Spatial variation in ä 13C and ä 15N of liver, muscle and bone in a rocky reef planktivorous fish: the relative contribution of sewage

Carbon and nitrogen stable isotope ratios in the liver, muscle and bone of a rocky reef planktivorous fish (Atypichthys strigatus) were compared amongst sewage impacted and non-impacted regions off eastern Australia. Fish were sampled in summer and autumn over 2 years from replicate sites across the three regions. Liver and muscle 15N ratios, representing short- (weekly) and long-term (yearly) nutrient integration, were depleted within the impacted region (10.0‰ and 12.0‰, respectively) relative to both non-impacted regions (11.4‰ and 13.6‰, respectively), consistent with the lighter 15N composition of primary treated sewage effluent (0.4‰), compared to marine derived particulate organic matter (POM, 8.2‰). Bone with the slowest turnover and longest integration time exhibited the least difference in 15N among regions (generally <1‰). Within fish variation in stable isotope ratios was found to differ among tissue types and between elements. Bone showed the least variation (5%), whereas liver had the greatest variation (29%). Up to 70% of the variance was observed amongst regions in almost all tissue/stable isotope combinations. With adequate replication at several spatial scales, small sample sizes had sufficient power to detect significant ecological variation. Compared to the massive but sporadic upwelling events, we find that the comparatively small yet continuous discharge of sewage off Sydney is an important nutrient source for planktivorous fish.