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Posts Tagged ‘biologically dead’

Photo: Diliff, David Iliff. License: CC BY-SA 3.0.
Sunset over the Thames River in London.

When I worry that humans may never repair any of the damage we have done to the environment, I remind myself that once upon a time the Erie River routinely caught fire and no longer does.

Similarly, as Veronica Edmonds-Brown, Senior Lecturer in Aquatic Ecology at the University of Hertfordshire, reports for the Conversation, fallible humans have brought back the Thames.

Edmonds-Brown writes, “It might surprise you to know that the River Thames is considered one of the world’s cleanest rivers running through a city. What’s even more surprising is that it reached that status just 60 years after being declared ‘biologically dead‘ by scientists at London’s Natural History Museum.

“Yet despite this remarkable recovery, there’s no room for complacency – the Thames still faces new and increasing threats from pollution, plastic and a rising population. …

“Where it bisects London, it has experienced pressures from expanding numbers of citydwellers since medieval times. The river became a repository for waste, with leaking cesspits and dumped rubbish reducing many of its tributaries to running sewers. [Read Dickens novel Our Mutual Friend.] Many of these small rivers now lie underneath the streets of London, long covered up to hide their foul smells. …

“The final straw was the hot summer of 1858 – referred to as the Great Stink – when the high levels of human and industrial waste in the river actually drove people out of London. The civil engineer Sir Joseph Bazelgette was commissioned to build a sewage network to alleviate the problem, which is still in use today. What followed was over a century of improvements to the network, including upgrading sewage treatment works and installing household toilets linked to the system.

“Bombings across the city during the second world war destroyed parts of the network, allowing raw sewage to again enter the river. What’s more, as the Thames widens and slows through central London, fine particles of sediment from its tributaries settle on the riverbed. These were, and remain, heavily contaminated with a range of heavy metals from roads and industry, creating a toxic aquatic environment.

“For most fish to thrive, the water they live in must contain at least 4-5 milligrams of dissolved oxygen per litre (mg/l). Measurements taken during the 1950s showed that dissolved oxygen (DO) levels in the Thames were at just 5% saturation: the rough equivalent of 0.5 mg/l. That meant the river could only support a few aquatic invertebrate species like midges and fly larvae. …

“From Kew to Gravesend, a 69km [a 43-mile] length of river, no fish were recorded in the 1950s. Surveys in 1957 found the river was unable to sustain life, and the River Thames was eventually declared ‘biologically dead.’

“With considerable effort from policymakers, the river’s fate began to change. From 1976, all sewage entering the Thames was treated, and legislation between 1961 and 1995 helped to raise water quality standards. …

“One of the main turning points in the Thames’ health was the installation of large oxygenators, or ‘bubblers,’ to increase DO levels. … The flounder was officially the first fish species to return to the Thames in 1967, followed by 19 freshwater fish and 92 marine species such as bass and eel into the estuary and lower Thames. The return of salmon during the 1980s was a thrilling marker for conservationists, and today around 125 species of fish are regularly recorded, with exotic species like seahorses even being occasionally sighted.”

Although, as Edmonds-Brown notes, this recovery is remarkable, “there remain deeper, unresolved issues relating to contaminated sediments still entering the river.” Read more at the Conversation, here.

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