup>Si data from the averaged 12% and did not change significantly between diatom bloom and non-bloom stations (<xref ref-type="bibr" rid="B31">Krause et&#xa0;al., 2021</xref>). Statistical analysis of trends in biomass-normalized bSiO<sub>2</sub> production among cruises were done using backward stepwise regression models in SigmaPlot software. Non-parametric comparisons of medians (Mann Whitney U Test) were used to evaluate differences in rates among prior studies and this one.</p>
</sec>
<sec id="s3" sec-type="results">
<title>Results</title>
<sec id="s3_1">
<title>Physical conditions</title>
<p>In the nGoM, atmospheric forcing (e.g. wind, precipitation) drives physical and biogeochemical variability in shelf waters. <xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al. (2017)</xref> observed that the induced storm surge following the passage of post-Hurricane Patricia (fall 2015) over the MB resulted in higher-than-normal and less variable salinity waters (ranging 32.1 to 35.6) drained from the regional estuaries. While most of the MB stations were surface-only, for two rate profiles at shallow stations (bottom depth 18 &#x2013; 21&#xa0;m), the irradiance rapidly attenuated and the euphotic zone depths ranged between 7 &#x2013; 12&#xa0;m. During the spring MB cruise, temperatures were similarly constrained and ranged from 19.3 to 21.2&#xb0;C. Due to the high discharge, as expected, surface salinity ranged from 17.9 to 33.5 and laterally increased away from Mobile Bay (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). The depth of the euphotic zone ranged from 4 &#x2013; 27&#xa0;m among five vertical profiles. The two shallowest euphotic zones were associated with the shallowest sites (8 and 12&#xa0;m) and the euphotic zones were 16 &#x2013; 27&#xa0;m (27 &#x2013; 36&#xa0;m bottom depth) among the three other sites.</p>
<fig id="f3" position="float">
<label>Figure&#xa0;3</label>
<caption>
<p>Surface properties for the spring cruises: LS during May 2017 and MB during March-April 2016. Properties include salinity <bold>(A)</bold>, CTD-fluorescence based chlorophyll <bold>(B)</bold>, nitrate+nitrite <bold>(C)</bold>, ammonium <bold>(D)</bold>, silicate <bold>(E)</bold>, biogenic silica standing stock <bold>(F)</bold>, production rate <bold>(G)</bold> and stock-normalized production rate <bold>(H)</bold>. Units are listed on the plot. Map made using Ocean Data View (<xref ref-type="bibr" rid="B54">Schlitzer, 2016</xref>).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g003.tif"/>
</fig>
<p>The LS physical conditions were similar to MB. Summertime LS surface temperatures were warmest among the four cruises, ranging from 29.0 to 30.3&#xb0;C. Surface salinities during this cruise ranged from 16.7 to 33.9. All summer stations sampled were between the 20 and 50&#xa0;m isobaths, and the euphotic zone depths ranged from 5 &#x2013; 48&#xa0;m; like the MB, the shallower euphotic zones on the LS were associated with the shallower stations. During the spring LS cruise, the surface temperatures were elevated compared to the spring MB cruise, ranging from 20.9 to 26.0, likely due to the May period LS cruise being later in spring (vs. late March &#x2013; April for the MB cruise). The surface salinity range sampled was comparable but slightly narrower than the summer cruise, 19.6 to 31.6, although one station was sampled in Mississippi Canyon (~1000 m bottom depth, hereafter referred to as the &#x201c;deep-water&#x201d; site) and surface salinity was 36.3 &#x2014;typical of deeper regional waters outside the direct freshwater influence. The spring-period LS euphotic zones were largely shallower than observed in summer, ranging from 7 &#x2013; 19&#xa0;m, and there was no clear relationship with bottom depth.</p>
</sec>
<sec id="s3_2">
<title>Dissolved nutrients</title>
<p>During autumn on the MB, dissolved nutrients showed typical ranges as observed in prior studies. NO<sub>3</sub>+NO<sub>2</sub> and NH<sub>4</sub> concentrations in the surface averaged 0.46 &#xb5;M (range: detection limit &#x2013; 2.1) and 0.33 &#xb5;M (range: detection limit &#x2013; 1.3), respectively (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). SRP was lower and less variable, averaging 0.28 &#xb5;M (range 0.15 &#x2013; 0.42). Si(OH)<sub>4</sub> concentrations were an order of magnitude higher, averaging 3.51 &#xb5;M (range: 1.31 &#x2013; 10.2). The highest concentrations corresponded to surface stations on the southern MB shelf just offshore and eastward of the MR delta. Vertically, there were few differences among stations, unsurprising given the storm passage which thoroughly mixed the water column. During the spring, nutrients in the MB surface concentrations were higher and more variable (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). NO<sub>3</sub>+NO<sub>2</sub> and NH<sub>4</sub> concentrations averaged 2.78 &#xb5;M (range: detection limit &#x2013; 13.1) and 0.95 &#xb5;M (range: detection limit &#x2013; 3.78), respectively (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). The average SRP concentration 0.39 &#xb5;M (range: 0.03 &#x2013; 3.1) was not as high as inorganic nitrogen forms; however, the dissolved N:P ratios were still below the canonical Redfield-Ketchum-Richards ratio (16:1). Si(OH)<sub>4</sub> was also higher than in autumn, averaging 9.33 &#xb5;M (range 0.42 &#x2013; 45.3, <xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). The highest concentrations for all nutrients were typically observed in the northern domain and a high station in the mid-shelf western domain (except for NH<sub>4</sub>) just offshore of the Chandeleur Islands (barrier island chain that forms a western boundary for the MB, <xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>).</p>
<fig id="f4" position="float">
<label>Figure&#xa0;4</label>
<caption>
<p>Surface box plots for nitrate+nitrite <bold>(A)</bold>, ammonium <bold>(B)</bold>, silicate <bold>(C)</bold>, biogenic silica standing stock <bold>(D)</bold>, production rate <bold>(E)</bold> and stock-normalized production rate <bold>(F)</bold> among all cruises. Box shows median (solid line) and average (dashed line), the box upper/lower boundaries denote the 25 and 75<sup>th</sup> percentiles with the whiskers denote the 10 and 90<sup>th</sup> percentiles.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g004.tif"/>
</fig>
<p>While clear seasonal differences were observed on the LS, there was also a strong difference between nutrient magnitude in the MB and LS. During the LS summer, surface NO<sub>3</sub>+NO<sub>2</sub> and NH<sub>4</sub> concentrations averaged 7.40 &#xb5;M (range: 0.30 &#x2013; 29.3) and 4.12 &#xb5;M (range: 1.94 &#x2013; 7.47), respectively (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). The average surface SRP was 0.64 &#xb5;M (range: 0.25 &#x2013; 2.25). Si(OH)<sub>4</sub> concentrations were higher than other nutrients, averaging 18.6 &#xb5;M (range: 2.40 &#x2013; 64.7). There was clear spatial variability among nutrients, with the highest concentrations in proximity to the MR outflow (see <xref ref-type="bibr" rid="B16">Dzwonkowski et&#xa0;al. (2018)</xref>). During spring, surface NO<sub>3</sub>+NO<sub>2</sub> and NH<sub>4</sub> concentrations averaged 5.20 &#xb5;M (range: 0.17 &#x2013; 14.9) and 5.59 &#xb5;M (range: 0.20 &#x2013; 15.8), respectively (<xref ref-type="fig" rid="f3">
<bold>Figures&#xa0;3</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). The surface SRP also was lower and less variable, averaging 0.14 &#xb5;M (range: 0.12 &#x2013; 0.17). At the deep-water station, surface nutrients were significantly lower than inshore with NO<sub>3</sub>+NO<sub>2</sub>, NH<sub>4</sub>, and SRP concentrations of 0.33, 0.34 and 0.18 &#xb5;M, in that order (<xref ref-type="fig" rid="f3">
<bold>Figures&#xa0;3</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). Surface Si(OH)<sub>4</sub> concentrations during the spring averaged 4.51 &#xb5;M (range: 0.17 &#x2013; 10.4) for the inshore stations, while the surface concentration at the deep-water station was 1.78 &#xb5;M (<xref ref-type="fig" rid="f3">
<bold>Figures&#xa0;3</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). There were strong spatial gradients associated with the river proximity, i.e. the highest concentrations observed near the river, except for NH<sub>4</sub> where the highest concentration observed in the surface was to the west and inshore (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>).</p>
</sec>
<sec id="s3_3">
<title>Biogenic silica stock and production rates</title>
