Linking nitrogen in estuarine producers to land-derived sources

Abstract

It is clear that anthropogenic nitrogen inputs from watersheds to estuaries stimulate eutrophication. It has been difficult, however, to explicitly link anthropogenic N entering estuaries to N found in estuarine producers. To explore this link, we compared stable isotope ratios of N in groundwater and producers from the Waquoit Bay watershed-estuary system, Cape Cod, Massachusetts. The δ15N values of groundwater nitrate within the Waquoit Bay watershed increase from -0.9‰ to +14.9‰ as wastewater contributions increase from 4 to 86% of the total N pool. As a result, the average δ15N of dissolved inorganic nitrogen (DIN, nitrate + ammonium) received by different estuaries around Waquoit Bay increases from +0.5‰ to +9.5‰. This increase is strongly correlated to increases in δ15N of eelgrass, macroalgae, cordgrass, and suspended particulate organic matter. The increase of all producers examined in Waquoit Bay with increasing δ15N of DIN in groundwater demonstrates a tight coupling between N contributed to coastal watersheds and N used by primary producers in estuaries. The ability to identify effects of increasing wastewater N loads on δ15N of estuarine producers may provide a means to reliably identify incipient eutrophication in coastal waters.