Linking nitrogen in estuarine producers to land‐derived sources

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 watershedestuary system, Cape Cod, Massachusetts. The δ 15 N 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 δ 15 N 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 δ 15 N of eelgrass, macroalgae, cordgrass, and suspended particulate organic matter. The increase of all producers examined in Waquoit Bay with increasing δ 15 N 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 δ 15 N of estuarine producers may provide a means to reliably identify incipient eutrophication in coastal waters.