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Changes in nitrogen isotope ratios in estuarine biota following nutrient reductions to Narragansett Bay
Taplin, B., R. Pruell, K. Miller, AND M. Charpentier. Changes in nitrogen isotope ratios in estuarine biota following nutrient reductions to Narragansett Bay. Coastal & Estuarine Research Federation (CERF) 24th Biennial Conference, Providence, Rhode Island, November 05 - 09, 2017.
Efforts are underway worldwide to reduce nutrient inputs to coastal systems by improving wastewater infrastructure and changing land management practices. This study used stable nitrogen isotopes (δ15N) to track anthropogenic sources of nitrogen to Narragansett Bay before and after tertiary upgrades to waste water treatment facilities (WWTFs). Results from our study show that reduced nutrients from tertiary treatment upgrades have significantly lowered nutrient levels in the upper part of Narragansett Bay. Based on our findings, nitrogen stable isotopes are a valuable tool in delineating anthropogenic nutrient sources to coastal water bodies.
Increased nutrient inputs globally have resulted in widespread eutrophication to many coastal water bodies including Narragansett Bay. Efforts to reduce point source nitrogen load¬ings from waste water treatment facilities (WWTFs) and combined sewer overflows (CSOs) started in 2006, with tertiary upgrades to 11 of the major WWTFs discharging to Narragansett Bay and its tributaries. One approach to monitoring nutrient inputs to coastal water bodies and biota include the use of stable isotopes. In this study, we monitored the changes in stable nitrogen isotopes( δ15N) in several estuarine marsh species (smooth cord grass, Spartina alterniflora; sea lettuce, Ulva lactuca; ribbed mussels Geukensia demissa; mud snails, Nassarus obsoletus; grass shrimp, Paleomonetes pugio and the common mummichog, Fundulus heteroclitus) before and after tertiary treatment upgrades to Rhode Island’s WWTFs. Results from our study showed a pronounced gradient down the bay for most species, except cord grass, with higher δ15N values measured in biota from the upper bay site (Passeonkquis cove) and lower δ15N values in biota from mid-bay (Prudence Island) and the lower-bay site (Fox Hill Marsh). Nitrogen isotope ratios in biota from the upper bay station (Passeonkquis cove) were on average 2‰ greater after WWTF upgrades than before, reflecting reduced preferential fractionation by biota and or an increase in the δ15N of NO3- and NH4+ released to the estuary by WWTFs. Based on our study, stable isotopes of nitrogen may be a good tool for identify nutrient sources and tracking anthropogenic changes to coastal water bodies.