Nitrogen isotopes in herbaria document historical nitrogen sewage pollution in the Mersey Estuary, England

A macroalgae (seaweed) herbarium nitrogen isotope (delta 15N) record is produced for the River Mersey and Liverpool South Docks (England) between 1821 and 2018. A modern macroalgae delta 15N record was also produced from September 2022. The herbaria delta 15N record shows a stark difference from 1821 to the present. Lower delta 15N in the early 1800s is attributed to agricultural and raw sewage pollution. From 1970 to the present the herbaria samples record very elevated delta 15N values - peaking in 1978 at +31 parts per thousand. The 1989 Water Act and privatisation of water companies in the UK had limited impact on the herbarium delta 15N record but indicated a dominance of sewage nitrogen in the River Mersey. Macroalgae delta 15N has become even more elevated since the last herbaria sample in 2013. The herbaria and modern data record some of the highest seaweed delta 15N values (and therefore, sewage nitrogen pollution) recorded to date. This study highlights a novel use of herbaria macroalgae to document past changes in nitrogen pollution in estuarine environments. More poignantly it highlights that the River Mersey - Mersey Estuary is heavily polluted with sewage nitrogen and requires immediate action to resolve this environmental issue. A schematic figure illustrating how nitrogen isotopes (delta 15N) from treated-raw sewage enters an estuary, is taken up by macroalgae (seaweed). Treated sewage produces more elevated delta 15N values compared to raw sewage.