Recovery of nitrogen stable isotope signatures in the food web of an intermittently open estuary following removal of wastewater loads

Nitrogen (N) stable isotope values (δ¹⁵N) were used to assess the removal of wastewater N from the food web within Tallow Creek, a small intermittently closed/open lake/lagoon (ICOLL) on the east coast of Australia, following the cessation of wastewater inputs in 2005. Current (2013) δ¹⁵N values of sediment organic carbon, plants, and animals within Tallow Creek were compared to values obtained before wastewater inputs ceased, and to values within a nearby near-pristine ICOLL (Jerusalem Creek). Most biota had significantly depleted δ¹⁵N values compared to conspecifics collected before wastewater inputs ceased (mean reduction of 6.0‰; 38% of impacted enrichment), indicating substantial loss of wastewater N since inputs ceased. However, δ¹⁵N values remained enriched compared to the near-pristine ICOLL for some components (mean enrichment of 3.3‰ or 38%), suggesting that some wastewater N remains. The δ¹⁵N recovery rate (decrease in δ¹⁵N as a percentage of the impacted enrichment) for Tallow Creek biota was slow compared to that of biota in more open systems. This slow recovery rate and the persistence of some wastewater N, even after 8 years without new inputs, reflects differences in hydrology and nitrogen cycling between permanently open and intermittently open estuarine systems and highlights the likely lower resilience of ICOLLs to anthropogenic N inputs.