Nutrient and Carbon Dynamics in the Chesapeake Bay Outflow Plume and Their Effect on the Coastal Ocean Environment

Seasonally resolved nutrient and carbon fluxes from estuaries to the coastal ocean are poorly constrained. Nutrient and carbon cycling in highly productive regions like the Chesapeake Bay outflow plume and surrounding coastal environments greatly affect our global understanding of carbon cycling. The overall questions for the research described in this dissertation stem from the need to close global carbon budgets, and obtain a fundamental understanding of nutrient dynamics in a coastal region heavily influenced by seasonality and human impacts. Within the framework of physical characteristics of the outflow plume and through the characterization of nutrient concentrations, primary productivity rates, and the uptake of nitrogen using stable isotopes, I identified three different plume types that differentially provided nutrients and created conditions either suitable or unsuitable for primary productivity in the coastal zone. A jet-like plume, where there were winds consistently from the north accompanied by high freshwater flow from the Bay, delivered high amounts of chlorophyll from the Bay. In contrast, two types of diffusive plumes occurred when winds came from the south accompanied with low freshwater discharge and were either influenced by estuarine or oceanic processes. The diffusive-estuarine plume delivered dissolved nutrients creating conditions suitable for high primary productivity rates in the coastal zone while the diffusive-oceanic plume generally had low primary productivity and nitrogen uptake rates. A secondary study compared and contrasted hydrography, nutrient availability, primary productivity rates and nitrogen uptake rates in three distinct regions of the Mid-Atlantic Bight: the plume regions influenced by the Delaware and Chesapeake Bays, the mid-shelf region between the Delaware and Chesapeake Bays influenced by both coastal and oceanic processes, and the southern shelf region below the Chesapeake Bay influenced by the Gulf Stream. Areal rates of carbon uptake were not significantly different among regions, and were higher than most published values of annual areal rates for the Mid-Atlantic bight. Annual areal nitrogen uptake rates were also calculated, providing carbon to nitrogen uptake ratios which were lower than the canonical Redfield ratio. These findings have implications regarding modeled estimates of carbon uptake based on nitrogen uptake and vice versa

Sources And Cycling of Carbonyl Sulfide in the Sargasso Sea

The cycling of the radiatively important gas carbonyl sulfide (OCS) was studied in surface waters of the Sargasso Sea. In August 1999, surface OCS concentrations averaged 8.6 pmol L-1, showed minor diel variations, and varied little with depth. An OCS precursor, total dissolved organic sulfur (DOS), was lowest at the surface (40 nmol L-1) and increased with depth. The photoproduction rate of OCS from in situ incubations averaged 9.6 pmol L-1 h-1, whereas dark production was 7.0 pmol L-1 h-1. Apparent quantum yields were 10-5-10-7 from 313-436 nm and varied with the water depth irradiated. In March 2000, there were strong diel variations in surface OCS (highest in late afternoon; overall average, 16.9 pmol L-1). Depth profiles in the afternoon showed surface water maxima and decreases with depth, whereas DOS had a surface maximum of 419 nmol L-1 and decreased with depth. Dark production was 4.0 pmol L-1 h-1. Modeling of the diel cycle suggested a photoproduction rate of 16.4 pmol L-1 h-1. Overall, the photochemical production of OCS strongly depended on DOS and chromophoric dissolved organic matter, whereas dark production was influenced by the presence of particles and perhaps microbial respiration, showing a direct biotic influence on OCS cycling

Effluent Organic Nitrogen (EON): Bioavailability and Photochemical and Salinity-Mediated Release

The goal of this study was to investigate three potential ways that the soluble organic nitrogen (N) fraction of wastewater treatment plant (WWTP) effluents, termed effluent organic N (EON), could contribute to coastal eutrophication - direct biological removal, photochemical release of labile compounds, and salinity-mediated release of ammonium (NH4+). Effluents from two WWTPs were used in the experiments. For the bioassays, EON was added to water from four salinities (∼0 to 30) collected from the James River (VA) in August 2008, and then concentrations of N and phosphorus compounds were measured periodically over 48 h. Bioassay results, based on changes in DON concentrations, indicate that some fraction of the EON was removed and that the degree of EON removal varied between effluents and with salinity. Further, we caution that bioassay results should be interpreted within a broad context of detailed information on chemical characterization. EON from both WWTPs was also photoreactive, with labile NH4+ and dissolved primary amines released during exposure to sunlight. We also present the first data that demonstrate that when EON is exposed to higher salinities, increasing amounts of NH4+ are released, further facilitating EON use as effluent transits from freshwater through estuaries to the coast

Remote sensing of phytoplankton community composition along the northeast coast of the United States

Satellite imagery has proven to be a powerful tool for measuring chlorophyll a in surface waters. While this provides an estimate of total phytoplankton biomass, it does not distinguish between phytoplankton groups, many of which have functional differences and therefore affect biogeochemical cycles differently. Phytoplankton pigment analysis has been used to quantify a wide range of photosynthetic and accessory pigments, and chemotaxonomic analysis (e.g. CHEMTAX) has been used to successfully quantify functional taxonomic groups in nature based on pigment distributions. Here, we combine CHEMTAX analysis with satellite-derived distributions of specific phytoplankton pigments to describe the distributions of particular components of the phytoplankton community in the northeast coast of the United States from space. The spatial and seasonal variations in phytoplankton community structure elucidated through satellite remote sensing methods generally agreed with observations of abundance estimates of cell counts. Diatoms were generally the most abundant phytoplankton in this region, especially during Winter–Spring and in the inner shelf, but phytoplankton populations shifted to increasing abundance of other taxa during Summer, especially offshore. While still preliminary, satellite-derived taxa-specific information with proper regional controls holds promise for providing information on phytoplankton abundance to a taxonomic group level which would greatly improve our understanding of the impacts of human activity and climate change on ecosystems

Effluent Organic Nitrogen (EON): Bioavailability and Photochemical and Salinity-Mediated Release

The goal of this study was to investigate three potential ways that the soluble organic nitrogen (N) fraction of wastewater treatment plant (WWTP) effluents, termed effluent organic N (EON), could contribute to coastal eutrophication - direct biological removal, photochemical release of labile compounds, and salinity-mediated release of ammonium (NH₄⁺). Effluents from two WWTPs were used in the experiments. For the bioassays, EON was added to water from four salinities (∼0 to 30) collected from the James River (VA) in August 2008, and then concentrations of N and phosphorus compounds were measured periodically over 48 h. Bioassay results, based on changes in DON concentrations, indicate that some fraction of the EON was removed and that the degree of EON removal varied between effluents and with salinity. Further, we caution that bioassay results should be interpreted within a broad context of detailed information on chemical characterization. EON from both WWTPs was also photoreactive, with labile NH₄⁺ and dissolved primary amines released during exposure to sunlight. We also present the first data that demonstrate that when EON is exposed to higher salinities, increasing amounts of NH₄⁺ are released, further facilitating EON use as effluent transits from freshwater through estuaries to the coast