Diffuse and concentrated nitrogen sewage pollution in island environments with differing treatment systems

Macroalgae is an under-utilised tool as a bioindicator of anthropogenic nitrogen loading to the coastal environment in the UK. This study compared two island systems — Jersey (Channel Islands) and St Mary’s (Isles of Scilly) to assess how differing sewerage infrastructure affects nitrogen loading. A total of 831 macroalgae samples of Fucus vesiculosus and Ulva sp. were analysed for nitrogen isotopes (δ¹⁵N). Elevated δ¹⁵N values were recorded for Jersey (>9.0 ‰) in St Aubin’s Bay – caused by the outflow from the Bellozanne Sewerage Treatment Works (STW). δ¹⁵N isoplots indicate low diffusion of nitrogen from St Aubin’s Bay. St Mary’s produced a varied δ¹⁵N isoplot in comparison. δ¹⁵N was typically lower and attributed to a smaller population and inefficient STW. Outflow of sewage/effluent at Morning Point, Hugh Town Harbour and Old Town produced elevated δ¹⁵N values in comparison to the island average. St Mary’s inefficient sewerage treatment and reliance on septic tanks/soakaways complicates δ¹⁵N interpretation although it suggests nitrogen pollution is an issue island wide. Future sewerage development and upgrades on islands are required to prevent the sewage environmental issue in St Aubin’s Bay. This study advocates the use of macroalgae as a bioindicator of nitrogen effluent in the marine environment.