Edited by: Tim Kalvelage, ETH Zurich, Switzerland
Reviewed by: Perran Cook, Monash University, Australia; Susanna Hietanen, University of Helsinki, Finland
*Correspondence: Stefan Sommer
This article was submitted to Marine Biogeochemistry, a section of the journal Frontiers in Marine Science
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Redox-sensitive mobilization of nutrients from sediments strongly affects the eutrophic state of the central Baltic Sea; a region associated with the spread of hypoxia and almost permanently anoxic and sulfidic conditions in the deeper basins. Ventilation of these basins depends on renewal by inflow of water enriched in oxygen (O2) from the North Sea, occurring roughly once per decade. Benthic fluxes and water column distributions of dissolved inorganic nitrogen species, phosphate (
The Baltic Sea is a landlocked marginal sea with a narrow connection to the North Sea through the Kattegat. It consists of a series of basins separated by shallow sills and narrow channels. Restricted water exchange with the North Sea and freshwater input from river run-off maintain a strong surface salinity gradient from around 3 in the Bothnian Bay at the northern end to 20 in the Kattegat (Samuelsson,
Ongoing chronic hypoxia in the Baltic Sea is partly due to a rapid turnover of phosphorus (P) from hypoxic and anoxic sediments (Conley et al.,
Natural ventilation of the deep central basins of the Baltic Sea exclusively depends on episodic inflow events from the North Sea (Matthäus and Franck,
Before ca. 1980, inflow events were relatively frequent and could be observed on average once a year (Matthäus and Franck,
Here, we report on benthic fluxes of nutrients and O2 in the EGB before and after the strong MBI in December 2014 and the moderate MBI in November 2015. Recent studies of nutrient release measured
Sampling campaigns in the EGB were conducted on the RV Alkor cruise AL422 in August/September 2013, RV Poseidon cruise POS487 in July/August 2015 and RV Alkor cruise AL473 in March 2016 (Table
651 | BIGO-II-6 | 57°26.26', 20°43.53′ | 65 | Oxycline | 08. Sep.2013 |
584 | BIGO-I-2 | 57°21.80', 20°35.87' | 80 | HTZ | 23. Aug.2013 |
561 | BIGO-II-1 | 57°20.76', 20°35.32' | 95 | HTZ | 19. Aug.2013 |
600 | BIGO-II-3 | 57°20.58', 20°34.32' | 110 | HTZ | 26. Aug.2013 |
658 | BIGO-I-6 | 57°20.59', 20°34.30' | 110 | HTZ | 09. Sep.2013 |
568 | BIGO-I-1 | 57°18.51', 20°32.99' | 123 | Anoxic basin | 20. Aug.2013 |
626 | BIGO-I-4 | 57°18.50', 20°33.01' | 123 | Anoxic basin | 05. Sep.2013 |
642 | BIGO-I-5 | 57°18.50', 20°33.04' | 123 | Anoxic basin | 07. Sep.2013 |
635 | BIGO-II-5 | 57°14.99', 20°27.13' | 140 | Anoxic basin | 06. Sep.2013 |
603 | BIGO-I-3 | 57°20.98', 20°28.99' | 151 | Anoxic basin | 27. Aug.2013 |
618 | BIGO-II-4 | 57°21.05', 20°27.97' | 173 | Anoxic basin | 04. Sep.2013 |
457 | BIGO-I-6 | 57°26.56′, 20°43.34' | 63 | Oxycline | 08. Aug.2015 |
453 | BIGO-II-6 | 57°21.81', 20°35.85' | 79 | Oxycline | 07. Aug.2015 |
318 | BIGO-I-1 | 57°21.06', 20°35.88' | 80 | HTZ | 18. Jul.2015 |
325 | BIGO-II-1 | 57°20.99', 20°35.12' | 94 | HTZ | 19. Jul.2015 |
410 | BIGO-II-4 | 57°20.86', 20°35.39' | 94 | HTZ | 01. Aug.2015 |
446 | BIGO-II-5 | 57°20.60', 20°34.35' | 108 | HTZ | 05. Aug.2015 |
