LINKING NITROGEN IN ESTUARINE PRODUCERS TO LAND-DERIVED SOURCES

It is clear that anthropogenic nitrogen inputs from watersheds to estu aries stimulate eutrophication. It has been difficult, however, to exp licitly Link anthropogenic N entering estuaries to N found in estuarin e producers. To explore this link, we compared stable isotope ratios o f N in groundwater and producers from the Waquoit Bay watershed-estuar y system, Cape Cod, Massachusetts. The delta(15)N values of groundwate r nitrate within the Waquoit Bay watershed increase from -0.9 parts pe r thousand to + 14.9 parts per thousand as wastewater contributions in crease from 4 to 86% of the total N pool. As a result, the average del ta(15)N of dissolved inorganic nitrogen (DIN, nitrate + ammonium) rece ived by different estuaries around Waquoit Bay increases from +0.5 par ts per thousand to +9.5 parts per thousand. This increase is strongly correlated to increases in delta(15)N of eelgrass, macroalgae, cordgra ss, and suspended particulate organic matter. The increase of all prod ucers examined in Waquoit Bay with increasing delta(15)N of DUN in gro undwater demonstrates a tight coupling between N contributed to coasta l watersheds and N used by primary producers in estuaries. The ability to identify effects of increasing wastewater N loads on delta(15)N of estuarine producers may provide a means to reliably identify incipien t eutrophication in coastal waters.