<p>bSiO<sub>2</sub> standing stocks in the MB and LS were high but variable. During the autumn MB cruise, surface bSiO<sub>2</sub> averaged 5.42 &#xb5;mol Si L<sup>-1</sup> (range: 0.16 &#x2013; 40.3), while average bSiO<sub>2</sub> in the surface was ~20% higher in spring, 6.18 &#xb5;mol Si L<sup>-1</sup> with a lower range (0.55 &#x2013; 30.7, <xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). On the LS, surface bSiO<sub>2</sub> were lower, averaging 2.53 &#xb5;mol Si L<sup>-1</sup> (range: 0.06 &#x2013; 9.14) during the summer cruise. Surface bSiO<sub>2</sub> at the shelf stations were elevated during LS spring and averaged 4.42 &#xb5;mol Si L<sup>-1</sup> (range: 1.37 &#x2013; 9.59, <xref ref-type="fig" rid="f3">
<bold>Figures&#xa0;3</bold>
</xref>, <xref ref-type="fig" rid="f4">
<bold>4</bold>
</xref>). At the deep-water station, surface bSiO<sub>2</sub> was lower (0.35 &#xb5;mol Si L<sup>-1</sup>), consistent with the lower observed nutrients (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). At the stations where Si(OH)<sub>4</sub> &lt;1 &#xb5;M, bSiO<sub>2</sub> was 2.6 &#x2013; 7.9 times higher than Si(OH)<sub>4</sub>, suggesting significant accumulation of diatom biomass. Vertically, bSiO<sub>2</sub> in the euphotic zone declined with depth on the MB, although there was considerable variation in the average for the base of the euphotic zone, consistent with the shallow water column depth sampled and therefore the proximity to the sediment/water interface and nepheloid layer (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). On the LS, bSiO<sub>2</sub> increased to a subsurface maximum in the mid-euphotic zone, with surface and deep average concentrations being similar (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). Between both regions, the average bSiO<sub>2</sub> among euphotic zone depths varied by less than a factor of two, much less than variability in rates of production (discussed below).</p>
<fig id="f5" position="float">
<label>Figure&#xa0;5</label>
<caption>
<p>Average ( &#xb1; standard error) for silicate <bold>(A)</bold>, biogenic silica standing stock <bold>(B)</bold>, production rate <bold>(C)</bold>, and stock-normalized production rate <bold>(D)</bold> per irradiance depth (LS and MB) or surface-only stations (MB).</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g005.tif"/>
</fig>
<p>Despite the apparent difference in surface bSiO<sub>2</sub> between regions, there was less variation in the euphotic zone integrated stock. &#x222b;bSiO<sub>2</sub> in the two autumn MB profile stations were 6.17 and 29.0 mmol Si m<sup>-2</sup>, this was lower than the &#x222b;bSiO<sub>2</sub> among the four spring MB profile stations which averaged 57.1 mmol Si m<sup>-2</sup> (range: 46.7 &#x2013; 70.1, <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). On the LS, &#x222b;bSiO<sub>2</sub> in the summer was similar to MB in the autumn, averaging 21.9 mmol Si m<sup>-2</sup> (range: 3.85 &#x2013; 48.9, <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). However, &#x222b;bSiO<sub>2</sub> on the LS during spring was nearly three-fold higher, averaging 64.1 mmol Si m<sup>-2</sup> (range: 31.8 &#x2013; 108.8). At the deep-water station, &#x222b;bSiO<sub>2</sub> was 18.7 mmol Si m<sup>-2</sup> (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>).</p>
<table-wrap id="T1" position="float">
<label>Table&#xa0;1</label>
<caption>
<p>Station locations and surface salinity, euphotic zone depth, and integrated Si(OH)<sub>4</sub>, bSiO<sub>2</sub> and &#x3c1; among cruises.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="middle" align="left">Region/<break/>Season</th>
<th valign="middle" align="center">Lat. (N)</th>
<th valign="middle" align="center">Long. (E)</th>
<th valign="middle" align="center">Surface Sal</th>
<th valign="middle" align="center">&#x222b;Z (m)</th>
<th valign="middle" align="center">&#x222b;Si(OH)<sub>4</sub> (mmol m<sup>-2</sup>)</th>
<th valign="middle" align="center">&#x222b;bSiO<sub>2</sub> (mmol m<sup>-2</sup>)</th>
<th valign="middle" align="center">&#x222b;&#x3c1; (mmol m<sup>-2</sup> d<sup>-1</sup>)</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="middle" rowspan="2" align="center">MB<break/>Autumn</td>
<td valign="bottom" align="center">29.85</td>
<td valign="bottom" align="center">-88.61</td>
<td valign="middle" align="center">34.2</td>
<td valign="middle" align="center">21</td>
<td valign="bottom" align="center">14.6</td>
<td valign="bottom" align="center">6.17</td>
<td valign="bottom" align="center">0.34</td>
</tr>
<tr>
<td valign="bottom" align="center">30.13</td>
<td valign="bottom" align="center">-88.13</td>
<td valign="middle" align="center">33.7</td>
<td valign="middle" align="center">18</td>
<td valign="bottom" align="center">28.7</td>
<td valign="bottom" align="center">29.0</td>
<td valign="bottom" align="center">1.95</td>
</tr>
<tr>
<td valign="middle" rowspan="4" align="center">MB<break/>Spring</td>
<td valign="middle" align="center">29.77</td>
<td valign="middle" align="center">-88.09</td>
<td valign="bottom" align="center">29.3</td>
<td valign="bottom" align="center">25</td>
<td valign="bottom" align="center">21.1</td>
<td valign="bottom" align="center">70.1</td>
<td valign="bottom" align="center">5.34</td>
</tr>
<tr>
<td valign="middle" align="center">29.80</td>
<td valign="middle" align="center">-88.13</td>
<td valign="bottom" align="center">29.7</td>
<td valign="bottom" align="center">27</td>
<td valign="bottom" align="center">85.2</td>
<td valign="bottom" align="center">50.8</td>
<td valign="bottom" align="center">4.86</td>
</tr>
<tr>
<td valign="middle" align="center">29.72</td>
<td valign="middle" align="center">-88.09</td>
<td valign="bottom" align="center">30.0</td>
<td valign="bottom" align="center">16</td>
<td valign="bottom" align="center">69.3</td>
<td valign="bottom" align="center">46.7</td>
<td valign="bottom" align="center">15.3</td>
</tr>
<tr>
<td valign="middle" align="center">31.20</td>
<td valign="middle" align="center">-88.14</td>
<td valign="bottom" align="center">26.2</td>
<td valign="bottom" align="center">9</td>
<td valign="bottom" align="center">122</td>
<td valign="bottom" align="center">60.8</td>
<td valign="bottom" align="center">12.1</td>
</tr>
<tr>
<td valign="middle" rowspan="12" align="center">LS<break/>Summer</td>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-90.48</td>
<td valign="bottom" align="center">30.0</td>
<td valign="bottom" align="center">14</td>
<td valign="bottom" align="center">234</td>
<td valign="bottom" align="center">3.85</td>
<td valign="bottom" align="center">1.35</td>
</tr>
<tr>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-90.47</td>
<td valign="bottom" align="center">31.1</td>
<td valign="bottom" align="center">18</td>
<td valign="bottom" align="center">187</td>
<td valign="bottom" align="center">7.17</td>
<td valign="bottom" align="center">0.27</td>
</tr>
<tr>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-90.48</td>
<td valign="bottom" align="center">30.4</td>
<td valign="bottom" align="center">14</td>
<td valign="bottom" align="center">113</td>
<td valign="bottom" align="center">18.8</td>
<td valign="bottom" align="center">6.54</td>
</tr>
<tr>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-89.46</td>
<td valign="bottom" align="center">16.7</td>
<td valign="bottom" align="center">14</td>
<td valign="bottom" align="center">242</td>
<td valign="bottom" align="center">21.7</td>
<td valign="bottom" align="center">6.16</td>
</tr>
<tr>
<td valign="middle" align="center">29.01</td>
<td valign="middle" align="center">-89.58</td>
<td valign="bottom" align="center">20.2</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">302</td>
<td valign="bottom" align="center">11.7</td>
<td valign="bottom" align="center">5.73</td>
</tr>
<tr>
<td valign="middle" align="center">29.01</td>
<td valign="middle" align="center">-89.58</td>
<td valign="bottom" align="center">21.7</td>
<td valign="bottom" align="center">13</td>
<td valign="bottom" align="center">397</td>
<td valign="bottom" align="center">7.99</td>
<td valign="bottom" align="center">3.85</td>
</tr>
<tr>
<td valign="middle" align="center">28.64</td>
<td valign="middle" align="center">-91.11</td>
<td valign="bottom" align="center">31.1</td>
<td valign="bottom" align="center">16</td>
<td valign="bottom" align="center">240</td>
<td valign="bottom" align="center">24.5</td>
<td valign="bottom" align="center">16.2</td>
</tr>
<tr>
<td valign="middle" align="center">28.45</td>
<td valign="middle" align="center">-91.61</td>
<td valign="bottom" align="center">32.7</td>
<td valign="bottom" align="center">39</td>
<td valign="bottom" align="center">111</td>
