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ASSESSING NITROGEN SOURCES IN LAKES WITH MUSSEL STABLE ISOTOPE RATIOS
Citation:
McKinney, R A., J L. Lake, M Charpentier, AND S A. Ryba. ASSESSING NITROGEN SOURCES IN LAKES WITH MUSSEL STABLE ISOTOPE RATIOS. Presented at 25th Annual Meeting of the New England Association of Environmental Biologists, Norwich, CT, April 4-6, 2001.
Description:
Despite progress made in the past several decades towards limiting nutrient inputs, human-induced eutrophication in lakes, ponds, and reservoirs continues to be of concern. Although many successes in reversing the effects of nutrient enrichment were realized by regulating phosphorous inputs to inland water bodies, investigations into the relationship between nutrient loading and productivity suggest the need to also consider nitrogen when assessing eutrophication in freshwater systems. In addition to atmospheric and autochthonous sources, watershed land-use practices can result in nitrogen inputs to receiving water bodies. For example, residential land use will result in the movement of septic wastewater nitrogen in groundwater which in turn feeds lakes and ponds, and agricultural land will lead to surface runoff of nitrogen from excess fertilizer application. In order to more effectively manage nutrient enrichment in freshwater systems, models are needed to help link watershed land use to changes in water quality.
In this study, we investigate the utility of stable nitrogen isotope ratios ( 15N) of freshwater mussels to trace inputs of nitrogen to inland water bodies. Mussel 15N values from 19 lakes and ponds in Rhode Island were related to watershed land use characteristics and were found to significantly correlate with the fraction of residential development in buffer zones around the ponds. Mussel 15N values also showed significant correlation with average dissolved nitrate concentrations. These observations, in light of previous studies which link elevated 15N values of nitrogen derived from septic wastewater with those seen in biota, suggest that mussel isotope ratios may reflect nitrogen sources in freshwater ecosystems. We developed a simple regression model to predict mussel 15N from the fraction of residential development in the 200 m buffer zone around the pond, and tested the model with data from 16 lakes in nearby Connecticut. Although this approach may be limited by differences in the scale and categorization of land-use data and in physical characteristics such as retention time, the model may ultimately provide information on nitrogen inputs which cannot be gleaned from available land-use data.