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

A macroalgae (seaweed) herbarium nitrogen isotope (δ¹⁵N) record is produced for the River Mersey and Liverpool South Docks (England) between 1821 and 2018. A modern macroalgae δ¹⁵N record was also produced from September 2022. The herbaria δ¹⁵N record shows a stark difference from 1821 to the present. Lower δ¹⁵N in the early 1800s is attributed to agricultural and raw sewage pollution. From 1970 to the present the herbaria samples record very elevated δ¹⁵N values – peaking in 1978 at +31‰. The 1989 Water Act and privatisation of water companies in the UK had limited impact on the herbarium δ¹⁵N record but indicated a dominance of sewage nitrogen in the River Mersey. Macroalgae δ¹⁵N has become even more elevated since the last herbaria sample in 2013. The herbaria and modern data record some of the highest seaweed δ¹⁵N 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.