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Variation in Nitrogen Isotope Ratios of Eelgrass to Detect Trends in Estuarine Nutrient Status

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The Challenge: Seagrasses are productive and important components of shallow coastal waters, and they have suffered extensive declines worldwide. Because seagrasses are directly in the path of watershed nutrient inputs, a major cause of habitat loss is coastal development and consequent water quality degradation. Improved approaches for detecting threats of nutrient enrichment are paramount to seagrass conservation.

The Science: Nitrogen isotope ratios (δ15N) of seagrasses typically reflect the isotopic composition of N sources, and enrichment of 15N is frequently associated with percent contribution of wastewater to N loads. We measured trends in δ15N of eelgrass in a groundwater-fed estuary (Pleasant Bay, MA, within Cape Cod National Seashore) associated with a change in estuarine nutrient status from 2003 to 2016. Early in the time series, enriched 15N values suggested high anthropogenic N inputs. Following increased flushing rates caused by formation of a new inlet, δ15N values in eelgrass have decreased. Stable sulfur isotopes are currently being examined to elucidate nutrient cycling patterns and verify causes of changing δ15N values.

The Future: Our time-series data suggest changes in eelgrass δ15N in relation to watershed nutrient inputs. Examining multiple lines of evidence will allow us to determine whether eelgrass δ15N is a reliable indicator of estuarine eutrophication.