Isotopic signatures in Mytilus galloprovincialis and Ulva latuca as bioindicators for assessing discharged sewage effluent in coastal waters along Otago Peninsula, New Zealand

Abstract

Sewage, waste organic matter from domestic and municipal wastewater, causes increased secondary productivity, eutrophication and trace metal contamination, reduced oxygen levels, and biodiversity which can lead to ecological disturbances in the natural aquatic ecosystem. The impact of sewage-derived organic matter (SDOM) on the nearshore marine ecosystem of the Otago Coast was assessed before, and 15 years after upgrade of the Dunedin sewage treatment plant. Carbon and nitrogen isotopic ratios in the tissues of sentinel organisms were used as bioindicators to elucidate the primary sources of nutrition the coastal environment. Mytilus galloprovincialis, a marine bivalve, exhibited a strong influence of SDOM from two sites in 2001. In 2015, M. galloprovincialis had a trophic enrichment factor of 3‰ (δ15N) and 1‰ (δ13C) when compared to the marine particulate organic matter (POM), suggestive of a dietary change away from the SDOM. Suspended POM collected from riverine and estuarine sources revealed other possible nitrogen sources from human-driven activities such as pastoral farming, application of organic manure and inorganic fertilisers, nitrification of ammonium from semi-urban septic tanks, and animal organic waste residues.