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

Science Center Objects

Seagrasses are among the most productive ecosystems on the planet. Worldwide losses of this important habitat have been caused by water quality degradation association with watershed development. Improved approaches to detect threats of nutrient enrichment are paramount to seagrass conservation.

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: The ratios of stable isotopes of nitrogen and sulfur in seagrass tissue often reflect environmental conditions. Most tests of stable isotope ratios as indicators of estuarine nutrient status are based on "snapshots" of different estuaries at a single point in time. We have measured stable nitrogen and sulfur isotope ratios of eelgrass in a groundwater-fed estuary (Pleasant Bay, MA, within Cape Cod National Seashore) from 2003 to 2018. Although the nitrogen isotope ratios suggested a possible change in estuarine nutrient status following formation of a new inlet, there is considerable variability in the data. Stable sulfur isotopes may help elucidate nutrient cycling patterns in the estuary.

The Future: Examining multiple lines of evidence over a long time series will allow us to determine whether eelgrass stable isotope ratios are reliable indicators of estuarine eutrophication.