River Thames: Biodiversity recovering despite more plastic pollution

Populations of birds, eels and seals living in the River Thames have risen steadily despite increasing plastic pollution and rising temperatures.

A survey by the Zoological Society London (ZSL) of the UK’s second longest river, which was declared biologically dead in 1957, found bird and mammal populations have improved over the past 10 years, although fish populations have deteriorated in the same period.

In both the upper and middle Thames, fish populations have gradually declined after bouncing back in the 1970s and 1980s. More than 115 fish species were recorded in the river, which ZSL said is a promising indication of ecosystem recovery. However, the most recent samples found fewer numbers of species per sample, which requires further research to understand.

The report said the river was “clearly impacted” by climate change, with water temperature and sea levels rising above historic baselines. ZSL said this would affect wildlife in the Thames estuary.

Rising sea levels have been seen in the Thames since 1911. The trend became more pronounced in recent years (1990-2018) when annual average rises of up to 4.26mm were recorded. Average temperatures have also been rising in both summer and winter, with average summer temperatures in upper tidal parts of the Thames increasing by 0.19 degree Celsius per year from 2008.

Phosphorus levels, which grew rapidly between 1950 and the 1980s, have seen a steady decline since 2008 thanks to removal by Thames Water at its sewage treatment works.

Nitrate concentrations from industrial and sewage effluent and agriculture gradually increased between 2000 and 2020, however, ZSL said shorter term data for the past 10 years suggested they have stabilised. Spikes in nitrate levels in tributary rivers were not recorded in the past 20 years, which ZSL attributed to improvements to sewage treatment works.

Analysis of plastic pollution in the river showed wet wipes were “overwhelmingly” the most common item found at survey sites, reflecting the challenge these pose to sewerage undertakers.

For both wildfowl and wading birds the picture is improving, with wading populations doubling from 1993 to 2017, contrasting with UK-wide general decline.

The Thames is home to approximately 900 harbour seals and 3,200 grey seals, which have both steadily increased since monitoring began in 2003. Both breeds of seal can be found upstream and in the estuary.

A 2020 report by the Environment Agency showed no English river was free from chemical pollution that sparked a wave of public concern into regulators’ monitoring practices. James Bevan, chief executive of the EA, attributed the problem partly to budgetary cuts after the agency had its annual funding slashed from £110 million in 2011 to £40 million in 2021.