At least 100 fish have died in the River Stort after being starved of oxygen by a rapid rise in algae levels.

An algal bloom along a ten-kilometre stretch of the river between Sawbridgeworth and Roydon last month is believed to have caused the deaths, the Environment Agency has said.

The government body has launched an investigation into the deaths.

However, department officials have confirmed the bloom will be left to disappear “naturally”.

An Environment Agency spokesman said: “We are investigating the deaths of at least 100 fish in the River Stort Navigation between Sawbridgeworth and Roydon last month.

“Our fisheries officers believe the hot weather at the time may have caused what’s known as an algal bloom along a ten-kilometre stretch, turning the water brown, depriving the fish of oxygen.”

Algal blooms – a rapid increase of algae in a body of water – are a natural phenomenon which cut off the oxygen supply in rivers and lakes.

Blooms can be the result of warm weather or a nutrient, like nitrogen or phosphorus from fertilizer run-off, causing excessive growth of algae.

Epping Forest District Council officials are monitoring the situation.

A spokesperson for the council said: “Members of the public are advised to stay away from any water courses showing signs of a bloom – typically cloudy or less clear green water and dead fish.

"It can also look blue-green or greenish-brown. Scums can form during calm weather when several bloom forming species rise to the surface. This can look like paint, mousse or small clumps."

Members of the public asked asked to report any change in colour of river water, dead fish, or fish struggling to breathe, to the Environment Agency on 0800 807060.

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