Carbon and nitrogen stable isotope analysis of three types of oyster tissue in an impacted estuary

The stable isotope ratios of carbon (d13C) and nitrogen (d15N) of the muscle, ctenidia and viscera of the Sydney rock oyster, Saccostrea glomerata, showed the dilution and assimilation of tertiary treated sewage along an estuarine gradient. The enriched 15N values of oyster ctenidia and viscera from within 50 m of the sewage outfall indicated the use of 15N-enriched tertiary treated sewage effluent (16G2.3&) as a nutrient source. The effect of sewage nitrogen on oyster d15N was localised, with oysters 5 km upstream and downstream of the outfall not significantly enriched. Viscera d15N was most sensitive to sewage nutrients and d13C significantly defined an ocean-to-estuarine gradient. High variance in isotope ratios of viscera compromised its use as an indicator of anthropogenic nutrients, and this also reduced the utility of whole-body stable isotope ratios. Ctenidia was the most useful indicator tissue of sewage discharge at the scale of this study, being consistently and significantly enriched in d15N close to the sewage outfall and d13C clearly defined an estuarine gradient with less internal variability than viscera. Muscle d15N was least sensitive to sewage effluent and showed the least variability, making it more suited to investigations of anthropogenic nutrient enrichment over larger spatio-temporal scales.