Deadly seaweed chokes Solent waterways

Farm runoffs and sewage outflows boost growth and puts birds and marine life at risk
Alok Jha
guardian.co.uk, Friday 21 August 2009 17.03 BST
Blooms of seaweed choking the waterways of the Solent have been caused by large amounts of untreated sewage and farm fertilisers dumped into the sea, according to the Environment Agency.
Extra nutrients in the pollutants combined with sunny weather has enabled the seaweed to grow out of control around Worthing in West Sussex, Ventnor in the Isle of Wight and at Langstone harbour. The growth has cut off access to food for local birds, fish and crustaceans and depleted oxygen in the water.
Large stretches of the mudflats and saltmarshes around the Solent are designated as areas of outstanding natural beauty. They are home to more than 7,500 migratory brent geese and tens of thousands of other local birds that visit the area throughout the year to feed on the plants and marine invertebrates such as crustaceans and molluscs.
But the biodiversity in the area's waterways and harbours is under threat from sewage and agriculture. "[Seaweed] growth is promoted by excess nutrients, mainly nitrogen, nitrates and ammonia," said Dave Lowthion, marine team leader at the Environment Agency. "We know that the two key sources to this are nitrogen in sewage discharges and nitrates runoff from agriculture into rivers and harbours."
This can lead to thick mats of seaweed growing between April and November each year which prevent migratory birds from getting at their invertebrate prey in the sediment. As the seaweed decomposes, it depletes the surrounding oxygen supply and produces hydrogen sulphide. This gas can be toxic to marine life and, in larger amounts, to people.
Parts of the UK under threat from algae blooms caused by agriculture and sewage pollution, according to the Environment Agency, include Cardiff Bay, Lougher estuary, Seal Sands Tees estuary, Chichester harbour, Hamble estuary, Pagham harbour and Portsmouth harbour. Lowthion said that more than 70% of the UK's rivers are designated as nitrate-vulnerable zones. "In the Solent that's virtually every stream on the mainland and the Isle of Wight," he said.
Seaweed build-up has also affected the northern coast of France, with excess blooms covering beaches all over Brittany. Release of hydrogen sulphide on the beaches there allegedly caused the recent death of a horse, which became overwhelmed by fumes on a beach in Saint Michel de Greve. Mounds of rotting seaweed have appeared at more than 80 locations on the coast this summer. They have a dry crust that can trap large concentrations of hydrogen sulphide, which builds up until the mound is disturbed.
Recent warm weather is also to blame for the death of thousands of fish along the coast of Cornwall. Around St Austell Bay and Tregantle beach at Antony, near Torpoint, the Food Standards Agency closed down shellfisheries this week as blooms of microscopic red algae appeared off the shore and dead fish washed up on the beaches. The so-called "red tide" produces toxins that are lethal to fish and shellfish and the sudden growth in the algae depletes oxygen in the water, with knock-on effects for marine life. The algae can cause skin irritation and harm pets — local authories have warned people to avoid going into the water wherever any red algae might be present.
"When it's eaten by shellfish, the toxin can persist in the flesh and can poison people if they eat the shellfish. They can be big blooms, half the size of the English Channel, but not very often do they go right up to the shore," said Lowthion.
He added that though the red tide blooms could be large, they would disperse quickly as the water temperatures drop in the run-up to winter.