Impact of Sewage and Agricultural Pollution on UK Underwater Forests
Sewage and agricultural pollution in rivers are causing an "alarming" impact on marine life within underwater seagrass forests along the British coastline, according to recent research.
Scientists discovered that seagrass meadows exposed to excessive nutrients—originating from sewage, fertilisers, manure, and industrial wastewater—host significantly fewer and less diverse small invertebrates such as crabs, shrimps, and snails.
"People don't want to swim in seas polluted by sewage," said Dr Benjamin Jones from Project Seagrass, which conducted the research in collaboration with Swansea University.
"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."
Role and Importance of Seagrass Meadows
Seagrasses are flowering plants inhabiting shallow, sheltered coastal areas, where they form dense underwater meadows. These meadows play a crucial role in combating climate change by absorbing large quantities of carbon dioxide and are regarded as vital habitats.
It is estimated that a single hectare of seagrass meadow can support up to 100 million invertebrates.
"If we think of seagrass meadows as forests, those invertebrates are basically insects that help it function in the marine environment," explained Jones.
Research Methodology and Findings
The research team examined 16 sites along the British coast, each affected to varying degrees by eutrophication—the enrichment of water by nutrients such as nitrogen and phosphorus.
Sources of nutrient enrichment include sewage discharges, runoff of fertilisers and manures from agricultural land, and wastewater from industrial sources, all contributing to elevated nutrient levels in rivers and estuaries.
Excess nutrients stimulate algae growth, which smothers seagrass beds, blocks sunlight, and depletes oxygen levels in the water.
In recent years, these environmental challenges have led to restrictions on housebuilding in some coastal areas and regulations on the spreading and storage of slurry on farmland.
"We picked particular areas to enable us to see a gradient of high nutrients to low nutrients," said Dr Richard Unsworth from Swansea University, noting that the team's findings were "stark."
Higher nitrogen concentrations were consistently linked with reductions in both animal abundance and species richness.
The study found that an increase in nitrogen could correspond to an approximately 90% decrease in the abundance of life per unit of available habitat area.
Elevated phosphorus levels were found to have a particularly devastating negative effect on life within lagoon environments.
Geographical Impact and Specific Sites
Seagrass meadows smothered by algae in the Thames estuary along the Essex coast and the Firth of Forth on Scotland's east coast were among the most severely affected sites.
In Wales, concerns were raised at Skomer Island, internationally known as a wildlife sanctuary, where seabird guano combined with "human impacts" were considered contributing factors.
Conversely, sites with "healthy, clear water" such as the Isles of Scilly off Cornwall and the Orkney Islands exhibited significantly greater marine life.
"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," Unsworth said.
"We want that biodiversity, we want that productivity in our oceans."
Calls for Integrated Environmental Management
Dr Jones emphasized the need for a broader perspective on sewage pollution.
"There is a lot of talk around sewage. 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," he argued.
The research has been published in the Global Ecology and Conservation journal.