<td valign="bottom" align="center">19.4</td>
<td valign="bottom" align="center">0.56</td>
</tr>
<tr>
<td valign="middle" align="center">28.50</td>
<td valign="middle" align="center">-91.61</td>
<td valign="bottom" align="center">33.9</td>
<td valign="bottom" align="center">48</td>
<td valign="bottom" align="center">144</td>
<td valign="bottom" align="center">26.3</td>
<td valign="bottom" align="center">1.14</td>
</tr>
<tr>
<td valign="middle" align="center">28.70</td>
<td valign="middle" align="center">-90.28</td>
<td valign="bottom" align="center">27.4</td>
<td valign="bottom" align="center">20</td>
<td valign="bottom" align="center">114</td>
<td valign="bottom" align="center">48.9</td>
<td valign="bottom" align="center">30.8</td>
</tr>
<tr>
<td valign="middle" align="center">28.72</td>
<td valign="middle" align="center">-90.17</td>
<td valign="bottom" align="center">28.7</td>
<td valign="bottom" align="center">35</td>
<td valign="bottom" align="center">205</td>
<td valign="bottom" align="center">48.1</td>
<td valign="bottom" align="center">15.9</td>
</tr>
<tr>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-89.45</td>
<td valign="bottom" align="center">22.7</td>
<td valign="bottom" align="center">5</td>
<td valign="bottom" align="center">132</td>
<td valign="bottom" align="center">24.0</td>
<td valign="bottom" align="center">26.0</td>
</tr>
<tr>
<td valign="middle" rowspan="14" align="center">LS<break/>Spring</td>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-90.50</td>
<td valign="bottom" align="center">27.6</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">89.7</td>
<td valign="bottom" align="center">57.7</td>
<td valign="bottom" align="center">26.2</td>
</tr>
<tr>
<td valign="middle" align="center">28.86</td>
<td valign="middle" align="center">-90.50</td>
<td valign="bottom" align="center">31.2</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">51.4</td>
<td valign="bottom" align="center">66.1</td>
<td valign="bottom" align="center">28.1</td>
</tr>
<tr>
<td valign="middle" align="center">28.87</td>
<td valign="middle" align="center">-90.49</td>
<td valign="bottom" align="center">27.0</td>
<td valign="bottom" align="center">7</td>
<td valign="bottom" align="center">68.9</td>
<td valign="bottom" align="center">36.8</td>
<td valign="bottom" align="center">39.8</td>
</tr>
<tr>
<td valign="middle" align="center">29.07</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">27.5</td>
<td valign="bottom" align="center">9</td>
<td valign="bottom" align="center">106</td>
<td valign="bottom" align="center">71.1</td>
<td valign="bottom" align="center">38.4</td>
</tr>
<tr>
<td valign="middle" align="center">29.07</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">26.3</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">116</td>
<td valign="bottom" align="center">86.5</td>
<td valign="bottom" align="center">54.9</td>
</tr>
<tr>
<td valign="middle" align="center">29.07</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">29.0</td>
<td valign="bottom" align="center">10</td>
<td valign="bottom" align="center">50.4</td>
<td valign="bottom" align="center">80.6</td>
<td valign="bottom" align="center">15.5</td>
</tr>
<tr>
<td valign="middle" align="center">28.50</td>
<td valign="middle" align="center">-90.83</td>
<td valign="bottom" align="center">30.5</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">18.8</td>
<td valign="bottom" align="center">31.8</td>
<td valign="bottom" align="center">5.85</td>
</tr>
<tr>
<td valign="middle" align="center">28.50</td>
<td valign="middle" align="center">-90.83</td>
<td valign="bottom" align="center">30.9</td>
<td valign="bottom" align="center">16</td>
<td valign="bottom" align="center">16.0</td>
<td valign="bottom" align="center">44.2</td>
<td valign="bottom" align="center">2.63</td>
</tr>
<tr>
<td valign="middle" align="center">28.50</td>
<td valign="middle" align="center">-90.83</td>
<td valign="bottom" align="center">31.6</td>
<td valign="bottom" align="center">16</td>
<td valign="bottom" align="center">13.6</td>
<td valign="bottom" align="center">45.4</td>
<td valign="bottom" align="center">9.78</td>
</tr>
<tr>
<td valign="middle" align="center">28.95</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">24.1</td>
<td valign="bottom" align="center">13</td>
<td valign="bottom" align="center">85.9</td>
<td valign="bottom" align="center">80.2</td>
<td valign="bottom" align="center">31.4</td>
</tr>
<tr>
<td valign="middle" align="center">28.96</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">24.4</td>
<td valign="bottom" align="center">11</td>
<td valign="bottom" align="center">47.5</td>
<td valign="bottom" align="center">109</td>
<td valign="bottom" align="center">21.5</td>
</tr>
<tr>
<td valign="middle" align="center">28.95</td>
<td valign="middle" align="center">-89.75</td>
<td valign="bottom" align="center">19.6</td>
<td valign="bottom" align="center">12</td>
<td valign="bottom" align="center">106</td>
<td valign="bottom" align="center">89.6</td>
<td valign="bottom" align="center">65.7</td>
</tr>
<tr>
<td valign="middle" align="center">28.72</td>
<td valign="middle" align="center">-90.18</td>
<td valign="bottom" align="center">28.5</td>
<td valign="bottom" align="center">19</td>
<td valign="bottom" align="center">54.6</td>
<td valign="bottom" align="center">34.6</td>
<td valign="bottom" align="center">10.6</td>
</tr>
<tr>
<td valign="middle" align="center">28.27</td>
<td valign="middle" align="center">-89.48</td>
<td valign="bottom" align="center">36.3</td>
<td valign="bottom" align="center">56</td>
<td valign="bottom" align="center">100</td>
<td valign="bottom" align="center">18.7</td>
<td valign="bottom" align="center">0.36</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>Note, because we did not do profiles among all sites in the MB, stations below represent a subset of the stations visualized in <xref ref-type="fig" rid="f2">
<bold>Figure&#xa0;2</bold>
</xref>.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>The minor disparities in surface bSiO<sub>2</sub> between regions were not apparent in the rate of biogenic silica production, &#x3c1;, in the surface. During autumn (MB), &#x3c1; averaged 0.28 &#xb5;mol Si L<sup>-1</sup> d<sup>-1</sup> (range: below detection &#x2013; 0.89). In the MB spring, surface &#x3c1; increased by an order of magnitude compared to the autumn, averaging 2.41 &#xb5;mol Si L<sup>-1</sup> d<sup>-1</sup> (range: 0.11 &#x2013; 8.11, <xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). On the LS during summer, the average &#x3c1; in the surface, 2.34 &#xb5;mol Si L<sup>-1</sup> d<sup>-1</sup>, and the range of &#x3c1; spanned over four orders of magnitude (range: below detection &#x2013; 17.4, <xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>). During spring, surface rates were similar, averaging 2.73 &#xb5;mol Si L<sup>-1</sup> d<sup>-1</sup> (range: 0.06 &#x2013; 8.26, <xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>) on the shelf, with a low rate in the deep-water station (&lt;0.01 &#xb5;mol Si L<sup>-1</sup> d<sup>-1</sup>, <xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). Vertically, &#x3c1; declined with depth in the euphotic zone for both regions (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>); however, there was a disparity at depth, as &#x3c1; in the mid euphotic zone on the LS was ~twice the rate at the same light depth on the MB. &#x222b;&#x3c1; during the autumn MB cruise stations were 6.17 and 29.0 mmol Si m<sup>-2</sup> d<sup>-1</sup>, this was elevated in the MB during the spring where the average was 9.39 mmol Si m<sup>-2</sup> d<sup>-1</sup> (range: 4.86 &#x2013; 15.3, <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). On the LS, &#x222b;&#x3c1; in the summer and spring averaged 9.54 mmol Si m<sup>-2</sup> d<sup>-1</sup> (range: 0.27 &#x2013; 30.8) and 27.0 mmol Si m<sup>-2</sup> d<sup>-1</sup> (range: 2.63 &#x2013; 65.7), respectively. In the deep-water station, &#x222b;&#x3c1; was 0.36 mmol Si m<sup>-2</sup> d<sup>-1</sup>.</p>
