The river Thames has a long history of pollution. What was once “The Jewel of London” soon became “The Great Stink”. The Thames was made host to an array of Victorian wastes. Industrial and household waste, sewage and the discards of slaughterhouses all made their way into the great London River. Despite this, the Thames remained the primary source of drinking water for England’s capital and consequently led to the great outbreak of cholera in 1832.

This pandemic brought on by the unsanitary conditions of the Thames persisted for 22 years and resulted in 32,000 deaths. Being 29 years short of Louis Pasteur’s germ theory of disease, citizens of London initially believed the outbreak to be a result of the foul smell in the air, known as miasma. The ignorance of the microworld was a detriment to the London populace.

‘A court for King Cholera’ showing the poor conditions in Victorian London during the Cholera epidemic. Image source.

The eruption of World War II only exacerbated the problem; frequent bombings from German forces resulted in the destruction of sewage treatment facilities and thus severe contamination of the Thames.

All of these effects culminated into the 1950s, when the river was deemed “biologically dead” by the Natural History Museum of London.

This was all until November 2021 when a comprehensive analysis, undertaken by the Zoological Society of London (ZSL), reported finding a multitude of aquatic life. Seahorses, sharks, eels, and seals have all been found living in the tidal Thames.

A seahorse found living in the Thames. Taken from ZSL.

The study undertaken by the ZSL enlisted the help of over 16 collaborating organisations, which together identified 17 indicators of Thames health. These indicators encompassed assessments of water quality, ecology, and human activities. In combination with available short- and long- term data series, the indicators provide a clear-cut view into the changes of the River Thames’ health and will help identify areas to improve.

As a result of updated sewage treatment, long-term water quality significantly improved. This was shown by improvements in dissolved oxygen availability, a necessity for biological functioning, as well as declines in phosphorus concentrations. High phosphorous concentrations as a result of agriculture and other human activities, leads to a phenomenon known as eutrophication, the formation of an extensive algal bloom and associated decline in subaquatic plants and animals.

The ZSL also reported drastic changes in animal life. A recent count confirmed that 900 harbour seals and 3,200 grey seals are living in the Thames; an undeniable increase since 2003 when the survey began.

Harbour seal mother and pup in the Thames estuary. Photograph by Graham Mee/ZSL.

The current estimate of fish species in the Thames is 115, which is associated with an unfortunate decline in fish abundance since the 1990s due to unknown reasons.

In addition to this, 92 species of birds and over 1000 acres of saltmarsh, both reliant on the Thames, have been identified. Saltmarshes themselves host a great diversity of plant and animal life and present another reason as to why the conditions of the Thames should be closely monitored.

The ZSL report details the benefits of the creation of new saltmarsh habitat; plans for re-wetting floodplains in the Upper Tidal Thames indicate a changing mindset and the accepted importance of habitat restoration in the Thames estuary.

Whilst the survey certainly revealed a glimpse of a promising future for “The Jewel of London”, some other findings were more disconcerting. Sewage effluent remains a leading cause in high nitrate concentrations, which poses a threat to aquatic life through eutrophication. In addition to this, the effects of climate change are on full display with rising sea levels (4.26 mm per year measured at Silvertown), as well as rising water temperatures (0.2 ⁰C per year in the Upper Thames).

Despite the inherent negative findings, the overall message of the report is a positive one. Life is returning to the Thames. The methods developed by the ZSL in conducting this survey may prove useful to estuarine environments worldwide.

The study doesn’t stop here. The ZSL plan ‘for this data to be revisited in 5-10 years… to once again observe how indicators have changed’.

Written by Lucas King