Sewage discharges, fertiliser runoff, manure and industry wastewater are driving an "alarming" knock-on impact on marine life in underwater forests along the British coastline, according to new research. The damage is not abstract: scientists found seagrass meadows exposed to excessive nutrients had far fewer and less variety of small invertebrates like crabs, shrimps and snails, showing how pollution pushed from land is reshaping life in the sea.
Who Pays for the Mess
Dr Benjamin Jones from Project Seagrass, which carried out the research with Swansea University, said: "People don't want to swim in seas polluted by sewage. But this is one of the first studies of its kind to show that some of those nutrient inputs... are having an impact on the animals too." That is the basic hierarchy on display here: waste from sewage systems, farms and industry is dumped into rivers and estuaries, and the costs are carried by coastal ecosystems and the creatures that depend on them.
Seagrasses are flowering plants that live in shallow, sheltered areas of the coast, forming dense underwater meadows. They help fight climate change by absorbing vast amounts of carbon dioxide and are considered incredibly important habitats. It is estimated that a single hectare can harbour as many as 100 million invertebrates. Jones said: "If we think of seagrass meadows as forests, those invertebrates are basically insects that help it function in the marine environment." When the water is fouled, the whole living structure starts to unravel.
What the Research Found
The researchers examined 16 different sites along the British coast which were affected to varying degrees by eutrophication, the enrichment of water by nutrients like nitrogen and phosphorus. Sewage discharges, runoff of fertilisers and manures from farms and wastewater from industry can all increase nutrient levels in rivers and estuaries. Extra nutrients lead to the growth of algae, smothering seagrass beds, blocking sunlight and depleting the water of oxygen.
Dr Richard Unsworth from Swansea University said the team picked particular areas to enable them to see a gradient of high nutrients to low nutrients, and that the findings were "stark". Higher concentrations of nitrogen were consistently associated with reductions in animal abundance and species richness. The research found "an increase of nitrogen could correspond to an approximately 90% decrease in the abundance of life per unit of available habitat area". Increased levels of phosphorus were shown to be having "a devastating negative effect on life within lagoon environments" in particular.
Algae-smothered seagrass meadows in the Thames estuary along the Essex coast and the Firth of Forth on the east coast of Scotland were among those worst affected. In Wales, issues were also flagged at Skomer Island, internationally renowned as a wildlife haven, though seabird poo alongside "human impacts" were thought to be part of the picture there. At sites with "healthy, clear water" like the Isles of Scilly off Cornwall and the Orkney Islands, "we were finding much more life," Unsworth said.
What They Call Management
In recent years the situation has led to limits on housebuilding in some coastal areas and restrictions on spreading and storing slurry on farmland. Those are the kinds of top-down fixes that arrive after the damage is already visible, while the same systems that produce the pollution keep operating.
Unsworth said: "I think it's quite alarming that all this riverine input in terms of sewage, in terms of poor fertiliser use... is all coming out onto our coasts and influencing the amount of food available for fish, the amount of food for birds. We want that biodiversity, we want that productivity in our oceans." Jones said there was a "lot of talk around sewage." He added: "If we want to protect the marine environment we need to look towards the land and there needs to be some integrated thinking - that's a conversation that's very rarely had."
The research is published in the Global Ecology and Conservation journal. The facts in the study point to a simple chain of command and consequence: pollution enters rivers from sewage, farms and industry, and the coastal life at the bottom of the pile absorbs the damage.