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‰) in St Aubin’s Bay—caused by the outflow of the Bellozanne Sewerage Treatment Works (STW). δ15N isoplots maps indicate low diffusion of nitrogen out of St Aubin’s Bay. St Mary’s produced a varied δ15N isoplot map in comparison. δ15N was typically lower and is attributed to a smaller population and inefficient STW. Outflow of sewage/effluent at Morning Point, Hugh Town 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 still indicates that nitrogen pollution is an island-wide issue. Future sewerage development and upgrades on islands are required to prevent similar effluent environmental issues as recorded in St Aubin’s Bay. This study advocates the use of macroalgae as a bioindicator of nitrogen effluent in the marine environment.

<p>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 <em>Fucus vesiculosus</em> and <em>Ulva</em> sp. were analysed for nitrogen isotopes (δ<sup>15</sup>N). Elevated δ<sup>15</sup>N values were recorded for Jersey (>9.0 ‰) in St Aubin’s Bay – caused by the outflow from the Bellozanne Sewerage Treatment Works (STW). δ<sup>15</sup>N isoplots indicate low diffusion of nitrogen from St Aubin’s Bay. St Mary’s produced a varied δ<sup>15</sup>N isoplot in comparison. δ<sup>15</sup>N 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 δ<sup>15</sup>N values in comparison to the island average. St Mary’s inefficient sewerage treatment and reliance on septic tanks/soakaways complicates δ<sup>15</sup>N 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.</p>