Carpets of toxic blue-green algae, some of which which can kill dogs within half an hour, are now blanketing many ponds, canals and other shallow water courses as the drought expands across Britain.
According to the UK Environment Agency, the effects of the drought can now be seen right across Britain, with more than 100 instances of algal blooms and fish suffocation reported in the past few weeks. Warm, shallow water holds less oxygen, leaving fish vulnerable to stress, suffocation and disease.
"These are the most suitable conditions for the formation of algal blooms since 1989 and it will get worse if the next few weeks are hot and dry," said Jonathan Newman, head of aquatic plant management at the the Center for Hydrology and Ecology.
"Blue-green algae can be very serious. They produce toxins that are especially nasty for dogs. If dogs lick it they can die in 30 minutes. The toxicity of Microcystis aeruginosa is somewhere between cobra venom and pufferfish," he said.
But he said that it was not particularly dangerous for humans, who would need drink about 250 liters of water containing the algae to be seriously harmed.
On Wednesday the UK's Royal Society for the Protection of Birds (RSPB) warned that without human intervention, many rivers and streams will be emptied of fish later this year as pollutants from farmland become concentrated in increasingly shallow river water.
Angling clubs and pond owners have installed installed aerators in some rivers and ponds to increase oxygen and reduce toxicity of algal blooms. Low flows in many rivers means that sea water is flowing further up the river than normal, causing other problems for freshwater wildlife.
According to the agency, drying up wetlands, ponds and rivers are affecting the breeding success of frogs, waders and insects.
"It's clear the impact of the drought is no longer contained to the southeast of England, where water shortages were impacting on people's water use," said David King, director of water management at the Environment Agency.
"We're all aware of the seriousness of this drought, especially in the southeast. But if we get a third dry winter in the southeast, and dry conditions continue across the rest of England and Wales, we're going to see our environment suffer even more," he said.
The RSPB has joined other environment organizations to demand an increase in the reuse and recycling of water, higher water efficiency for new homes and greater investment to reduce leaks.