INVESTIGATION OF NITROGEN CYCLING USING STABLE NITROGEN AND OXYGEN ISOTOPES IN NARRAGANSETT BAY, RI

摘要: 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 last century with evidence for eutrophication in some regions of Bay. Increased concerns prompted upgrades at wastewater treatment facilities (WWTFs) decrease amount discharged. The upgrade tertiary – where bioavailable is reduced removed denitrification occurred multiple throughout Bay’s watershed. Nitrogen remains limiting primary production Bay proper which led speculation that system may result large part high nutrient loads north. Tracing sources essential attributing drivers it previously been done using stable isotopes. However, little information on isotopic composition inputs available no data are assess impact dissolved inorganic or producers. objective this dissertation explore spatial temporal distribution isotopes forms, organic, their compositions, role plays driving within Samples were collected 2007 2012, before during treatment. rivers WWTFs characterize potential source nitrate (NO3) variability. Surface water NO3 samples a north-south transect trace spatially. Finally, summers 2011 additional subsurface nitrate, chlorophyll a, macroalgae (2012 only) relative importance All analyzed concentrations (NO3 , PO4, NH4) (δN) oxygen (δO) compositions NO3. Chlorophyll content compound specific bulk macroaglae, respectively, measured. Between 2009 by 30 % but impacts regional minimal. During same time period, overall surface maintained decreasing gradient downstream toward ocean, summer remain relatively static bay. Estimates availability phosphate (N*) suggest bay switches having excess exhibiting deficit what phytoplankton require 41.7°N (the boundary between Providence River Estuary Bay), regardless upstream. On other hand, significant shift was observed. Tertiary one WWTF effluent δN δO values ~16 ‰ both isotopes, 4 ‰. North Estuary) significantly 2 ‰, not south point (Narragansett proper). increase attributed δN-NO3 δNchlorophyll peaked mid-bay while macroalgal decreased linearly differences imply supporting production. Phytoplankton vertically horizontally tides/currents mixing events. exact position they incorporate unknown, appear be supported nitrate. This runs counter idea harvest nutrients upstream carried into advection. Macroalgae incorporating fixed position, small, consistent, benthic fluxes. conclusion, river when seasonal means different sources, useful tracer replete region Estuary. Beyond Estuary, we find anthropogenically-derived mixed offshore and/or recycled quite efficiently, overprinting any signal. Primary producers rely also derive vertical dynamics proper. future, likely input reductions will continue north recycling dominate processes point.