Macroalgae Stable Isotope Analysis to Trace Sewage Nitrogen Pollution in Estuarine Environments

Abstract

Sewage pollution is a widespread issue across the UK causing significant ecological damage to rivers, estuaries, and coastal environments. The issue has drawn widespread media attention over the high number of sewage spills and illegal discharges. Use of δ15N values in macroalgae tissue can discriminate between pollutant sources and has been successfully used to identify sewage nitrogen inputs across the globe. This method could provide a cheap and easy method of tracing sewage inputs to UK waterbodies. High δ15N values indicate an anthropogenic source (i.e., sewage) whereas lower values (~ 0 ‰) suggest an artificial source (i.e., fertiliser). Macroalgae is considered a reliable bio-indicator of nitrogen sources, although the technique is limited to only a handful of studies in the UK. A methodological approach was taken sampling Fucus vesiculosus and Ulva sp. at Staithes (North Yorkshire, UK) to understand δ15N variation within a small bay. High δ15N values were linked to a sewage treatment facility upstream. The harbour recorded relatively homogenous δ15N values suggesting sites were representative of the whole harbour. Only one exception recorded a significantly different δ15N value within the harbour; attributed to a WWTW Pump. Fucus vesiculosus and Ulva sp. were also used to record seasonal δ15N variation for the highly populated Mersey Estuary in the UK. Consistently high δ15N values (~ 14–18 ‰) revealed widespread sewage nitrogen loading across the entire estuary. Ulva sp. showed significant enrichment in summer, although values were high for all collection months and suggest sewage nitrogen is a year-round issue for the Mersey Estuary. Further work is required to produce a monthly data set and extend δ15N to include samples upstream. The Mersey record was extended to include historical δ15N data from herbaria macroalgae donated by the World Museums Liverpool. This is the first study using herbaria δ15N analysis in the UK. A 200-year δ15N record was produced for the estuary, revealing a shift from industrial and/or raw sewage nitrogen inputs in the early 1800s to a treated sewage nitrogen source into present day. Variation in δ15N appeared to coincide with policy changes regarding water quality and sewage infrastructure although more data is required. Herbaria was shown to be a useful tool when tracing historical nitrogen pollutants and further analysis of other herbaria records could reveal similar trends for other UK estuaries. Fucus vesiculosus and Ulva sp. are reliable bio-monitors, they are widespread making results comparable to other European studies; δ15N analysis of these species should be considered as an alternative method for identification of sewage nitrogen across the UK.