345 | BIGO-I-2 | 57°20.58', 20°34.32' | 111 | HTZ | 22. Jul.2015 |
354 | BIGO-II-2 | 57°18.42', 20°33.12' | 124 | Oxygenated | 23. Jul.2015 |
450 | BIGO-I-5 | 57°20.49', 20°29.12' | 142 | Oxygenated | 06. Aug.2015 |
373 | BIGO-I-3 | 57°20.98', 20°29.03' | 151 | Oxygenated | 25. Jul.2015 |
420 | BIGO-I-4 | 57°21.05', 20°28.56' | 161 | Oxygenated | 02. Aug.2015 |
401 | BIGO-II-3 | 57°21.07', 20°27.94' | 178 | Oxygenated | 27. Jul.2015 |
138 | BIGO-II-3 | 57°21.70', 20°35.83′ | 81 | HTZ | 22. Mar.2016 |
92 | BIGO-II-1 | 57°20.81', 20°35.26′ | 95 | HTZ | 12. Mar.2016 |
87 | BIGO-I-1 | 57°20.59', 20°34.29′ | 109 | HTZ | 11. Mar.2016 |
105 | BIGO-I-2 | 57°18.47', 20°33.00′ | 123 | Oxygenated | 14. Mar.2016 |
123 | BIGO-I-3 | 57°20.96', 20°29.00′ | 151 | Oxygenated | 20. Mar.2016 |
115 | BIGO-II-2 | 57°21.04', 20°27.96′ | 174 | Oxygenated | 19. Mar.2016 |
For this study we will adopt the definitions suggested by Noffke et al. (
Conductivity, temperature, depth (CTD) measurements were performed during casts of a Seabird CTD system equipped with a water-sampling rosette. These casts were made at water depths between 50 and 223 m along the redox depth transect shown in Table
Sea floor images were obtained using the towed camera system OFOS (Ocean Floor Observation System) equipped with a video and still camera (Nikon D70s), two Xenon lights (Oktopus) and a flashlight (Benthos). The system was towed ~1.5 m above the sea floor at ~0.3 knots. Seven OFOS deployments were conducted during the pre-inflow late summer cruise along the depth transect where in situ fluxes were determined (Figure
The landers are also equipped to recover the upper incubated sediment layers (~10–15 cm), which serves as a check for sediment disruption during seafloor operations and chamber insertion. The sediment surface for all lander deployments during the pre-inflow (late summer) cruise was intact and undisturbed. On post-inflow cruises the sediments close to the chamber wall of BIGO-II-3 at the 178 m site and of BIGO-I-3 at 151 m were slightly disturbed. It is not known whether this was caused during insertion of the chambers into the sediment or during lander retrieval. The concentration data did not indicate any artifacts during the flux measurements, and the initial concentrations inside the chamber at the start of the incubation were close to the bottom-water concentrations. Consequently, we have no reason to disregard the fluxes determined from these deployments.
Concentration measurements of dissolved inorganic nitrogen (
DIC measurements were performed using a quadrupole membrane inlet mass spectrometer (MIMS, GAM200, In Process Instruments). The instrument was equipped with inline sample acidification to shift the carbonate system entirely to the volatile CO2 species, which then was measured on the MIMS at a mass to charge ratio of 44 (Bell et al.,
The O2 concentrations in the pre-inflow phase (late summer) were similar to those previously described by Noffke et al. (
The strong MBI triggered in 2014 oxygenated the deep basin, leading to dissolved O2 concentrations of up to ~70 μM at 173 m water depth (Figures
The MBI caused a massive perturbation to nutrient distributions.
