, Volume 74, Issue 2, pp 167-192

Using Mussel Isotope Ratios to Assess Anthropogenic Nitrogen Inputs to Freshwater Ecosystems

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Stable nitrogen isotope ratios (δ15N) of freshwater mussels from a series of lakes and ponds wererelated to watershed land use characteristics to assess their utility in determining the source ofnitrogen inputs to inland water bodies. Nitrogen isotope ratiosmeasured in freshwater musselsfrom 19 lakes and ponds in Rhode Island, U.S.A., ranged from4.9–12.6‰ and were found tosignificantly correlate with the fraction of residential development in 100 and 200 m bufferzones around the ponds. Mussel δ15N values in 12 of the 19 ponds also showed significantcorrelation with average dissolved nitrate concentrations, which ranged from 23–327 μg L-1.These observations, in light of previous studies which link elevatedδ15N values of nitrogenderived from septic wastewater with those seen in biota, suggest that mussel isotope ratios mayreflect nitrogen source in freshwater ecosystems. We followed aniterative approach usingmultiple regression analysis to assess the relationship between musselδ15N and the land usecategories fraction residential development, fraction feedlotagriculture, fraction row-cropagriculture, and fraction natural vegetation in 100 and 200 m bufferzones and pond watersheds.From this we developed a simple regression model to predict musselδ15N from the fraction ofresidential development in the 200 m buffer zone around the pond.Subsequent testing with datafrom 16 additional sites in the same ecoregion led us to refine themodel by incorporating thefraction of natural vegetation. The overall average absolute differencebetween measured andpredicted δ15N values using the two-parameter model was 1.6‰. Potential sources of error inthe model include differences in the scale and categorization ofland-use data used to generate andtest the model, differences in physical characteristics, such asretention time and range ofresidential development, and exclusion of sources of enrichednitrogen such as runoff from feedlot operations or increased nitrogen loading from inefficient or failed septic systems.