Food webs of two intermittently open estuaries receiving 15N-enriched sewage effluent

Carbon and nitrogen stable isotope signatures were used to assess the response of food webs to sewage effluent discharged into two small intermittently open estuaries in northern New South Wales, Australia. One of these systems, Tallows Creek, has a history of direct sewage inputs, whilst the other, Belongil Creek, receives wastewater via an extensive wetland treatment system. The food webs of both systems were driven by algal sources of carbon, reflecting high autotrophic productivity in response to the nutrients entering the system front sewage effluent. All aquatic biota collected front Tallows Creek had significantly enriched delta N-15 signatures relative to their conspecifics from Belongil Creek, indicating that sewage nitrogen had been assimilated and transferred throughout the Tallows Creek food web. These delta N-15 values were higher than those reported from studies in permanently open estuaries receiving sewage effluent. We Suggest that these enriched signatures and the transfer of nitrogen throughout the entire food web reflect differences in hydrology and associated nitrogen cycling processes between permanently open and intermittently open estuaries. Although all organisms in Tallows Creek were generally N-15-enriched, isotopically light (less N-15-enriched) individuals of estuary perchlet (Anibassis marianus) and sea mullet (Miwil cephalus) were also collected. These individuals were most likely recent immigrants into Tallows Creek, as this system had only recently been opened to the ocean. This isotopic discrimination between resident (enriched) and immigrant (significantly less enriched) individuals can provide information on fish movement patterns and the role of heavily Polluted intermittently open estuaries in supporting commercially and recreationally valuable estuarine species. (c) 2006 Elsevier Ltd. All rights reserved.