As a result of the moderate MBI that was triggered in November 2015, oxygenated water masses were detected in the deep basin although with lower O2 levels compared to the post-inflow (summer) cruise (Figures
Seafloor imaging during all cruises showed that the sediment surface in the HTZ was densely covered with white filamentous microbial mats, tentatively identified as belonging to the family
BIGO−II−6 | CH1 | 65 | 30.0 | 1.0 | −0.22 | 0.78 | 0.04 | bdl | nd | −12.6 |
CH2 | 1.0 | −0.21 | 0.79 | 0.06 | bdl | nd | −14.6 | |||
BIGO−I−2 | CH1 | 80 | 31.5 | 0.8 | −0.69 | 0.11 | 0.22 | bdl | nd | −0.37 |
CH2 | 0.3 | −0.50 | −0.20 | 0.26 | bdl | nd | −0.44 | |||
BIGO−II−1 | CH1 | 96 | 30.0 | 1.0 | −0.52 | 0.48 | 0.08 | bdl | nd | bdl |
CH2 | 1.3 | −0.72 | 0.58 | 0.19 | bdl | nd | bdl | |||
BIGO−II−3 | CH1 | 110 | 31.0 | 1.1 | −0.72 | 0.38 | 0.13 | bdl | 15.3 | bdl |
CH2 | 1.5 | −0.72 | 0.78 | 0.20 | bdl | 16.1 | bdl | |||
BIGO−I−6 | CH1 | 110 | 29.0 | nd | nd | nd | nd | nd | nd | nd |
CH2 | 1.3 | −0.13 | 1.17 | 0.27 | bdl | nd | bdl | |||
BIGO−I−5 | CH1 | 123 | 30.0 | nd | nd | nd | nd | nd | nd | nd |
CH2 | 1.2 | −0.35 | 0.85 | 0.21 | bdl | 3.6 | bdl | |||
BIGO−I−4 | CH1a | 123 | 30.0 | nd | nd | nd | nd | nd | nd | nd |
CH2 | 1.5 | −0.84 | 0.66 | 0.27 | bdl | nd | bdl | |||
BIGO−I−1 | CH1 | 124 | 30.0 | 1.4 | bdl | 1.40 | 0.24 | bdl | nd | bdl |
CH2 | 1.6 | bdl | 1.60 | 0.19 | bdl | nd | bdl | |||
BIGO−II−5 | CH1 | 140 | 29.0 | 0.7 | bdl | 0.7 | 0.09 | 7.61 | nd | bdl |
CH2 | 0.1 | bdl | 0.1 | 0.00 | 5.27 | 3.6 | bdl | |||
BIGO−I−3 | CH1 | 152 | 35.0 | 0.5 | bdl | 0.5 | 0.1 | 3.24 | 13.2 | bdl |
CH2 | 0.6 | bdl | 0.6 | 0.1 | 4.11 | 10.9 | bdl | |||
BIGO−II−4 | CH1 | 173 | 31.0 | 0.9 | bdl | 0.9 | 0.19 | 10.15 | 7.0 | bdl |
CH2 | 1.3 | bdl | 1.3 | 0.22 | 9.81 | 2.9 | bdl | |||
BIGO−I−6 | CH1 | 63 | 32.0 | 0.07 | 0.13 | 0.20 | −0.03 | bdl | 6.7 | 8.3 |
CH2 | 0.27 | 0.24 | 0.51 | 0.02 | bdl | 5.2 | 8.8 | |||
BIGO−II−6 | CH1 | 80 | 32.0 | 0.57 | −0.57 | 2.35 | 0.23 | bdl | 6.2 | 2.4 |
CH2 | 0.76 | −0.56 | 2.35 | 0.30 | bdl | 5.8 | 2.1 | |||
BIGO−II−1 | CH1 | 94 | 30.6 | 1.40 | −1.69 | −0,29 | 0.38 | bdl | nd | nd |
CH2a | nd | nd | nd | nd | nd | nd | nd | |||
BIGO−II−4 | CH1 | 95 | 35.0 | 1.63 | −0.64 | 0.99 | 0.20 | bdl | 11.8 | −2.8 |
CH2 | 1.13 | −0.48 | 0.65 | 0.16 | bdl | 11.5 | −3.0 | |||
BIGO−I−2 | CH1 | 111 | 36.1 | 0.59 | −0.62 | −0.03 | 0.11 | bdl | 6.4 | −1.0 |
CH2b | 0.50 | −0.50 | 0.00 | 0.05 | bdl | nd | nd | |||
BIGO−II−5 | CH1 | 111 | 34.0 | 0.52 | −0.51 | 0.01 | 0.07 | bdl | 4.2 | −0.7 |
CH2b | 0.72 | −0.78 | −0.06 | 0.08 | bdl | 3.7 | −1.0 | |||
BIGO−II−2 | CH1 | 124 | 36.0 | 1.14 | −0.99 | 0.15 | 0.21 | bdl | 15.7 | 0.8 |
CH2a | nd | nd | nd | nd | nd | nd | nd | |||
BIGO−I−5 | CH1 | 142 | 35.0 | 0.77 | −0.81 | −0.04 | −0.01 | bdl | bdl | 2.9 |
CH2b | 0.80 | −0.77 | 0.03 | −0.08 | bdl | nd | nd | |||
BIGO−I−3 | CH1 | 151 | 56.0 | 0.33 | −0.41 | −0.08 | −0.02 | bdl | 4.0 | 5.2 |
CH2 | 0.20 | −0.30 | −0.10 | −0.03 | bdl | 2,2 | 3.5 | |||
BIGO−I−4 | CH1 | 161 | 36.0 | 0.41 | −0.43 | −0.02 | −0.07 | bdl | 4.1 | 4.3 |
CH2 | 0.40 | −0.38 | 0.02 | −0.04 | bdl | 2.8 | 3.2 | |||
BIGO−II−3 | CH1 | 178 | 30.0 | 0.87 | −0.54 | 0.33 | 0.19 | bdl | 9.9 | 3.8 |
CH2 | 0.78 | −0.59 | 0.19 | 0.15 | bdl | 6.8 | 3.8 | |||
BIGO−II−3 | CH1 | 81 | 30.7 | 0.29 | −0.03 | 0.59 | −0.07 | bdl | nd | −4.5 |
CH2 | 0.36 | −0.49 | 0.72 | −0.07 | bdl | nd | −4.3 | |||
