How the Distribution of Anthropogenic Nitrogen Has Changed in Narragansett Bay (RI, USA) Following Major Reductions in Nutrient Loads

Abstract

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre-and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ 15 N) and carbon (δ 13 C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ 15 N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1-2‰, on average. In contrast, the δ 13 C values measured in particulate matter and hard clams have declined by about the same amount. The δ 15 N results indicated that, even after the N-reductions, sewage N still plays an important role in supporting primary and secondary

Carbon Stable Isotope Values in Plankton and Mussels Reflect Changes in Carbonate Chemistry Associated with Nutrient Enhanced Net Production

“…Thus, the stronger influence of organic matter on carbonate chemistry in the Providence River and Upper Bay is logical (Narragansett Bay Estuary Program, 2017). Sewage effluent is still an important source of nutrients to the whole Bay, with nitrogen stable isotope values characteristic of sewage effluent reflected in primary producers and consumers throughout the system (Oczkowski et al, 2017). But, in the overall less productive Mid and Lower Bay, the amount of organic matter (detritus and plankton) in these regions had less of an influence on the model.…”

Pelagic N2 fixation dominated by sediment diazotrophic communities in a shallow temperate estuary

“…Pelagic N 2 fixation contributes minimally to nitrogen loading in Narragansett Bay Narragansett Bay is a highly dynamic environment, influenced by multiple environmental factors such as runoff, anthropogenic nitrogen loading, and freshwater input from rivers and salt water inputs that invade the bay through a semidiurnal tide regime (Li and Smayda 1998). Nitrogen loading has decreased notably over the past decades (Oczkowski 2018), which may provide a spatiotemporally variable niche for diazotrophs in Narragansett Bay as well as in other nitrogen loading managed estuaries (Fulweiler et al 2007;Messer et al 2021). Indeed, we found measurable N 2 fixation rates in the bay (0.02-9.41 nmol N L À1 d À1 ).…”

Marine macroalgae are an overlooked sink of silicon in coastal systems

“…A subset of the Waquoit Bay macroalgae samples were analyzed for percentage C and N using standard methods (Dalsgaard et al ., 2000) and a Eurovector CHN elemental analyzer at the Boston University Stable Isotopes Lab (Boston, MA, USA). All of the Narragansett Bay samples were analyzed for percent C and N and δ 13 C on a Carlo‐Erba NA 1500 Series II elemental analyzer interfaced with a Micromass Optima mass spectrometer (Oczkowski et al ., 2018).…”