<p>bSiO<sub>2</sub> normalized &#x3c1;, V<sub>b</sub>, suggest many zones had active diatom assemblages (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). During autumn on the MB, surface V<sub>b</sub> averaged 0.36 d<sup>-1</sup> (range: below detection to 1.27 d<sup>-1</sup>), while in the spring, the average V<sub>b</sub> was similar 0.40 d<sup>-1</sup> (range: 0.05 to 1.27 d<sup>-1</sup>). These average specific rates imply doubling times approximately 1.5 &#x2013; 1.9 days. On the LS during summer, surface V<sub>b</sub> exceeded the spring values in the MB, averaging 0.50 d<sup>-1</sup> (range: &lt;0.05 to 1.56 d<sup>-1</sup>). The summer surface rate was also higher than the average V<sub>b</sub> during the spring for the shelf stations, 0.33 d<sup>-1</sup> (range: 0.02 to 0.84 d<sup>-1</sup>). These LS rates imply doubling times between 1.4 &#x2013; 2.1 days. At the deep-water station, V<sub>b</sub> was low (0.03 d<sup>-1</sup>) and suggests a doubling time of ~23 days. The similarity in V<sub>b</sub> between regions also manifested in vertical trends, as V<sub>b</sub> declined precipitously in the lower euphotic zone (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>).</p>
<p>Statistical analysis on the drivers of V<sub>b</sub> (to account for biomass differences) were done using surface data. During the autumn on the MB, backward stepwise regression model retained Si(OH)<sub>4</sub> and salinity as the best predictors of V<sub>b</sub> (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>); however, in the spring, the NO<sub>3</sub>+NO<sub>2</sub> alone was retained (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). On the LS (excluding the deep-water site), the model retained Si(OH)<sub>4</sub> in both seasons, but in summer and spring, SRP and NO<sub>3</sub>+NO<sub>2</sub> were also retained, respectively, along with salinity in the spring (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>).</p>
<table-wrap id="T2" position="float">
<label>Table&#xa0;2</label>
<caption>
<p>Backwards stepwise regression analysis to explore drivers of V<sub>b</sub> in the surface waters among stations and cruises.</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="top" align="left">Region</th>
<th valign="top" align="left">Season/Year</th>
<th valign="top" align="left">Retained parameters: coefficient &#xb1; SE</th>
<th valign="top" align="left">Adjusted R<sup>2</sup>
</th>
<th valign="top" align="left">F-value</th>
<th valign="top" align="left">P-value</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="top" align="left">MB</td>
<td valign="top" align="left">Autumn 2015</td>
<td valign="top" align="left">Constant: 11.7 &#xb1; 3.38<break/>Si(OH)<sub>4</sub>: -0.14 &#xb1; 0.07<break/>Salinity: -0.31 &#xb1; 0.09</td>
<td valign="top" align="left">0.55</td>
<td valign="top" align="left">6.38</td>
<td valign="top" align="left">0.03</td>
</tr>
<tr>
<td valign="top" align="left">MB</td>
<td valign="top" align="left">Spring 2016</td>
<td valign="top" align="left">Constant: 0.21 &#xb1; 0.05<break/>NO<sub>3</sub>+NO<sub>2</sub>: 0.14 &#xb1; 0.02</td>
<td valign="top" align="left">0.50</td>
<td valign="top" align="left">34.2</td>
<td valign="top" align="left">&lt;0.01</td>
</tr>
<tr>
<td valign="top" align="left">LS</td>
<td valign="top" align="left">Summer 2016</td>
<td valign="top" align="left">Constant: 0.58 &#xb1; 0.16<break/>Si(OH)<sub>4:</sub> 0.05 &#xb1; 0.02<break/>SRP: -1.42 &#xb1; 0.56</td>
<td valign="top" align="left">0.59</td>
<td valign="top" align="left">5.74</td>
<td valign="top" align="left">0.03</td>
</tr>
<tr>
<td valign="top" align="left">LS</td>
<td valign="top" align="left">Spring 2017</td>
<td valign="top" align="left">Constant: -1.14 &#xb1; 0.44<break/>Si(OH)<sub>4:</sub> 0.10 &#xb1; 0.02<break/>NO<sub>3</sub>+NO<sub>2</sub>: -0.02 &#xb1; 0.01<break/>Salinity: 0.04 &#xb1; 0.01</td>
<td valign="top" align="left">0.97</td>
<td valign="top" align="left">86.2</td>
<td valign="top" align="left">&lt;0.01</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>Models started with all parameters and the most predictive were retained (coefficient shown). Also reported are the adjusted R<sup>2</sup> for the model and the associated F- and P-values from an Analysis of Variance.</p>
</fn>
</table-wrap-foot>
</table-wrap>
</sec>
</sec>
<sec id="s4" sec-type="discussion">
<title>Discussion</title>
<sec id="s4_1">
<title>High silica production in the northern Gulf of Mexico: physics and allochthonous factors</title>
<p>River plumes are among the most highly productive ecosystems in the ocean. The nGoM shelf waters have been dubbed the &#x2018;Fertile Fisheries Crescent&#x2019; (<xref ref-type="bibr" rid="B23">Gunter, 1963</xref>) due to the exceptional productivity of upper trophic level biomass. In many productive fisheries environments (e.g. Monterey Bay, Bering Sea), diatom production can be exceptional (<xref ref-type="bibr" rid="B7">Brzezinski et&#xa0;al., 2003</xref>; <xref ref-type="bibr" rid="B31">Krause et&#xa0;al., 2021</xref>). To our knowledge, this study reports the first &#x222b;&#x3c1; rates within the euphotic zone for the nGoM. Despite the backdrop of a highly productive pelagic ecosystem, the measured &#x3c1; on the LS and MB are not among the highest in systems studied to date. Among a survey of 24 studies within many high and low productivity regions, the average &#x222b;&#x3c1; rate on the LS shelf was among the upper 33% whereas MB is closer to the median (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6A</bold>
</xref>). &#x222b;&#x3c1; in the highest productivity systems (e.g. coastal upwelling zones, Southern Ocean sectors) exceed averages of 50 mmol Si m<sup>-2</sup> d<sup>-1</sup>, whereas the average for the LS during spring was 25 mmol Si m<sup>-2</sup> d<sup>-1</sup> (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>) and for MB was 10 mmol Si m<sup>-2</sup> d<sup>-1</sup> (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). While the spring cruises were in different years (2016, MB; 2017, LS), the spread of the &#x222b;&#x3c1; and the consistently higher values quantified on the LS suggests this may be a consistent difference; we discuss the potential reason(s) for this below (final discussion section). However, the magnitude of diatom production, while enhanced relative to many other systems, is not so exceptionally high as to be a clear factor leading to the stimulation of fisheries production in this region. Therefore, we propose that the unique hydrographic factors working in this system, help to physically concentrate diatom biomass to make it more assessable to higher trophic organisms. Some potential mechanisms are explored below:</p>
<fig id="f6" position="float">
<label>Figure&#xa0;6</label>
<caption>
<p>Survey of integrated euphotic zone biogenic silica production rate ranged from highest to lowest <bold>(A)</bold> among 24 studies and the euphotic-zone depth normalized production <bold>(B)</bold>, termed euphotic zone density (mathematically equivalent single-depth units) for the same studies arranged from high to low. Average values per study are reported from the literature including the following systems: coastal/upwelling (C/U), river plume (RP), open ocean (Open), Southern Ocean (SO), and the Arctic region (RU); studies from mid-ocean gyres (e.g. Hawaii Ocean Time-series, Bermuda Atlantic Time Series study) were omitted due to very low absolute and depth-normalized rates. The average LS and MB value are denoted by the arrows in each plot. Data and original studies are listed in <xref ref-type="supplementary-material" rid="SM1">
<bold>Supplementary Table S1</bold>
</xref>.</p>
</caption>
<graphic mimetype="image" mime-subtype="tiff" xlink:href="fmars-10-1162685-g006.tif"/>
</fig>
<p>Given the river plume forcing, we posit that the high system productivity in the LS and MB can be partially explained by diatom productivity being highly accessible to higher trophic organisms. When normalizing the &#x222b;&#x3c1; depth to the depth of the euphotic zone (i.e. mathematically equivalent to volumetric rates, e.g. &#xb5;mol L<sup>-1</sup> d<sup>-1</sup> or mmol m<sup>-3</sup> d<sup>-1</sup>), the LS is the 5<sup>th</sup> highest, and the MB is in the upper 50% for &#x222b;&#x3c1; among our survey (<xref ref-type="fig" rid="f6">
<bold>Figure&#xa0;6B</bold>
