Dead fish have washed up on the shores of a London lake.

Residents spotted the washed up fish carcasses at popular beauty spot Ruislip Lido on Monday.

Local councillor Philip Corthorne confirmed the issue in an online community group, telling residents that fish have died due to low oxygen levels in the water.

This is likely down to an algal bloom according to the Environment Agency (EA), which itself is likely to have been triggered by the prolonged heat this summer.

He said: “We’re advised the lake is now closed to fishing to preserve stocks and the EA (Environment Agency) say it may take 3/4 days to stabilise the lake, and they’ll reassess. We’ve had the below;

“The EA have attended and tested the water. The oxygen levels are at 20% which is the cause of the fish mortalities. The EA are lending us 3 aerators to re-oxygenate a portion of the lake where the most fish are gathered.

“This will be run during the day. Our team will monitor the units and top up fuel when necessary.”

A spokesperson for the Environment Agency told the Local Democracy Reporting Service (LDRS): “Following the report of a fish kill at Ruislip Lido, our fisheries officers went to test the water. We believe the recent algal bloom, which is starting to die off, has caused a drop in oxygen in the water.

“The fishing lake is privately owned and therefore the responsibility of the owner to manage. We have agreed to provide them with aeration equipment to help restore oxygen levels in the lido and will continue to provide advice and guidance.”

A lake aerator adds oxygen and moves water. This helps fish breathe and prevents them from dying during hot weather.

It keeps the ecosystem healthy and stops the water from becoming stagnant. It also helps good bacteria survive by controlling excessive algae growth and it can prevent foul smells.

Cool water is able to hold more dissolved oxygen than warmer water. As water heats up, the molecules within it move faster – pushing some of the free oxygen into the atmosphere. As the temperature of the water increases, the amount of oxygen that water can hold decreases, resulting in less oxygen for aquatic life.

In August, the Met Office announced that this summer is on course to be the hottest since records began. There have been 37 consecutive heatwave days somewhere in the UK this year.

Several long-standing heat records have been broken this summer, including the driest July in England and Wales since records began 190 years ago. By July 15, the UK had already recorded more days above 30 degrees than in the whole of 1976 – remembered by many for its iconic heatwave.

Ruislip Lido is managed and owned by Hillingdon Council. Swimming has long been banned in the lido, yet, thousands still flock there every summer with many entering the water against the rules, ignoring warning signs dotted around the lake.

The presence of blue-green algae poses a significant toxic threat to both visitors and their pets. The risk of drowning is also heightened by cold water shock, especially as the lake’s depth fluctuates unexpectedly.

Visitors are also warned of ‘swimmer’s itch’, a condition sparked by an allergic response to tiny parasites found within the water.

The Environment Agency was contacted for further information.