BIGO−II−1 | CH1 | 95 | 31.7 | 1.65 | −0.82 | 0.83 | 0.19 | bdl | 16.4 | −3.5 |
CH2 | 1.29 | −0.64 | 0.65 | 0.16 | bdl | 15.1 | −2.9 | |||
BIGO−I−1 | CH1 | 109 | 29.7 | 1.48 | −0.74 | 0.74 | 0.17 | bdl | nd | −1.4 |
CH2 | 1.31 | −0.65 | 0.66 | 0.16 | bdl | nd | −2.1 | |||
BIGO−I−2 | CH1 | 123 | 33.7 | 1.23 | −0,55 | 0.68 | 0.14 | bdl | 15.1 | −2.5 |
CH2 | 1.08 | −0.73 | 0.34 | 0.17 | bdl | 8.2 | −2.2 | |||
BIGO−I−3 | CH1 | 151 | 30.7 | 0.44 | −0.34 | 0.10 | 0.04 | bdl | 5.1 | −1.4 |
CH2 | 0.62 | −0.42 | 0.20 | −0.001 | bdl | 5.2 | −1.4 | |||
BIGO−II−2 | CH1 | 174 | 30.7 | 2.14 | −0.88 | 1.26 | 0.20 | bdl | 13.9 | −2.4 |
CH2 | 2.42 | −0.75 | 1.71 | 0.26 | bdl | 13.2 | −3.0 |
During both post-inflow cruises, ventilation resulted in elevated TOU rates in the deep basin ranging from 0.8 to 5.2 mmol m−2 d−1 (post-inflow, summer) and from 1.4 to 3.0 mmol m−2 d−1 (post-inflow, winter) (Table
Despite oxygenated bottom waters, the depth distribution of
The objective of this study is to first identify changes in benthic nutrient fluxes in response to two MBIs. These inflows led to increased availability of O2 and
As known from previous MBIs in 1993 and 2003 (e.g., Nausch and Nehring,
Oxycline (60-<80 m) | 11.5 ± 19.9 | 14.8 ± 4.2 | −1.5 ± 10.4 | nd |
HTZ (80–120 m) | 66.4 ± 78.5 | 102.4 ± 28.3 | 106.3 ± 52.5 | 48.1 ± 74.1 |
Deep basin (>120 m) | 21.4 ± 9.3 | 32.3 ± 13.8 | 21.6 ± 13.7 | 31.4 ± 5.7 |
Grand total |
87.9 ± 43.9 | 134.8 ± 21.1 | 128.0 ± 33.1 | 79.5 ± 39.9 |
Oxycline | 31.0 ± 10.7 | 133.3 ± 0 | 22.6 ± 18.6 | nd |
HTZ | 191.9 ± 83.9 | 241.3 ± 95.8 | 211.7 ± 106.9 | 256.6 ± 154.6 |
Deep basin | 34.9 ± 19.1 | 81.6 ± 42.4 | 73.9 ± 17 | 93.2 ± 21.5 |
Grand total |
226.7 ± 51.5 | 322.9 ± 69.1 | 285.5 ± 62 | 349.9 ± 88.1 |
Oxycline | 30.0 ± 13.5 | −28.7 ± 0.9 | 24.7 ± 10.4 | nd |
HTZ | −178.6 ± 101.6 | −139.6 ± 12.6 | −169.4 ± 49.3 | −135.6 ± 63.2 |
Deep basin | −12.6 ± 21.8 | −5.8 ± 11.5 | −32.9 ± 12.9 | −31.9 ± 5.1 |
Grand total |
−191.2 ± 61.7 | −145.3 ± 12.0 | −202.3 ± 31.1 | −167.5 ± 34.2 |
In contrast to elevated P release under euxinic bottom water conditions (this study; Jilbert et al.,
At the upper boundary of the deep basin at 124 m water depth, the
Seafloor imaging revealed that sulfur bacteria belonging to the family
Filamentous HS− oxidizing microbes may also play an important role in benthic P cycling. They are able to perform luxury uptake of
A plot of the
The O2 depth profiles obtained during post inflow summer conditions indicate that the MBI only affected water masses deeper than ~120 m. Above this, the O2 levels were close to zero and reached ~30 μM at 80 m at the base of the oxycline. During winter inflow the O2 profile shows a similar trend, yet with slightly higher O2 levels in the HTZ. An identical O2 distribution has been previously described during euxinia and, along with the distribution of microbial mats, was used as a major criterion to define the HTZ (Noffke et al.,
The above discussion has demonstrated that the deep basin and HTZ behaved differently during the most recent MBI events. In order to assess the importance of the expected basin wide reduction of nutrient release during ventilation, we approximated the entire benthic nutrient load for the Baltic Proper before and after ventilation (Table
For ease of comparison with other published rates (e.g., Viktorsson et al.,
