Date of Award
Doctor of Philosophy in Oceanography
Marine and Atmospheric Chemistry
Rebecca S. Robinson
Estuaries regulate nitrogen (N) fluxes transported from land to the open ocean through uptake and denitrification. In Narragansett Bay, anthropogenic N loading has increased over the last century with evidence for eutrophication in some regions of Narragansett Bay. Increased concerns over eutrophication prompted upgrades at wastewater treatment facilities (WWTFs) to decrease the amount of nitrogen discharged. The upgrade to tertiary treatment – where bioavailable nitrogen is reduced and removed through denitrification – has occurred at multiple facilities throughout Narragansett Bay’s watershed. Nitrogen remains limiting to primary production in the Bay proper which led to speculation that primary production throughout the system may result in large part from the high nutrient loads in the north. Tracing N sources is essential for attributing drivers of primary production and it has previously been done using stable isotopes. However, little information on the isotopic composition of the nutrient inputs is available and no data are available to assess the impact of upgrades to tertiary treatment on the isotopic composition of dissolved inorganic nitrogen or primary producers. The objective of this dissertation is to explore the spatial and temporal distribution of the stable isotopes of nitrogen in multiple forms, inorganic and organic, the impact of upgrades at the wastewater treatment facilities on nutrient fluxes and their isotopic compositions, and the role anthropogenic N plays in driving primary production within Narragansett Bay.
Samples were collected from 2007 through 2012, before and during upgrades to tertiary treatment. Samples from rivers and WWTFs were collected to characterize the potential impact of upgrades and anthropogenic source nitrate (NO3-) isotopic variability. Surface water NO3- samples were collected from a north-south transect to trace the impact of the upgrades spatially. Finally, during the summers of 2011 and 2012, additional samples of subsurface nitrate, chlorophyll a, and macroalgae (2012 only) were collected to assess the relative importance of anthropogenic nitrogen to primary producers. All water samples were analyzed for nutrient concentrations (NO3-, PO43-, NH4+) and stable nitrogen (δ15N) and oxygen (δ18O) isotopic compositions of NO3-. Chlorophyll a content and the compound specific and bulk N isotopic composition of chlorophyll a and macroaglae, respectively, were measured.
Between 2009 and 2012, upgrades to tertiary treatment reduced nitrogen inputs to Narragansett Bay by 30 % but the impacts on regional concentrations were minimal. During that same time period, overall nitrate concentrations in surface water maintained a decreasing gradient downstream toward the ocean, and summer subsurface nitrate concentrations remain relatively static throughout the bay. Estimates of nitrogen availability relative to phosphate (N*) suggest that the bay switches from having excess N to exhibiting a deficit relative to what phytoplankton require at 41.7°N (the boundary between the Providence River Estuary and Narragansett Bay), regardless of N inputs upstream. On the other hand, a significant shift in the isotopic composition of the sources was observed. Tertiary treatment at one WWTF increased effluent nitrate δ15N and δ18O values by ~16 ‰ for both isotopes, and increased rivers by 4 ‰. North of 41.7°N (the Providence River Estuary) δ15N values increased significantly by 2 ‰, but not south of this point (Narragansett Bay proper). The increase in δ15N is attributed to the increased δ15N from upgrades to tertiary treatment.
During the summers of 2011 and 2012, the subsurface δ15N-NO3- and δ15N-chlorophyll a peaked mid-bay while macroalgal δ15N decreased linearly throughout the bay. The differences between the δ15N of macroalgae and chlorophyll a imply multiple sources of nitrogen supporting primary production. Phytoplankton are transported vertically and horizontally by tides/currents and mixing events. The exact position where they incorporate N is unknown, but they appear to be supported by subsurface nitrate. This runs counter to the idea that phytoplankton harvest nutrients upstream and are carried into the Bay by advection. Macroalgae are incorporating N at a fixed position, and may be supported by small, but consistent, benthic fluxes.
In conclusion, the river and WWTF data suggest that when seasonal means are significantly different from other sources, δ15N-NO3- may be a useful tracer of inputs in the nutrient replete region of the Providence River Estuary. Beyond the Providence River Estuary, we find that anthropogenically-derived nitrate is mixed with offshore water and/or is recycled quite efficiently, overprinting any anthropogenic tracer signal. Primary producers rely on anthropogenic nutrients within the Providence River Estuary, but also derive nitrogen from vertical mixing and benthic nutrient dynamics within the bay proper. In the future, it is likely that anthropogenic N input reductions will continue to impact north of 41.7°N while mixing and recycling will dominate processes south of this point.
Schmidt, Courtney Elizabeth, "INVESTIGATION OF NITROGEN CYCLING USING STABLE NITROGEN AND OXYGEN ISOTOPES IN NARRAGANSETT BAY, RI" (2014). Open Access Dissertations. Paper 209.