Wastewater nitrogen and trace metal uptake by biota on a high-energy rocky shore detected using stable isotopes

On high-energy rocky shores receiving treated wastewater, impacts are difficult to distinguish against a highly variable background and are localised due to rapid dilution. We demonstrate that nitrogen stable isotope values (δ¹⁵N) of rocky shore biota are highly sensitive to wastewater inputs. For macroalgae (Ulva lactuca and Endarachne binghamiae), grazing snails (Bembicium nanum and Nerita atramentosa), and predatory snails (Morula marginalba), δ¹⁵N was enriched near a wastewater outfall and declined with distance, returning to background levels within 290 m. Any of these species therefore indicates the extent of influence of wastewater, allowing identification of an appropriate scale for studies of ecosystem impacts. For M. marginalba, significant regressions between δ¹⁵N and tissue copper, manganese, and zinc concentrations indicate a possible wastewater source for these metals. This suggests that δ¹⁵N is a proxy for exposure to wastewater contaminants, and may help to attribute variations in rocky shore communities to wastewater impacts.