Archive for nitrate

The role of nutrient trapping in promoting shelf hypoxia in the southern Benguela upwelling system

Posted by mmaheigan 
· Thursday, September 3rd, 2020 

The southern Benguela upwelling system (SBUS) off southwest Africa is an exceptionally fertile ocean region that supports valuable commercial fisheries. The productivity of this system derives from the upwelling of nutrient-rich Subantarctic Mode Water, and from the concurrent entrainment of nutrients regenerated proximately on the expansive continental shelf. The SBUS is prone to severe seasonal hypoxic events that decimate regional fisheries, occurrences of which are inextricably linked to the inherent nutrient dynamics. In a study recently published in JGR Oceans, the authors sought to understand the mechanisms sustaining elevated concentrations and seasonally-variable distributions of nutrients in the SBUS, in relation to the subsurface oxygen content. Inter-seasonal measurements of nutrients and nitrate isotope ratios across the SBUS in 2017 revealed that upwards of 48% (summer) and 63% (winter) of the on‐shelf nutrients derived from regeneration in situ.  The severity of hypoxia at the shelf bottom, in turn, correlated with the incidence of regenerated nutrients. The accrual of nutrients at the shelf bottom appears to be aided by hydrographic fronts that restrict offshore transport, trapping regenerated nutrients on the SBUS shelf and increasing the pool of nutrients available for upwelling – ultimately contributing to hypoxic events. This study underscores the need – if we are to develop a mechanistic and predictive understanding of hypoxia in the SBUS and elsewhere – to elucidate the role of shelf circulation in promoting the accrual of regenerated nutrients on the continental shelf. The next step is to combine new and existing observations with quantitative simulations to further interrogate the coupled physical-biogeochemical mechanisms that modulate the intensity of hypoxia.

Figure caption: Schematic of proposed nutrient-trapping mechanism: Deep nutrient-rich Subantarctic Mode Water (SAMW) acquires more nutrients as it passes over the shelf sediments from the regeneration of exported particulate organic material (POM). The production of this POM is fueled by nutrients stripped from the surface waters advecting back off-shore. The thickness of the arrows represents nutrient concentrations. Triangles indicate the positions of the Shelf Break Front (SBF) and Columbine Front (CF), coincident with an observed subduction of the Ekman layer and downwelling at the inner front boundary.

Authors
Raquel Flynn (University of Cape Town)
Julie Granger (University of Connecticut)
Jennifer Veitch (South African Environmental Observation Network)
Samantha Siedlecki (University of Connecticut)
Jessica Burger (University of Cape Town)
Keshnee Pillay (South Africa Department of Environment, Forestry and Fisheries)
Sarah Fawcett (University of Cape Town)

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan 
· Tuesday, September 1st, 2020 

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

 

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

 

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

Physics vs. biology in Southern Ocean nutrient gradients

Posted by mmaheigan 
· Tuesday, June 16th, 2020 

In the Southern Ocean, surface water silicate (SiO4) concentrations decline very quickly relative to nitrate concentrations along a northward gradient toward mode water formation regions on the northern edge (Figure 1a, b). These mode waters play a critical role in driving global nutrient concentrations, setting the biogeochemistry of low- and mid-latitude regions around the globe after they upwell further north. To explain this latitudinal surface gradient, most hypotheses have implicated diatoms, which take up and export silicon as well as nitrogen: (1) Diatoms, including highly-silicified species such as Fragilariopsis kerguelensis, are more abundant in the Southern Ocean than elsewhere; (2) Iron limitation, which is prevalent in the Southern Ocean, elevates the Si:N ratio of diatoms; (3) Mass export of empty diatom frustules pumps silicate but not nitrate to deeper waters.

Figure 1: (a) and (b) nitrate and silicate concentrations in surface waters of the Southern Ocean (GLODAPv2_2019 data). (c) Model results of a standard run (black diamonds), a run without biology (red diamonds) and a run without mixing (blue diamonds).