</xref>). The local river plume forcing facilitates water-column stability (stratification) and thereby allows phytoplankton to consume nutrients and avoid light limitation due to deep mixing. The degree of stratification in this system is extreme, as density differences between the surface and main pycnocline due to salinity are typically much higher (&gt;&gt;1&#xa0;kg m<sup>-3</sup>) than open ocean metrics used define mixed layers (e.g. &gt;0.1&#xa0;kg m<sup>-3</sup> differences from the surface). Thus, residence in this stable water column facilitates phytoplankton consumption of nutrients to exhaustion &#x2014;if other ecological or physical factors are favorable. Indeed, the lowest surface Si(OH)<sub>4</sub> observed during the LS spring bloom was 0.17 &#xb5;M. This concentration is multiple factors lower than Si(OH)<sub>4</sub> observed (i.e. ~0.6 &#x2013; 0.8 &#xb5;M) in the low-diatom-biomass oligotrophic gyres (<xref ref-type="bibr" rid="B8">Brzezinski et&#xa0;al., 2011</xref>; <xref ref-type="bibr" rid="B28">Krause et&#xa0;al., 2017</xref>) and is similar to intense diatom blooms observed on the LS (<xref ref-type="bibr" rid="B42">Nelson and Dortch, 1996</xref>) or in productive upwelling systems (<xref ref-type="bibr" rid="B43">Nelson et&#xa0;al., 1981</xref>; <xref ref-type="bibr" rid="B29">Krause et&#xa0;al., 2015</xref>; <xref ref-type="bibr" rid="B11">Closset et&#xa0;al., 2021</xref>). We suggest that the LS and MB physics allow for diatom production to be more accessible to higher trophic organisms compared to other systems where the integrated productivity may be higher, but more diffusely distributed within the water column. This is consistent with ideas concerning the importance of spatial aggregations as predictors of higher trophic level variation, e.g. <xref ref-type="bibr" rid="B3">Benoit-Bird and McManus (2012)</xref>, opposed to typical volumetric abundances. Specifically, the ideas that have emerged in the last decade are not that volume-based rates or standing stock matter for ecological productivity, but instead how aggregated and concentrated production is (or can be at times), as highly concentrated production can be more easily accessed by higher trophic levels.</p>
<p>Beyond basic river-plume stratification dynamics (discussed above), there is regional evidence that small scale and/or ephemeral physical mechanisms in the nGoM may also help efficiently funnel diatom productivity to higher trophic levels. Using an <italic>in situ</italic> imaging system, <xref ref-type="bibr" rid="B21">Greer et&#xa0;al. (2020)</xref> observed a ~2.3 km (lateral extent) thin layer, dominated by the diatom <italic>Odontella</italic> sp., during July 2016 in the southwestern domain of the MB. <italic>Odontella</italic> species typically have large cell sizes compared to other diatom genera, especially slender pennate species which are abundant regionally (<xref ref-type="bibr" rid="B38">MacIntyre et&#xa0;al., 2011</xref>; <xref ref-type="bibr" rid="B2">Bargu et&#xa0;al., 2016</xref>). <xref ref-type="bibr" rid="B21">Greer et&#xa0;al. (2020)</xref> suggested the growth and grazing decoupling and motility (i.e. <italic>Odontella</italic> sp. is not a swimmer) in the thin layer were not major factors contributing to its formation. Specifically, <italic>Odontella</italic> sp. abundances outside of the thin layer were so low the disparity in abundances within and outside of the layer could not be solely from increased growth rates within the layer; therefore, these authors concluded diatom cells were concentrated by physical processes. Leveraging a high-resolution physical model, <xref ref-type="bibr" rid="B21">Greer et&#xa0;al. (2020)</xref> suggested the thin-layer feature was driven by surface convergence and vertical shear. Furthermore, their model outputs suggested that similar convergences occur frequently in the MB region and may contribute to efficient trophic transfer of phytoplankton organic matter to higher trophic levels. Such an idea is consistent with other studies in warm-water regions where standing stocks of phytoplankton may be relatively low but can be enhanced by similar physical mechanisms and fuel efficient passage of producer organic matter to higher trophic level organisms, e.g. Hawaii (<xref ref-type="bibr" rid="B39">McManus et&#xa0;al., 2012</xref>).</p>
<p>Despite the potential importance for thin layers regionally, it is unlikely that such dynamics were resolved in our study. Our vertical sampling was limited to four or five depths, which were predetermined based on the irradiance levels required for our deck board incubator. Consequently, it is unlikely that such thin-layer biomass would have been accurately captured in our rates. Specifically, the vertical orientation of the Niskin bottle would likely have sampled a component of such a layer (if any at all). However, this may not matter when trying to quantify the regional &#x3c1;. Our bottle incubations clearly captured the vertical extent of euphotic-zone productivity (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>). Such short-duration physical dynamics acting to concentrate (opposed to stimulate) diatom production within a vertical layer would not radically alter the total euphotic zone integrated production, but redistribute (i.e. concentrate) the biomass. Thus, these features may not affect total &#x3c1; or annual productivity but could be important to facilitate more efficient transfer of this material up the foodweb.</p>
<p>Comparison of &#x222b;&#x3c1; rates with riverine Si(OH)<sub>4</sub> delivery show differences in the river effect between systems. Given our vertical sampling was confined to the euphotic zone, we consider stocks and rates in a euphotic-zone integrated context (e.g. one euphotic-zone box opposed to a two-box water column). In the MB, the delivery of riverine Si(OH)<sub>4</sub> to the CONCORDE sampling domain (area of the three corridors) would support ~33-40% of the &#x222b;&#x3c1; (<xref ref-type="table" rid="T3">
<bold>Table&#xa0;3</bold>
</xref>); the proportional importance decreases if the entire MB shelf area is considered (i.e. Si in discharged water would be spread over a larger area). This implies that other Si(OH)<sub>4</sub> sources are necessary to sustain the production rates of diatoms in the water column. Water-column recycling of bSiO<sub>2</sub> can be quantitatively important. <xref ref-type="bibr" rid="B7">Brzezinski et&#xa0;al. (2003)</xref> reported that even during blooms in productive systems (e.g. Monterey Bay), 10-20% of &#x222b;&#x3c1; was supported by euphotic-zone bSiO<sub>2</sub> remineralization. Furthermore, the proportional support of &#x222b;&#x3c1; from dissolution increases during non-bloom periods (e.g. 60-70% of &#x3c1;) or in regions with warmer water temperatures (e.g. ~30% in a Gulf Stream Warm Core Ring). Given surface temperatures in the MB were ~20&#xb0;C and 24&#xb0;C for the spring and autumn cruises, respectively, and the specific dissolution rate for bSiO<sub>2</sub> in slightly cooler (~15-20&#xb0;C) subtropical waters was reported to be ~0.15 d<sup>-1</sup> (<xref ref-type="bibr" rid="B9">Brzezinski and Nelson, 1989</xref>), such a dissolution rate applied to our measured &#x222b;bSiO<sub>2</sub> standing stock would be sufficient to meet 65-100% of the &#x222b;&#x3c1; during the MB cruises (<xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>). Note, this would be an upper estimate if most of the bSiO<sub>2</sub> standing stock dissolves below the euphotic zone. Furthermore, benthic flux, while not reported for Si(OH)<sub>4</sub> in the MB is likely similar to that observed on the LS given the similarity in bSiO<sub>2</sub> content in sediments between these regions (<xref ref-type="bibr" rid="B47">Pickering et&#xa0;al., 2020</xref>; <xref ref-type="bibr" rid="B26">Kemp et&#xa0;al., 2021</xref>). <xref ref-type="bibr" rid="B33">Lehrter et&#xa0;al. (2012)</xref> reported benthic Si flux rates ranging from 0.3 &#x2013; 4.4 mmol Si m<sup>-2</sup> d<sup>-1</sup> on the LS in spring and summer. This represents a major pool of usable Si but given the shallow euphotic zones and surface layers, such a pool may not be accessible to diatoms until tropical cyclones (e.g. summer) or cold fronts (e.g. autumn) break down this stratification and mix the water column. Taken together, the MB data suggest that over the entirety of the shelf, riverine Si and internal recycling may meet the Si demand for diatoms; however, within this broad area, there are ephemeral conditions where diatoms can exhaust Si faster than replenishment, as has been observed offshore of coastal Alabama in the northern domain of the MB, e.g. <xref ref-type="bibr" rid="B38">MacIntyre et&#xa0;al. (2011)</xref>.</p>
<table-wrap id="T3" position="float">
<label>Table&#xa0;3</label>
<caption>
<p>Comparison of riverine Si(OH)<sub>4</sub> flux relative to &#x222b;&#x3c1; (e.g. <xref ref-type="table" rid="T1">
<bold>Table&#xa0;1</bold>
</xref>) on the MB and LS during each cruise (error is standard deviation).</p>
</caption>
<table frame="hsides">
<thead>
<tr>
<th valign="top" align="left">Region</th>
<th valign="top" align="left">Season/Year</th>
<th valign="top" align="left">Cruise-period discharge (m<sup>3</sup> s<sup>-1</sup>)</th>
<th valign="top" align="left">Shelf Area (km<sup>2</sup>)</th>
<th valign="top" align="left">Riverine Si(OH)<sub>4</sub> (mmol Si m<sup>-2</sup> d<sup>-1</sup>)</th>