Ventilation during the MBI from December 2014 (post-inflow summer cruise POS487) reduces the yearly
Importantly, since the fluxes in the HTZ were apparently not greatly affected by the inflows, the HTZ remained a major nutrient release site. With the exception of data from the pre-inflow early summer cruise AL355 (Noffke et al.,
The analysis shows that seabed nutrient release in the basin is controlled by processes in the HTZ rather than the deep basin as previously assumed. Even the largest MBI recorded from December 1951 (225 km3), which compares to 198 km3 of the MBI in 2014, would be very likely insufficient to ventilate the HTZ and suppress seafloor nutrient release. Despite the uncertainties involved, this simple extrapolation highlights that the effect of deep basin ventilation on the reduction of benthic nutrient release can be considered as minor in the context of the entire nutrient budget. It should be noted that the above extrapolation was conducted under the assumption that the HTZ is present throughout the Baltic Proper (Figure
The intrusion of O2 and
The presence of the HTZ, which has been identified recently as a second major zone for rapid nutrient recycling and nutrient release (Noffke et al.,
Ventilation events suppress HS− toxification and nutrient release only for short time periods of several months. If the inflow events occur as infrequently in the future as during the past decade they have only limited impact to sustainably reduce internal nutrient loading in the EGB. In the long-term, eutrophication will not be diminished by these events because recycling of P (and N) between the water column and surface sediments is relatively rapid compared to slower sequestration of P by burial in the sediments.
SS, OP, and AD designed the study, coordinated ship operations, lander deployments, sediment sampling, and the data selection process; SS, DC, MY, HS, and AD took and processed samples; All authors contributed ideas and wrote the manuscript.
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.
We very much thank Captain J. Lass and officers and crew of RV Alkor, Captain K. Ricke, and officers and crew of RV Poseidon for their excellent support during cruises AL422, AL473, and POS487. Many thanks are due to A. Beck, T. Berghäuser, J. Braasch, E. Fabrizius, S. Cherednichenko, S. Kriwanek, N. Meides, A. Petersen, M. Steffen, A. Stephan, M. Türk, and K. Stolpovsky for technical support deploying the benthic landers, the Ocean Floor Observation System (OFOS), the CTD water sampling rosette and for taking care of water and sediment samples retrieved by the landers. We thank A. Bleyer, B. Domeyer, C. Laudan, K. Qelaj, G. Schüßler, R. Surberg, V. Thoenissen, S. Trinkler, and J. Wemhöner for the excellent biogeochemical analyses onboard and in the home laboratory. We are grateful for the support of J. Wölfel and L. Bryant at sea. The Technology and Logistics Centre at GEOMAR and C. Utecht are acknowledged for logistical support. We are grateful for the very helpful and constructive reviews of two reviewers. Funding was provided by the Helmholtz Alliance “ROBEX-Robotic Exploration of Extreme Environments” and the Sonderforschungsbereich 754 “Climate-Biogeochemistry Interactions in the Tropical Ocean” supported by the Deutsche Forschungsgemeinschaft. This work was further supported financially by the Swedish Research Council (VR).