Nitrogen is a key limiting nutrient that influences marine productivity and carbon sequestration in the ocean via the biological pump. In this study, we present the first estimates of nitrogen cycling in a coupled 3D ocean-biogeochemistry-isotope model forced with realistic boundary conditions from the Last Glacial Maximum (LGM) ~21,000 years before present constrained by nitrogen isotopes. The model predicts a large decrease in nitrogen loss rates due to higher oxygen concentrations in the thermocline and sea level drop, and, as a response, reduced nitrogen fixation. Model experiments are performed to evaluate effects of hypothesized increases of atmospheric iron fluxes and oceanic phosphorus inventory relative to present-day conditions. Enhanced atmospheric iron deposition, which is required to reproduce observations, fuels export production in the Southern Ocean causing increased deep ocean nutrient storage. This reduces transport of preformed nutrients to the tropics via mode waters, thereby decreasing productivity, oxygen deficient zones, and water column N-loss there. A larger global phosphorus inventory up to 15% cannot be excluded from the currently available nitrogen isotope data. It stimulates additional nitrogen fixation that increases the global oceanic nitrogen inventory, productivity, and water column N-loss. Among our sensitivity simulations, the best agreements with nitrogen isotope data from LGM sediments indicate that water column and sedimentary N-loss were reduced by 17–62% and 35–69%, respectively, relative to preindustrial values. Our model demonstrates that multiple processes alter the nitrogen isotopic signal in most locations, which creates large uncertainties when quantitatively constraining individual nitrogen cycling processes. One key uncertainty is nitrogen fixation, which decreases by 25–65% in the model during the LGM mainly in response to reduced N-loss, due to the lack of observations in the open ocean most notably in the tropical and subtropical southern hemisphere. Nevertheless, the model estimated large increase to the global nitrate inventory of 6.5–22% suggests it may play an important role enhancing the biological carbon pump that contributes to lower atmospheric CO2 during the LGM.
At the end of 2014, a Major Baltic Inflow (MBI) brought oxygenated, salty water into the Baltic proper and reached the long-term anoxic Eastern Gotland Basin (EGB) by March 2015. In July 2015, we measured benthic fluxes of phosphorus (P), nitrogen (N) and silicon (Si) nutrients and dissolved inorganic carbon (DIC) in situ using an autonomous benthic lander at deep sites (170–210 m) in the EGB, where the bottom water oxygen concentration was 30–45 μM. The same in situ methodology was used to measure benthic fluxes at the same sites in 2008–2010, but then under anoxic conditions. The high efflux of phosphate under anoxic conditions became lower upon oxygenation, and turned into an influx in about 50% of the flux measurements. The C:P and N:P ratios of the benthic solute flux changed from clearly below the Redfield ratio (on average about 70 and 3–4, respectively) under anoxia to approaching or being well above the Redfield ratio upon oxygenation. These observations demonstrate retention of P in newly oxygenated sediments. We found no significant effect of oxygenation on the benthic ammonium, silicate and DIC flux. We also measured benthic denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) rates at the same sites using isotope-pairing techniques. The bottom water of the long-term anoxic EGB contained less than 0.5 μM nitrate in 2008–2010, but the oxygenation event created bottom water nitrate concentrations of about 10 μM in July 2015 and the benthic flux of nitrate was consistently directed into the sediment. Nitrate reduction to both dinitrogen gas (denitrification) and ammonium (DNRA) was initiated in the newly oxygenated sediments, while anammox activity was negligible. We estimated the influence of this oxygenation event on the magnitudes of the integrated benthic P flux (the internal P load) and the fixed N removal through benthic and pelagic denitrification by comparing with a hypothetical scenario without the MBI. Our calculations suggest that the oxygenation triggered by the MBI in July 2015, extrapolated to the basin-wide scale of the Baltic proper, decreased the internal P load by 23% and increased the total (benthic plus pelagic) denitrification by 18%.
Diverse planktonic microorganisms play a crucial role in mediating methane flux from the ocean to the atmosphere. The distribution and composition of the marine methanotroph community is determined partly by oxygen availability. The low oxygen conditions of oxygen minimum zones (OMZs) may select for methanotrophs that oxidize methane using inorganic nitrogen compounds (e.g., nitrate, nitrite) in place of oxygen. However, environmental evidence for methane-nitrogen linkages in OMZs remains sparse, as does our knowledge of the genomic content and metabolic capacity of organisms catalyzing OMZ methane oxidation. Here, binning of metagenome sequences from a coastal anoxic OMZ recovered the first near complete (95%) draft genome representing the methanotroph clade OPU3. Phylogenetic reconstruction of concatenated single copy marker genes confirmed the OPU3-like bacterium as a divergent member of the type Ia methanotrophs, with an estimated genome size half that of other sequenced taxa in this group. The proportional abundance of this bacterium peaked at 4% of the total microbial community at the top of the anoxic zone in areas of nitrite and nitrate availability but low methane concentrations. Genes mediating dissimilatory nitrate and nitrite reduction were identified in the OPU3 genome, and transcribed in conjunction with key enzymes catalyzing methane oxidation to formaldehyde and the ribulose monophosphate (RuMP) pathway for formaldehyde assimilation, suggesting partial denitrification linked to methane oxidation. Together, these data provide the first field-based evidence for methanotrophic partial denitrification by the OPU3 cluster under anoxic conditions, supporting a role for OMZs as key sites in pelagic methane turnover.
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 (), dissolved inorganic carbon (DIC), sulfide (HS−), and total oxygen uptake (TOU) were measured along a depth gradient in the Eastern Gotland Basin (EGB). Campaigns were conducted during euxinic conditions of the deep basin in Aug./Sept. 2013 and after two inflow events in July/Aug. 2015 and March 2016 when O2 concentrations in deep waters reached 60 μM. The intrusion of O2-rich North Sea water into the EGB led to an approximate 33 and 10% reduction of the seabed and ammonium () release from deep basin sediments. Post-inflow, the deep basin sediment was rapidly colonized by HS− oxidizing bacteria tentatively assigned to the family Beggiatoaceae, and HS− release was completely suppressed. The presence of a hypoxic transition zone (HTZ) between 80 and 120 m water depth was confirmed not only for euxinic deep-water conditions during 2013 but also for post-inflow conditions. Because deep-water renewal did not ventilate the HTZ, where and fluxes were highest, high seabed nutrient release there was relatively unchanged. Extrapolation of the in situ nutrient fluxes indicated that, overall, the reduction in and release in response to deep-water renewal can be considered as minor, reducing the internal nutrient load by 2 and 12% only, respectively. Infrequent inflow events thus have a limited capacity to sustainably reduce internal nutrient loading in the EGB and mitigate eutrophication.