Seagrass meadow

Seagrass: The ocean superhero at risk from sewage

2 minute read

Seagrass meadows are a key player in helping to combat climate change – but untreated sewage pollution in our seas is threatening their future.

Seagrass meadows are the Swiss army knife of marine habitats. They create hotspots for biodiversity and provide vital nursery habitats for various fish species.

Long seagrass blades buffer wave energy, protecting our shores against coastal erosion and storms. Their canopies slow the flow of water, drawing down suspended matter like pollutants and excess nutrients from the water column and burying it in the sediment below.

This also makes them one of the oldest and most effective carbon storage technologies, accounting for an estimated 10-18% of ocean carbon storage while occupying only 0.1% of the seafloor.

Unlike terrestrial habitats like forests, seagrass doesn't release the carbon it has captured back into the atmosphere when it decomposes. If undisturbed, seagrass can store carbon for thousands of years.

Sand eels in seagrass meadow

Credit: Heather Hamilton

Seagrasses do a lot of heavy lifting in mitigating the stress that we inflict on the ocean. As ecosystem engineers, they’re skilled at adapting their environment to suit their needs. However, the flow of untreated sewage discharges into UK seas is posing a problem for seagrass.

Untreated sewage discharges contain excess nutrients and pathogens, which  encourage faster-growing macroalgae which reduce light availability and epiphytic algae which smother the seagrass leaves.

Seagrass underwater Bembridge

Credit: Theo Vickers

Research by Cardiff University and Swansea University indicates that insufficient monitoring and management of sewage and wastewater treatment threatens seagrass meadows around the UK.

Each of the 11 sites sampled in the study, ten of which were within marine protected areas, contained seagrass that was contaminated by nutrients “of a human and livestock waste origin”.

The findings show that sewage pollution is a stressor to seagrass – one whose effects are far-reaching and continues to have an impact far from its source.

The only effective way to protect seagrass and the whole marine environment from this stress is to tackle the issue at source.

Seagrass Isle Of Wight

Credit: Laura McConnell

We have already lost 92% of seagrass meadows in the UK, and their survival and recovery is further undermined by poor water quality. However, we can reverse this trend.

Removing stressors, such as untreated sewage pollution, is the most important factor in allowing seagrass to recover and we have seen seagrass successfully recolonise areas which were previously wiped out by sewage outfall.

Our seagrass meadows are an essential ally against global warming, a biodiversity crisis, and pervasive pollution. These superhero habitats need our help and a first major step towards this is to stop releasing untreated sewage into our seas.

Learn more about our work to achieve sewage-free seas

Sewage pollution