Chemical Indicators of Anthropogenic Nitrogen-Loading in Four Pacific Estuaries

Abstract

Watershed inputs of anthropogenic nitrogen ( N) are altering the trophic status of estuaries worldwide. In this study we compared two chemical approaches for assessing watershed N inputs to estuaries: (1) use of conventional nutrient concentration measurements, and (2) use of nitrogen isotope ( 15N) measurements in estuarine sediments and biota. Of special interest was testing whether d 15N assays were generally robust tracers of watershed N across different estuarine systems. Four Pacific estuaries were chosen for study at widely spaced intervals on the U.S. West Coast: Padilla Bay (northern Washington State), South Slough (southern Oregon), Elkhorn Slough (central California), and Tijuana River (southern California). These estuaries are part of the National Estuarine Research Reserve (NERR) system. They are relatively small and shallow, are well flushed by tides, and can receive substantial natural N-loading from seasonally upwelled offshore waters. Results showed that none of the estuaries was truly pristine, with high watershed DIN (dissolved inorganic nitrogen) concentrations >500 mM especially in Elkhorn and Tijuana estuaries that respectively received high agricultural and sewage inputs. Nitrogen isotope assays failed to detect N-loading under conditions of very high ammonium inputs from sewage, but were otherwise useful indicators of estuarine N status in all four estuaries. Overall, using a combination of nutrient and isotope measurements was the best strategy for detecting watershed N-loading in these estuaries. The combination approach could be used to generate maps of low, medium, and high inputs to each of the four study estuaries. The N isotope measurements appear to be useful especially for tracing historical development of N-based eutrophication and for showing entry of pollutant N into local food webs.