Stable isotopes in biota reflect the graduated influence of sewage effluent along a tropical macro-tidal creek

In hydrodynamically complex environments, where conventional water-quality parameters may not adequately quantify sewage influence, stable isotopes in time-integrating biological tissues may provide an alternative monitoring tool. We measured nitrogen and carbon isotope ratios and concentrations in mangrove leaves and muscle tissues of two species of gastropod snails to determine the dispersion and biological assimilation of sewage-derived nutrients in a macro-tidal creek. The values of stable isotope of nitrogen (δ15N) in mangrove leaves and gastropods from the affected creek were significantly higher than those in samples from an unaffected creek, reflecting a graduated influence of sewage-derived N. The δ15N values in mangrove leaves showed high repeatability between sampling rounds and this, coupled with ease of sampling, makes them an effective monitoring tool to trace the influence of sewage effluent in receiving waters. The combined use of values of δ15N and stable isotope of carbon in gastropods showed some promise as a monitoring tool, but intra- and inter-specific variations in isotope values due to spatial differences in available food sources may affect their reliability in tracing sewage influence.