ENVIRONMENT chiefs are probing the deaths of fish in York’s River Foss – but say the green duckweed which is currently carpeting the surface is not to blame.
The Environment Agency says its fisheries officers are investigating reports of recent fish deaths in the river but says low oxygen levels or a pollution incident may be the cause, and it is unlikely to be directly related to the presence of duckweed.
In a statement tweeted by the River Foss Society, the agency said many people thought that duckweed was unsightly – which was a matter of opinion, and a sign of pollution – which was untrue.
“It is a sign of low river flows, partly due to lack of rain, and a river that is enriched by nutrients,” it said.
“All of these are issues that people should be aware of as part of public environmental awareness – the situation is there for all to see.
“Duckweed, which is not algae but a flowering plant, is usually found in ponds and still water sites. It can grow in rivers when flows are very low and the river begins to behave like a pond and water temperature rise (at least at the surface). This summer has been hot and rainfall has been low, conditions that suit duckweed growth.”
It said the Foss was also slow flowing at its junction with the River Ouse because the lock gates at the Foss Basin acted as a barrier and because the Ouse had a larger volume of water and faster flows, causing the smaller river to ‘back-up.’
It said duckweed was not the result of pollution, although it thrived in water that was enriched, and it was not seen as a major problem that needed action to be taken to remove it.
“It should die back in autumn and be flushed away when river flows improve,” said the agency. “If it is treated with a chemical herbicide it will die and decompose but this will use up much of the oxygen in the water and cause fish and other aquatic species to die.”
The agency admitted duckweed could cause problems for anglers when it formed a thick mat across the river, in that fish were no longer visible and it clogged up fishing line, landing and keep nets and other equipment.
However, it did provide cover for fish and smothered algae, which used up oxygen and could produce toxins.
“Duckweed produces oxygen during daylight hours and it take in nutrients from the water reducing certain pollution levels (such as nitrates and phosphates). It also provides food for some fish and bird species.”