Algal and invertebrate bioindicators detect sewage effluent along the coast of Titahi Bay, Wellington, New Zealand

Abstract

Sewage effluent contains both dissolved and particulate matter that can affect coastal ecosystems and cascade through food webs. We used δ15N and δ13C ratios to explore performance of a kelp (Carpophyllum maschalocarpum) and two invertebrates (a grazing isopod, Amphoroidea media, and a filter-feeding crab, Petrolisthes elongatus) as bioindicators along a gradient of sewage exposure. These species vary in trophic status and, consequently, are likely to assimilate sewage constituents differently into food webs. δ15N values in C. maschalocarpum and A. media were highest near the sewage outfall and decreased with increasing distance from the outfall. δ13C values in these two species did not appear to be reliable predictors of sewage concentrations in seawater. In contrast, δ13C ratios for P. elongatus were consistently depleted at sites nearest the sewage outfall, indicating the consumption of 13C-depleted sewage particulates. These results suggest there is value in multiple-isotope and multi-species bioindicator approaches both for detecting sewage dispersal patterns and understanding the incorporation of sewage-derived nutrients into food webs.