The wholesale destruction of the United Kingdom's estuary wetlands has raised the prospect of harmful algal blooms one day forming off the east coast.
Vast amounts of nitrates and phosphates used in agriculture and derived from sewage have been flowing directly into the North Sea instead of being naturally reduced by salt marshes and intertidal sediments.
A team of scientists from the University of East Anglia and the Centre for Environment, Fisheries and Aquaculture Science, a government laboratory in Lowestoft, has revealed the extent to which the wetlands around the Humber estuary - Britain's largest estuary - have been destroyed and the quantities of nutrients now pouring into the sea.
It concludes that the situation is likely to be the same for other east coast estuaries. This encourages algal blooms, which already appear periodically off Germany and Holland.
Tim Jickells, at UEA's School of Environmental Sciences, said: "There has been an absolutely huge loss of prime ecological habitat, wetland forest and peat bogs, which could have taken out a great deal of the nutrients."
By analysing sediment cores taken from across the region, the team has been able to reconstruct how the shape of the Humber estuary has evolved over the past 10,000 years.
The research, which will be published in the next issue of the environmental journal Ambio, has shown that more than 90 per cent of the wetlands has vanished over the past 300 years, mostly as reclaimed farmland or for quayside development.
Professor Jickells said this natural environment could effectively remove the nutrients by either turning them into organic matter bound up in plants that subsequently get buried or through biochemical decomposition processes that convert them into gases that are released into the atmosphere.
The scientists measured flow rates of the chemicals and have calculated that some 50,000 tonnes of nitrates and 5,000 tonnes of phosphates reach the North Sea from the Humber estuary each year.
Algae can feed off these nutrients and could form vast blooms in the sea, stripping oxygen from the water and killing other forms of marine life.
Despite heightened levels of nutrients, this has so far not happened in the UK as the amount of sediment suspended in the water has prevented the algae from photosynthesising, but the scientists are still concerned that levels of phosphates and nitrates should be reduced.
One way to achieve this is to abandon some of the nation's coastal defences and allow the sea to recapture some of the wetlands it has lost.