Stable isotopes in mangrove leaves and gastropod snails reflect exposure to treated sewage effluent in a tropical macro-tidal ecosystem (Buffalo Creek, Darwin, Australia)

In order to assess suitable bio-indicators of spatial and temporal patterns of treated sewage effluent in a tropical macro-tidal mangrove ecosystem, carbon and nitrogen stable isotope signatures together with carbon and... [ view full abstract ]

In order to assess suitable bio-indicators of spatial and temporal patterns of treated sewage effluent in a tropical macro-tidal mangrove ecosystem, carbon and nitrogen stable isotope signatures together with carbon and nitrogen concentrations were measured in leaves of Avicenia marina mangroves and foot muscle of two gastropod snails Telescopium telescopium and Terebralia semistriata sampled over a one year period. The δ15N values of mangrove leaves and snails were significantly enriched in the receiving creek compared to the reference creek (p<0.001). We observed a systematic decrease in δ15N values and N concentration in mangrove leaves with increasing distance from the treatment ponds outfall towards the creek mouth indicating uptake by trees of a diminishing proportion of sewage-derived N from mangrove soil. The δ13C values of mangrove leaves were not reliable indicators of sewage-derived inputs. In the receiving creek, both snail species showed elevated and nearly constant δ15N values from the outfall to the creek mouth where values abruptly decreased and approached the values recorded in the reference creek. The δ13C values in snails indicated that benthic microalgae containing sewage-derived nutrients were a partial food source for the snails in the upper reaches of the receiving creek. In conclusion, δ15N values in mangrove leaves and gastropod snails proved to be a reliable tool to trace the dispersion and uptake of nutrient inputs from sewage effluent, while δ13C values in snails were more indicative of the relative contributions of different food sources in their diet. The δ15N and δ13C values of mangrove leaves and snails did not vary significantly through the year.

Key words: Treated sewage effluent, tropical macro-tidal estuaries, stable carbon and nitrogen isotopes, bio-indicators