By Christopher Doble
River catchments represent rich sources of biodiversity and provide a wide range of services for society. As a result, a global pattern of urbanization around major river systems has developed, leading to the modification and degradation of these waterways. The observed decline in the state of urban rivers has been so consistent globally that it has led to the development of the term ‘urban stream syndrome’ to describe the decrease in diversity and habitat quality that exists within these waterways1. With both the global population and rates of urbanisation increasing, so are the pressures facing urban rivers.
Looking out over the River Thames, its murkiness can give the impression of a polluted and lifeless river. This couldn’t be further from the truth however, as the river provides a broad range of habitats for a large number of species that many people would not expect to find in London. Regular sightings of grey and harbour seals, as well as porpoises, are made each year, and surveys have even found short-snouted seahorses. In fact, the Thames not only represents one of the cleanest urban river catchments in Europe, it is also the result of one of the most successful large-scale river restorations in history
As recently as 1957, the Thames was in a very different state. Extensive pollution since the industrial revolution had degraded the river to the extent that it was declared biologically dead by scientists at the Natural History Museum. London saw a rapid population growth from 2 million in 1850 to 4,750,000 in 1880, as well as a large expansion in industry. While waste from industries such as tanneries caused localised drops in water quality, it was the vast quantity of organic matter from untreated sewage that had the most devastating impact on the river. Bacteria decompose this organic matter by stripping the oxygen from the water, and producing toxic substances such as carbon dioxide, ammonia and phosphates, creating conditions that are inhospitable to life.
Improvements were made in the 1960s through the development of new sewage works, which increased the capacity of London’s sewage system and reduced the amount of untreated sewage entering the river. Industrial effluents were also reduced thanks to technological developments and stricter legislation. Instead of discharging by-products directly into the river, a number of industries introduced their own treatment plants, or ensured that effluents were taken away for special disposal. Immediate benefits in water quality were seen, with no oxygen-depleted river reaches being recorded in 1966. As the water quality of the river began to improve, the life within it started to return. Recreational and commercial fisheries re-established themselves, and 125 species of fish have now been recorded in the tidal Thames.
While the river’s restoration should certainly be seen as a success story, there are still a number of pressures facing it. The city’s sewage system has struggled to keep up with its continued growth, leading to overflows of raw sewage into the river as often as five times a month. Further pollution sources come from plumbing misconnections present in 10% of British houses, and rain runoff from roads. While it is hoped the future introduction of two major sewage tunnels will help reduce the number of outflows, present day pollution still leads to a deterioration in water quality as the river passes through central London.
Another major pollutant has been the ever-increasing quantity of plastic found in the Thames. Roughly 250 tons of debris is collected from the river every year, but the majority goes undetected, beneath the surface2. A recent study has found plastic in the guts of smelt and flounder fish in the Thames, suggesting that plastic litter could be affecting the river’s wildlife.
While many species have successfully recolonized the river, this is hampered by the arrival of a large number of invasive species. Nearly 100 non-native species are found in the river, over half of which arrived within the past half century3. A number of these, such as the Chinese mitten crab, have been found to negatively impact our native species, and their combined effect is likely to be much greater. Invasive species are continuing to arrive, with the Quagga mussel being discovered in the Thames at the end of 2014. It is therefore vital that good biosecurity measures are implemented for recreational activities within and around the Thames, to prevent future spreads.
Working to address these issues and creating more riparian habitat will help further improve biodiversity in the River Thames. While there are improvements to be made, the success of restoration efforts over the past 50 years provides hope for urban river conservation worldwide, demonstrating that it is possible for urban waterways to support a diversity of habitats and a rich array of life.
Want to get involved?
- Thames21 run a number of volunteer projects throughout the year helping to clean up and restore habitats within the Thames.
- ZSL run a number of citizen science projects within the Thames. Here is a link to their long-running European Eel monitoring programme with sites across London.
- You can record your marine mammal sightings in the Thames here.
1. Walsh. C. J, Roy. A. H, Feminella. J. W, Cottingham. P. D, Groffman. P. M and Morgan. R. P. (2005) The urban stream syndrome: current knowledge and the search for a cure. Journal of the North American Benthological Society 24(3), 706-723.
2. Morritt. D, Stefanoudis. P. V, Crimmen. O. A. (2014) Plastic in the Thames: A river runs though it. Marine Pollution Bulletin 78, 196-200.
3. Jackson. M. C, Grey. J. (2012) Accelerating rates of freshwater invasions in the catchment of the River Thames. Biological Invasions 15(5), 945-951
Francis. R. A, Hoggart. S. P. G, Gurnell. A. M and Coode. C. (2008) Meeting the challenges of urban river habitat restoration: Developing a methodology for the River Thames through central London. Area, 40(4), 435-445.
Wheeler. A. (1979) Tidal Thames: History of a River and Its Fishes. Routledge & Keegan Paul Books.