<th valign="top" align="left">&#x222b;&#x3c1; (mmol Si m<sup>-2</sup> d<sup>-1</sup>)</th>
<th valign="top" align="left">Riverine Si(OH)<sub>4</sub>/&#x222b;&#x3c1;</th>
</tr>
</thead>
<tbody>
<tr>
<td valign="top" align="left">MB</td>
<td valign="top" align="left">Autumn 2015</td>
<td valign="top" align="left">866 &#xb1; 365</td>
<td valign="top" align="left">9000</td>
<td valign="top" align="left">0.47 &#xb1; 0.35</td>
<td valign="top" align="left">1.15 &#xb1; 1.14</td>
<td valign="top" align="left">0.41 &#xb1; 0.51</td>
</tr>
<tr>
<td valign="top" align="left"/>
<td valign="top" align="left">Spring 2016</td>
<td valign="top" align="left">3487 &#xb1; 1730</td>
<td valign="top" align="left">9000</td>
<td valign="top" align="left">2.49 &#xb1; 0.75</td>
<td valign="top" align="left">7.44 &#xb1; 5.22</td>
<td valign="top" align="left">0.33 &#xb1; 0.26</td>
</tr>
<tr>
<td valign="top" align="left">LS</td>
<td valign="top" align="left">Summer 2016</td>
<td valign="top" align="left">12987 &#xb1; 1259</td>
<td valign="top" align="left">16500</td>
<td valign="top" align="left">8.78 &#xb1; 1.16</td>
<td valign="top" align="left">9.54 &#xb1; 10.3</td>
<td valign="top" align="left">0.92 &#xb1; 1.01</td>
</tr>
<tr>
<td valign="top" align="left"/>
<td valign="top" align="left">Spring 2017</td>
<td valign="top" align="left">25786 &#xb1; 5316</td>
<td valign="top" align="left">16500</td>
<td valign="top" align="left">15.7 &#xb1; 3.62</td>
<td valign="top" align="left">25.1 &#xb1; 19.7</td>
<td valign="top" align="left">0.62 &#xb1; 0.51</td>
</tr>
</tbody>
</table>
<table-wrap-foot>
<fn>
<p>MB discharge data are from Mobile Bay were aggregated from United States Geological Survey stations 02428400 (Alabama River) and 02469761 (Tombigbee River); these two rivers represent ~90% of the Mobile Bay discharge and were corrected to account for this volume difference as in <xref ref-type="bibr" rid="B15">Dykstra and Dzwonkowski (2020)</xref>. LS discharge data are from the Mississippi River at United States Geological Survey station 7374000. The MB and LS areas were estimated based on the main area encompassing the three CONCORDE meridional transects offshore of Mobile Bay (MB) and the Mississippi River plume affected area of the LS (<xref ref-type="bibr" rid="B18">Ebner, 2019</xref>) without affects from the Atchafalaya River. Riverine Si(OH)<sub>4</sub> for MB was estimated using the highest values quantified during CONCORDE and average values for months at the FOCAL Mobile Bay stations (<xref ref-type="bibr" rid="B56">Sutton et&#xa0;al., 2023</xref>) from 2008-2012.</p>
</fn>
</table-wrap-foot>
</table-wrap>
<p>Unlike the delivery of Si(OH)<sub>4</sub> to the MB, the MR delivers an order of magnitude more Si to the LS where it is used in an area less than twice that considered for the MB (<xref ref-type="table" rid="T3">
<bold>Table&#xa0;3</bold>
</xref>). During the summer and spring, the riverine flux averaged 62% and 92% of the measured &#x222b;&#x3c1;. Thus, over the entirety of the shelf, the river flux combined with internal recycling of bSiO<sub>2</sub> (discussed above) should have been sufficient to meet the Si demand for diatoms. Furthermore, consistent with calculations by <xref ref-type="bibr" rid="B33">Lehrter et&#xa0;al. (2012)</xref>, benthic flux of Si could also have been a significant source of Si to help diatom avoid limitation, especially for any diatom production occurring deeper than the main pycnocline. While many studies have suggested that Si limitation may be more prevalent now than before eutrophication (<xref ref-type="bibr" rid="B59">Turner et&#xa0;al., 1998</xref>), when considering bulk rates of delivery and shelf-wide production, any limitation by Si is not due to a lack of Si within the integrated system (i.e. upper water column over the shelf area) but must be due to the balance between diatom growth and the Si(OH)<sub>4</sub> supply to the euphotic zone. While the <xref ref-type="bibr" rid="B59">Turner et&#xa0;al. (1998)</xref> study (among others), inferred diatom Si limitation by nutrient ratios (i.e. Si depleted before N), there is more direct evidence showing that diatoms can be limited by Si(OH)<sub>4</sub> on the LS. <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> reported many instances where Si(OH)<sub>4</sub> in the upper water column was &lt;1.5 &#xb5;M in the summer and &lt;0.4 &#xb5;M in the spring. We did not observe such low concentrations during the summer, but during the spring on the LS six (of 14) stations had surface Si(OH)<sub>4</sub> &lt;1 &#xb5;M with the lowest concentration of 0.17 &#xb5;M (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). Thus, both our study and <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> demonstrate diatoms can effectively exhaust Si(OH)<sub>4</sub> faster than local physical delivery (e.g. mixing) or remineralization scales in the euphotic zone, despite considerable pools of Si which are separated by relatively short vertical scales (e.g. &lt;5&#xa0;m) &#x2014;especially compared to gradients in deep-water systems. In the absence of exceptionally concentrated diatom production, which can deplete Si(OH)<sub>4</sub> to levels that likely limit diatom growth, e.g. <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> (and this study), it appears that most of the Si production on the shelf can be met by external Si delivered by the MR with the rest being met by internal recycling or benthic flux (after the exceptional water column stratification is broken down).</p>
</sec>
<sec id="s4_2">
<title>Biogenic silica production drivers</title>
<p>While many factors potentially affect the rate of biogenic silica production (i.e. V<sub>b</sub>), our analysis focused on a common subset of parameters available for all cruises. The models all explain at least a majority of the variance (i.e. R<sup>2&#xa0;</sup>= 0.50 to 0.97), but only one explains nearly all the variance (i.e. LS Spring). Thus, for the other three cruises (LS Summer, MB cruises), there is a quantitatively significant portion of variance which cannot be explained by the variables considered. Despite this, the model results provide an empirical approach to evaluate the factors which best correlated to V<sub>b</sub> during these cruises.</p>
<p>A significant number of studies reporting &#x3c1; are from systems with relatively invariant salinity. Even in coastal systems with such data, e.g. Bay of Brest (<xref ref-type="bibr" rid="B4">Beucher et&#xa0;al., 2004</xref>), salinity ranges are relatively minor (e.g. 32 &#x2013; 36) compared to the nGoM. During our study, we observed considerable variability in surface salinity during spring (both regions, <xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>) and summer (LS). For the MB, the effect of the estuarine condition differed between autumn and spring. The autumn cruise occurred after the passage of the remnants from Hurricane Patricia (<xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al., 2017</xref>); note, given the lack of historical data, we cannot disentangle the effect of any prior anthropogenic changes that may alter the response following the storm. The storm surge flooded the nGoM estuaries, and as the waters flowed back into the Gulf, the post-storm stratification led to increased rates of primary production, especially among larger cells (e.g. <italic>Trichodesmium</italic>, diatoms) within 15&#xa0;km of the Mobile Bay outflow (<xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al., 2017</xref>). The regression model for these autumn 2015 data shows that the strongest predictors for V<sub>b</sub> were salinity and Si(OH)<sub>4</sub> (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). Both parameters had negative coefficients, which means they scaled in the same direction. Si(OH)<sub>4</sub> in the waters feeding MB (e.g. Mobile Bay) behaves semi-conservatively, especially compared to other dissolved nutrients, e.g. NO<sub>3</sub>+NO<sub>2</sub>, NH<sub>4</sub>, SRP (<xref ref-type="bibr" rid="B56">Sutton et&#xa0;al., 2023</xref>), and thus we would expect Si(OH)<sub>4</sub> and salinity to be negatively correlated (i.e. higher Si(OH)<sub>4</sub> at lower salinities). Our model instead showed these parameters trended the same direction and V<sub>b</sub> was highest when Si(OH)<sub>4</sub> and salinity were lower. Such a trend was likely driven by the receding storm surge. Thus, the importance of the salinity effect in this specific season was the ability for the low salinity water to stabilize the water column (which had been well mixed) post storm thereby allowing phytoplankton (e.g. diatoms) to grow; this is consistent with interpretations by <xref ref-type="bibr" rid="B17">Dzwonkowski et&#xa0;al. (2017)</xref>. During spring in the MB, salinity was not a significant predictor, as V<sub>b</sub> was positively associated with NO<sub>3</sub>+NO<sub>2</sub> (<xref ref-type="table" rid="T2">
