Can nitrogen loading affect trophic structure in estuarine food webs?
Using nitrogen stable isotopes to determine trophic length in nutrient enriched waters In this project, food webs in areas of Puget Sound characterized by nitrogen loading (Nisqually Reach, Poverty Bay) were studied and compared to similar areas not affected by nitrogen loading (Murden Cove, Hood Canal). Trophic structure was determined using stable isotope analysis of nitrogen and carbon in tissues of collected zooplankton, using the ratio of the heavier isotope to the lighter isotope (given in d notation). There is a 2-4 enrichment per trophic level due to fractionation, which allows for determining the trophic position of organisms. Nutrient and POC/PON samples were taken as well, to determine relative concentrations of nitrate, nitrite, ammonium, and POM. The surface concentration of nitrate was highest in Hood Canal, and lowest in Murden Cove. Poverty Bay had the highest concentration of nitrite, whereas Hood Canal had the lowest concentration. Ammonium was highest at Nisqually Re