In a recent paper published in Biogeosciences, the authors use an idealized model to explore the relative roles of biological vs. physical processes in driving the observed latitudinal surface nutrient gradients. Over timescales of a few years, removing the effects of biology (no SiO4 uptake or export) from the model elevates silicate concentrations slightly over the entire latitudinal range, but does not remove the strong latitudinal gradient (Figure 1c). However, if the effects of vertical mixing processes such as upwelling and entrainment are removed from the model by eliminating the observed deep [SiO4] gradient, the observed surface nutrient gradient is greatly altered (Figure 1c). These model results suggest that, over short timescales, physics is more important than biology in driving the observed surface water gradient in SiO4:NO3 ratios and forcing silicate depletion of mode waters leaving the Southern Ocean. These findings add to our understanding of Southern Ocean dynamics and the downstream effects on other oceans.

 

Authors:
P. Demuynck (University of Southampton)
T. Tyrrell (University of Southampton)
A.C. Naveira Garabato (University of Southampton)
C.M. Moore (University of Southampton)
A.P. Martin (National Oceanography Centre)

Nitrate enrichment may threaten coastal wetland carbon storage

Posted by mmaheigan 
· Thursday, February 27th, 2020 

With their high primary productivity and slow decomposition in anoxic soils, salt marshes and other coastal wetlands can store carbon more efficiently than terrestrial uplands. These wetlands also provide critical ecosystem services such as interception of land-derived nutrients before they can enter the coastal ocean. Therefore, it is important to understand how anthropogenic supplies of nitrate (NO3–) affect marsh sustainability and carbon storage.

In marsh sediment studies, the most common form of experimental nitrogen enrichment uses pelletized fertilizer composed of ammonium, urea, or other organic based fertilizers. Authors of a recent study published in Global Change Biology hypothesized that when nutrients were instead added in the form of nitrate (NO3–), the most common form of nitrogen enrichment in coastal waters, it would stimulate microbial decomposition of organic matter by serving as an electron acceptor for microbial respiration in anoxic salt marsh sediments. Furthermore, decomposition would vary with sediment depth, with decreased decomposition at greater depths, where less biologically available organic matter accumulated over time.

Figure 1: DIC production as a proxy for microbial respiration in salt marsh sediments from three distinct depth horizons (shallow 0-5cm, mid 10-15cm, deep 20-25cm) that span a range of biological availability. The addition of NO3- (green) stimulated DIC production relative to unenriched sediments, regardless of sediment depth. All samples were run under anoxic conditions (without the presence of oxygen), closely matching that of normal salt marsh sediments.

Surprisingly, NO3– addition stimulated decomposition of organic matter at all depths, with the highest decomposition rates in the surface sediments. This suggests that there is a pool of “NO3–-labile” organic matter in marsh sediments that microbes can decompose under high-NO3– conditions that would otherwise remain stable. As human activities continue to enrich our coastal waters with NO3– through agricultural runoff, septic systems, and other pathways, it could inadvertently decrease coastal wetlands’ carbon storage capacity, with negative consequences for both blue carbon offsets and marsh sustainability in the face of sea level rise.

 

Authors:
Jennifer Bowen (Northeastern University)
Ashley Bulseco (MBL/WHOI)
Anne Giblin (MBL)