<bold>Table&#xa0;2</bold>
</xref>). Such a result is consistent with the differences in the Si:N ratio of waters feeding the MB and LS, where Si:N in the MB is high (&gt;1) and optimal for diatoms compared to the LS (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>, discussed below). In this case, the quantity of NO<sub>3</sub>+NO<sub>2</sub> to the MB was the best predictor of the quantified diatom silica production rate, suggesting a potential stimulatory effect near the plume edge where nutrients are still elevated relative to the offshore waters.</p>
<p>Drivers on the LS differed from those on the MB. The loading of nutrients in the MR watershed increased both N and P in the river over time (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref> for nitrate). Determining what nutrient may limit primary production for the community has important regional implications for management actions in the watershed. For instance, much of the regional regulatory effort to combat hypoxia on the LS focuses on the reduction of N in the river. But while N can limit primary productivity on the LS, e.g. <xref ref-type="bibr" rid="B48">Rabalais et&#xa0;al. (2002)</xref> and references therein, there are times when P can be limiting, e.g. <xref ref-type="bibr" rid="B57">Sylvan et&#xa0;al. (2006)</xref>. The LS cruise effort was more focused on benthic-pelagic coupled processes vs. the cruises in the MB with more fine-scale spatial sampling; therefore, in the LS we have only ~25% of the spatial stations compared to the MB: although unlike the MB, we have full vertical profiles for each occupied station.</p>
<p>The backward stepwise regression model analysis identified that the strongest nutrient predictors for V<sub>b</sub> during the summer and spring cruises were NO<sub>3</sub>+NO<sub>2</sub> (spring, negative coefficient), SRP (summer, negative coefficient), silicate (both seasons, positive coefficient), and salinity (spring, positive coefficient). Thus, our data analysis suggests that diatom productivity (V<sub>b</sub>) was responding more to NO<sub>3</sub>+NO<sub>2</sub> in the spring and SRP in the summer, while there is a persistent underlying response to Si(OH)<sub>4</sub> in both seasons. The latter is expected as Si(OH)<sub>4</sub> affects both diatom growth rates and their rate of Si uptake (e.g. nutrient kinetics). The positive correlation between Si(OH)<sub>4</sub> and V<sub>b</sub> in our statistical analysis infers some degree of kinetic limitation, i.e. Si(OH)<sub>4</sub> is suboptimal for uptake (V<sub>b</sub> at ambient Si(OH)<sub>4</sub> &lt; V<sub>b</sub> at non-limiting Si(OH)<sub>4</sub>) and under these conditions diatoms alter physiology to compensate (<xref ref-type="bibr" rid="B40">McNair et&#xa0;al., 2018</xref>). Such kinetic limitation is also consistent with previous LS data from the early 1990s (<xref ref-type="bibr" rid="B42">Nelson and Dortch, 1996</xref>).</p>
<p>Model results indicate that the sign of the model coefficients for NO<sub>3</sub>+NO<sub>2</sub> in the spring and SRP in summer are negative. These data infer that V<sub>b</sub>, a proxy for diatom growth, is stimulated at lower NO<sub>3</sub>+NO<sub>2</sub> and SRP concentrations than the high-nutrient riverine endmember (if this were the case, then the model coefficient for both would be positive). This is consistent with both remote data and shipboard incubation work showing that primary production and phytoplankton biomass accumulation rates are not highest in the main part of the river discharge zone but downstream, likely due to improved light conditions (reduced turbidity) and stratification which facilitates growth (<xref ref-type="bibr" rid="B35">Lohrenz et&#xa0;al., 1997</xref>; <xref ref-type="bibr" rid="B34">Lehrter et&#xa0;al., 2009</xref>). Our project results highlight that Si(OH)<sub>4</sub> always plays a role in diatom Si uptake (and potentially becomes a growth-limiting factor when concentrations are very low, e.g. &lt;1 &#xb5;M), but typically diatom activity switches from being more correlated to NO<sub>3</sub>+NO<sub>2</sub> in the spring and SRP in the summer. Such empirical connection of diatom activity to three different nutrients suggests modeling diatom growth in this region over time may be challenging. Identifying the underlying factors affecting these trends (e.g. changes in diatom assemblage, direct evidence of nutrient limitation) should be a priority given diatoms critical role in system productivity.</p>
</sec>
<sec id="s4_3">
<title>Has diatom silica production changed due to eutrophication?</title>
<p>As discussed, there have been significant changes to Si(OH)<sub>4</sub> and NO<sub>3</sub> concentrations in the MR (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>) which feeds the nGoM (<xref ref-type="bibr" rid="B60">Turner and Rabalais, 1991</xref>; <xref ref-type="bibr" rid="B61">Turner and Rabalais, 1994a</xref>; <xref ref-type="bibr" rid="B62">Turner and Rabalais, 1994b</xref>; <xref ref-type="bibr" rid="B63">Turner et&#xa0;al., 2008</xref>). These results compelled a United States Environmental Protection Agency Science Advisory Board (<xref ref-type="bibr" rid="B12">Dale et&#xa0;al., 2007</xref>) to recommend that:</p>
<p>
<italic>&#x201c;. the potential for silicate limitation and its effects on phytoplankton production and composition on the Louisiana-Texas continental shelf should be explored when carrying out experiments on the importance of N and P as limiting factors and when considering nutrient management scenarios.&#x201d;</italic>
</p>
<p>The relationship between eutrophication and diatom production could be evaluated by understanding how diatom production rates respond to eutrophied conditions. However, the lack of diatom-specific rate information, facilitated by using isotope-addition methods (e.g. <sup>30</sup>Si, <sup>32</sup>Si), has hindered understanding the quantity of production attributed to diatoms in this system. While our production rate data set lacks a robust baseline, i.e. one study reporting data on &#x3c1; during the early 1990s (<xref ref-type="bibr" rid="B42">Nelson and Dortch, 1996</xref>), the comparison between the LS and MB offers potential insight regarding how the system operates now and we can speculate whether this may have changed in the last half century.</p>
<p>Compared to a past study in the nGoM (<xref ref-type="bibr" rid="B42">Nelson and Dortch, 1996</xref>), we find no evidence that diatom growth (i.e. V<sub>b</sub> is proxy) in the upper depths is significantly different. Working in the same region of the LS during the early 1990s, rates reported by <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> during the summer and spring fell within the range for our study on the LS during the same seasons (<xref ref-type="fig" rid="f4">
<bold>Figure&#xa0;4</bold>
</xref>) with no significant differences (Mann Whitney U Test (Summer, Spring), U = 25, 47, p = 0.29, 0.88), albeit these earlier data are less variable. <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> also observed a significant difference between LS rates during spring (lower) and summer (higher), such a trend was not resolved during this present study. With the caveat that <xref ref-type="bibr" rid="B42">Nelson and Dortch (1996)</xref> did not report vertically integrated rates, the comparison of our data with those in the early 1990s does not suggest any significant change to the specific rate of diatom bSiO<sub>2</sub> production.</p>
<p>Comparison of the &#x222b;&#x3c1; data during spring between the MB and LS are consistent with a eutrophication effect suggested in previous studies. There is relatively low nitrate in the Mobile Bay waters, &lt;20 &#xb5;M (<xref ref-type="bibr" rid="B46">Pennock et&#xa0;al., 1999</xref>), compared to the &gt;100 &#xb5;M concentrations in the MR derived from eutrophication in the watershed (<xref ref-type="fig" rid="f1">
<bold>Figure&#xa0;1</bold>
