Holly and Reide collecting samples in the Pawcatuck River
Nitrogen cycling and hypoxia in the Southern Benguela Upwelling (NSF-OCE)
The Southern Benguela Upwelling System in the eastern Atlantic Ocean ranks among the most fertile regions in the world ocean, host to economically important fishing grounds that comprise important sectors of the national economies of Namibia and South Africa. It is also characterized by seasonal hypoxia punctuated by episodic severe hypoxic events that can be harmful to these fisheries. It has been proposed that the severity of hypoxia is related to the degree to which nutrients get "trapped" on the shelf due to the release of nutrients following productive blooms and subsequent degradation of phytoplankton combined with the physical conditions preventing cross-shelf water exchange. Using the stable isotope ratios of nitrate (Granger) along with a coupled physical-biogeochemical model (Siedlecki) we hope to identify the conditions leading to nutrient trapping and its role in modulating the severity of hypoxia on the shelf.
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Nitrogen fixation in the Atlantic Gulf Stream
In collaboration with Drs. Jamie Palter (URI), Pia Moisander (UMass Dartmouth), Angel White (OSU), and Mar Benavides (Mediterranean Institute of Oceanography, France) we are investigating the hypothesis that nutrient phosphate in excess of nitrate entrained in the North Atlantic Gulf Stream fuel substantial rates of biological N2 fixation in the gyre boundary. During a recent expedition across the Atlantic Gulf Stream aboard the R.V. Endeavor, we performed on-deck incubations to quantify N2 fixation in surface and subsurface waters, paired with hydrographic measurements and a concurrent investigation of N2 fixer abundances and of the presence of nitrogenase genes.
Limit of detection of N2 fixation measurements (NSF-EAGER)
In collaboration with Pia Moisander, Angel White, and the OCB N-fixation working group, we are conducting a thorough examination of potential experimental and analytical errors inherent to the 15N2 tracer incubation fixation method, in tandem with comprehensive molecular measurements in the deep ocean waters, in order to (1) determine the minimum quantifiable rates of 15N2 fixation based on incubations of mesopelagic waters via full characterization of sources of experimental error and mass spectrometer performance, and (2) seek evidence of expression and presence of nitrogen fixation genes via comprehensive molecular approaches on corresponding samples.
Courtesy of A. White
GEOTRACES: Nitrogen biogeochemistry of the Arctic Ocean (NSF-OCE/PLR)
The Arctic Ocean has been a difficult place to access due to ice cover, such that it is under-studied relative to ocean basins. Through the umbrella of of the US and Canada GEOTRACES programs, we are generating the first comprehensive survey of the nitrate stable isotope distributions in different regions of the Arctic Ocean, which is providing an incomparable synoptic view of nitrogen cycling therein.
Nitrate isotope dynamics in groundwater aquifers (NSF-EAR)
Why does denitrification appear to impart a different isotopic imprint on nitrate in freshwater aquifers compared to ocean systems? To answser this perplexing question, in June of 2016, we injected nitrate and acetate in the anoxic portion of a USGS experimental aquifer in Cape Cod, in order to monitor the evolution of nitrate N and O isotope ratios when denitrification by heterotrophic bacteria is stimulated (by acetate) over oxidative nitrate production (by anammox and/or nitrification). To do so, we first pumped aquifer-water to the surface into a steel drum containing nitrate, acetate and bromide (a conductivity tracer), whilst keeping groundwater anoxic, then the mixture was pumped back into the ground, and the evolution of the nitrate plume and its isotope was tracked down-gradient for a month. Measurements from this experiment are providing exciting and unexpected answers.
Dick overseeing extraction of 200 gallons of anoxic water from the aquifer.
The oxygen isotope composition of nitrate produced by nitrification in freshwater (NSF-EAR)
Nitrate produced by nitrification, the biological oxidation of ammonium and nitrite to nitrate, can be identified in environmental pools based on the oxygen isotope ratios of nitrate.... Or so we think, because freshwater and marine communities have different rubrics from which to interpret isotopic signals ascribed to nitrification. For her Masters research, Danielle has investigated the oxygen isotope composition of nitrate produced associated with nitrification in freshwater, thus providing a robust basis to distinguish biologically produced nitrate from pollutant sources.
Reactive nitrogen sources to the Pawcatuck River and Little Narragansett Bay (CT Sea Grant)
Sea grass beds that once thrived in Little Narragansett Bay have been replaced by glutinous and noxious mats of invasive macroalgae of the genus Cladophora. Excess nutrient loading from the Pawcatuck River is suspected to exacerbate the extent of macroalgal cover. From measurements of the stable isotope ratios of reactive nitrogen pools, Veronica is investigating the dominant sources of nutrients to the bay, as well as the cycling and fate of reactive nitrogen in the bay. She intends to establish whether point sources, such as the waste water treatment plants contribute disproportionately to river-borne nutrients, and whether reductions in nutrient loading could help reduce macroalgal density in the bay.
Nitrogen isotope fractionation during ammonium assimilation by phytoplankton (NSF-CAREER)
While the isotopic discrimination associated with the assimilation of nitrate by phytoplankton is fairly well understood, isotopic discrimination during ammonium assimilation is not well characterized. Importantly, nitrogen isotopic discrimination appears to be negligible at lower ammonium concentrations expected for most ocean environments. As part of his doctoral research, Michael is investigating the physiological basis of N isotopic discrimination during ammonium assimilation by phytoplankton in cultures, focusing on a lower range of ammonium substrate concentrations than have typically been studied.
Contribution of N2 fixation to export production in the North Pacific Subtropical Gyre (NPSG)
In collaboration with Angel White, we are constraining the contribution of N2 fixation to export production in the NPSG. Water samples were collected during the cruise MESO-SCOPE in 2017. We are measuring δ15N of nitrate in water samples, to compare to that of the sinking particulate material collected in shallow sediment traps deployed during the cruise, to estimate the contribution of N2 fixation to the sinking flux and how mesoscale features affect this contribution.