Filter by Keyword

234Th disequilibrium abundance acidification africa air-sea flux air-sea interactions air-sea interface algae alkalinity allometry ammonium AMOC anoxia anoxic Antarctic anthro impacts anthropogenic carbon aquaculture aragonite saturation arctic Argo argon arsenic artificial seawater Atlantic Atlantic modeling atmospheric carbon atmospheric CO2 atmospheric nitrogen deposition authigenic carbonates autonomous platforms bacteria BATS benthic bgc argo bio-go-ship bio-optical bioavailability biogeochemical cycles biogeochemical cycling biogeochemical models biogeochemistry Biological Essential Ocean Variables biological pump biological uptake biophysics bloom blooms blue carbon bottom water boundary layer buffer capacity C14 CaCO3 calcification calcite calcium carbonate carbon-climate feedback carbon-sulfur coupling carbon budget carbon cycle carbon dioxide carbon export carbon sequestration carbon storage Caribbean CCA CCS changi changing marine ecosystems changing marine environments changing ocean chemistry chemical oceanographic data chemical speciation chemoautotroph chesapeake bay chl a chlorophyll circulation climate change climate variability CO2 CO2YS coastal darkening coastal ocean cobalt Coccolithophores community composition conservation cooling effect copepod coral reefs CTD currents cyclone data data access data management data product Data standards DCM dead zone decadal trends decomposers decomposition deep convection deep ocean deep sea coral deoxygenation depth diagenesis diatoms DIC diel migration diffusion dimethylsulfide dinoflagellate discrete measurements dissolved inorganic carbon dissolved organic carbon DOC DOM domoic acid dust DVM earth system models ecology ecosystems ecosystem state eddy Education Ekman transport emissions ENSO enzyme equatorial regions error ESM estuarine and coastal carbon estuarine and coastal carbon fluxes estuary euphotic zone eutrophication evolution export export fluxes export production EXPORTS extreme events extreme weather events faecal pellets filter feeders filtration rates fire fish Fish carbon fisheries floats fluid dynamics fluorescence food webs forage fish forams freshening freshwater frontal zone fronts functional role future oceans geochemistry geoengineering geologic time GEOTRACES glaciers gliders global carbon budget global ocean global warming go-ship grazing greenhouse gas Greenland groundwater Gulf of Maine Gulf of Mexico Gulf Stream gyre harmful algal bloom high latitude human food human impact hurricane hydrogen hydrothermal hypoxia ice age ice cores ice cover industrial onset inverse circulation ions iron iron fertilization isotopes jellies katabatic winds kelvin waves krill kuroshio laboratory vs field land-ocean continuum larvaceans lateral transport LGM lidar ligands light light attenuation lipids mangroves marine carbon cycle marine heatwave marine particles marine snowfall marshes Mediterranean meltwater mesopelagic mesoscale metagenome metals methane methods microbes microlayer microorganisms microscale microzooplankton midwater mixed layer mixed layers mixing mixotrophy modeling models mode water molecular diffusion MPT multi-decade n2o NAAMES NASA NCP net community production net primary productivity new ocean state new technology Niskin bottle nitrate nitrogen nitrogen fixation nitrous oxide north atlantic north pacific nuclear war nutricline nutrient budget nutrient cycling nutrient limitation nutrients OA ocean-atmosphere ocean acidification ocean acidification data ocean carbon uptake and storage ocean color ocean observatories ocean warming ODZ oligotrophic omics OMZ open ocean optics organic particles oscillation overturning circulation oxygen pacific paleoceanography particle flux pCO2 PDO peat pelagic PETM pH phenology phosphorus photosynthesis physical processes physiology phytoplankton PIC plankton POC polar regions pollutants precipitation predation prediction primary production primary productivity Prochlorococcus proteins pteropods pycnocline radioisotopes remineralization remote sensing repeat hydrography residence time resource management respiration resuspension rivers rocky shore Rossby waves Ross Sea ROV salinity salt marsh satell satellite scale seafloor seagrass sea ice sea level rise seasonal patterns seasonal trends sea spray seaweed sediments sensors shelf system shells ship-based observations shorelines silicate silicon cycle sinking particles size SOCCOM soil carbon southern ocean south pacific spatial covariations speciation SST stoichiometry subduction submesoscale subpolar subtropical sulfate surf surface surface ocean Synechococcus teleconnections temperate temperature temporal covariations thermocline thermodynamics thermohaline thorium tidal time-series time of emergence top predators total alkalinity trace elements trace metals trait-based transfer efficiency transient features Tris trophic transfer tropical turbulence twilight zone upper ocean upper water column upwelling US CLIVAR validation velocity gradient ventilation vertical flux vertical migration vertical transport volcano warming water clarity water quality waves western boundary currents wetlands winter mixing world ocean compilation zooplankton

Copyright © 2023 - OCB Project Office, Woods Hole Oceanographic Institution, 266 Woods Hole Rd, MS #25, Woods Hole, MA 02543 USA Phone: 508-289-2838  •  Fax: 508-457-2193  •  Email: ocb_news@us-ocb.org

link to nsflink to noaalink to WHOI

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.