</xref>). The high nitrate and relative lack of light limitation as the MR discharges onto a deeper continental shelf, potentially facilitates Si(OH)<sub>4</sub> uptake while waters are diluted on the LS (consistent with reported non-conservative behavior for Si(OH)<sub>4</sub>). This situation (i.e. high NO<sub>3</sub>+NO<sub>2</sub> and NO<sub>3</sub>+NO<sub>2</sub>&gt;Si(OH)<sub>4</sub>) does not appear to occur in the MB (consistent with reported conservative behavior for Si(OH)<sub>4</sub>). During spring, the LS had higher surface concentrations and ranges for NO<sub>3</sub>+NO<sub>2</sub> and NH<sub>4</sub> relative to those in the MB, but surface concentrations of Si(OH)<sub>4</sub> were higher in the MB than in LS (<xref ref-type="fig" rid="f3">
<bold>Figure&#xa0;3</bold>
</xref>). Such differences are consistent with the well-reported anthropogenic factors in the MR watershed compared to the MB watersheds. For example, the increased nitrogen has been linked to fertilizer use in the MR watershed (<xref ref-type="bibr" rid="B59">Turner et&#xa0;al., 1998</xref>). Whereas the reduced Si(OH)<sub>4</sub> over time has been attributed to factors like damming (e.g. bSiO<sub>2</sub> retained in reservoirs behind dams vs. being carried downstream and dissolving en route), as reported in the Black Sea (<xref ref-type="bibr" rid="B25">Humborg et&#xa0;al., 1997</xref>) and other systems (<xref ref-type="bibr" rid="B24">Humborg et&#xa0;al., 2000</xref>). In the MB, the watersheds which provide fluvial input onto the shelf ecosystem are much smaller, with comparatively less developed urban and agricultural regions, and have the opposite nutrient trends (i.e. relatively lower NO<sub>3</sub>+NO<sub>2</sub> and higher Si(OH)<sub>4</sub>) than the MR. Although urbanization is occurring within the MB watershed, the degree of eutrophication is much lower than for the MR (<xref ref-type="bibr" rid="B46">Pennock et&#xa0;al., 1999</xref>). Such watershed differences would be expected to affect diatom processes downstream in the coastal domain.</p>
<p>The integrated diatom productivity appears to be significantly higher in the LS compared to the MB during spring, the season we can compare directly. Despite comparable V<sub>b</sub> in the MB surface waters, the average &#x222b;&#x3c1; was 2.5x higher in the euphotic zone of the LS than the MB. This difference is stark considering the average euphotic zone depth on the LS in spring was ~12 m vs. ~19 m for the MB. While both euphotic zones are relatively shallow compared to deep-water systems, there is likely a vertical zone in both systems where cells switch from nutrient limitation (under high irradiance) to light limitation &#x2014;sensu <xref ref-type="bibr" rid="B14">Dugdale (1967)</xref>. Phytoplankton cells can compensate for the lower light by increasing their light harvesting capacity (e.g. increase pigment per cell); however, pigment complexes have higher N requirements relative to P (<xref ref-type="bibr" rid="B19">Geider et&#xa0;al., 1996</xref>). Thus, given that the integrated standing stock of NO<sub>3</sub>+NO<sub>2</sub> on the LS in spring was double (~90 mmol m<sup>-2</sup>) that in the euphotic zone of the MB (~45 mmol m<sup>-2</sup>, data not shown), the additional NO<sub>3</sub>+NO<sub>2</sub> may have helped diatoms in the lower euphotic zone avoid reducing growth rates due to rapidly attenuating light in the turbid waters. Hence, we posit that eutrophication sets a higher potential for diatom biomass due to higher total N to exploit (if sufficient Si is available) and increases the euphotic-zone &#x222b;&#x3c1; via facilitating higher uptake rates at depth (<xref ref-type="fig" rid="f5">
<bold>Figure&#xa0;5</bold>
</xref>).</p>
<p>Due to lack of baseline data for the water column on the effect of eutrophication on diatom production, our interpretation that &#x3c1; on the LS has increased in response to eutrophication cannot be tested directly. However, we can glean insights from local sediment records or use experimental approaches (e.g. bioassays) in future studies. For example, on the LS, there has been an increased preservation of diatom valves (especially the genera <italic>Pseudo-nitzschia</italic> which can have toxic species) in sediments through the end of the 20<sup>th</sup> century (<xref ref-type="bibr" rid="B45">Parsons et&#xa0;al., 2002</xref>). On a similar timescale, albeit not directly synchronized with <italic>Pseudo-nitzschia</italic>, there have also been increases in sediment bSiO<sub>2</sub> (<xref ref-type="bibr" rid="B63">Turner et&#xa0;al., 2008</xref>). It is unknown if these trends have continued in the last two decades since these reports. The general trend of increased bSiO<sub>2</sub> preservation in sediments after eutrophication, due to increased diatom &#x3c1; in the water column, has been observed in limnic systems (<xref ref-type="bibr" rid="B53">Schelske et&#xa0;al., 1987</xref>), suggesting some fundamental similarities in the responses of diatoms assemblages to eutrophication among environments. Given the lack of baseline data to assess the general trends in &#x3c1; during and before eutrophication in the 20<sup>th</sup> century, new methods (e.g. silicon stable isotopes) could be employed to fill these temporal gaps. Comparison of trends over time between the LS and MB sediments could provide a means to better understand whether eutrophication has changed diatom silica utilization and production regionally or if similar spatial differences observed in this study reflect general subregional differences driven by the size of the watersheds feeding each site.</p>
</sec>
</sec>
<sec id="s5" sec-type="data-availability">
<title>Data availability statement</title>
<p>The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: <ext-link ext-link-type="uri" xlink:href="https://www.bco-dmo.org/project/712667">https://www.bco-dmo.org/project/712667</ext-link>, <ext-link ext-link-type="uri" xlink:href="http://data.gulfresearchinitiative.org">http://data.gulfresearchinitiative.org</ext-link> (doi: 10.7266/N78050N9), <ext-link ext-link-type="uri" xlink:href="http://data.gulfresearchinitiative.org">http://data.gulfresearchinitiative.org</ext-link> (doi: 10.7266/N70K2738).</p>
</sec>
<sec id="s6" sec-type="author-contributions">
<title>Author contributions</title>
<p>All authors were involved in the collection of samples, sample analyses, archiving data, and assisted with editing the manuscript. The study was conceived by JK with input from KM. The data analysis and initial manuscript was done by JK. All authors contributed to the article and approved the submitted version.</p>
</sec>
</body>
<back>
<sec id="s7" sec-type="funding-information">
<title>Funding</title>
<p>The Louisiana Shelf work was funded by the United States National Science Foundation Chemical Oceanography (OCE&#x2010;1558957 to JK and KM). The Mississippi Bight research was made possible by a grant from the Gulf of Mexico Research Initiative through the CONCORDE Consortium and ACER Consortium programs (both to JK). Data associated with this work are publicly available through the Biological &amp; Chemical Oceanography Data Management Office (<ext-link ext-link-type="uri" xlink:href="https://www.bco-dmo.org/project/712667">https://www.bco-dmo.org/project/712667</ext-link>) and Gulf of Mexico Research Initiative Information &amp; Data Cooperative (GRIIDC) at <ext-link ext-link-type="uri" xlink:href="http://data.gulfresearchinitiative.org">http://data.gulfresearchinitiative.org</ext-link> (doi: 10.7266/N78050N9, 10.7266/N70K2738).</p>
</sec>
<ack>
<title>Acknowledgments</title>
<p>We dedicate this work to the memory of our friend and colleague, Sydney Acton; her technical efforts before, during, and after these four cruises was critical to their success. We also thank the captains, crew, and science parties aboard the R/V Point Sur and R/V Pelican (especially Kevin Martin, Allison Mojzis, Wokil Bam, Neha Ghaisas), Brian Dzwonkowski and Alan Shiller for helpful discussions, and John Perry, Eric Lachenmyer, Grant Lockridge, Yantzee Hintz, and Laura Linn for technical assistance. </p>
</ack>
<sec id="s8" sec-type="COI-statement">
<title>Conflict of interest</title>
<p>The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.</p>
</sec>
<sec id="s9" sec-type="disclaimer">
<title>Publisher&#x2019;s note</title>
<p>All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.</p>
</sec>
<sec id="s10" sec-type="supplementary-material">
<title>Supplementary material</title>
<p>The Supplementary Material for this article can be found online at: <ext-link ext-link-type="uri" xlink:href="https://www.frontiersin.org/articles/10.3389/fmars.2023.1162685/full#supplementary-material">https://www.frontiersin.org/articles/10.3389/fmars.2023.1162685/full#supplementary-material</ext-link>
</p>
<supplementary-material xlink:href="DataSheet_1.pdf" id="SM1" mimetype="application/pdf"/>
</sec>
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