Beneath the surface of coastal waters, there are landscapes that resemble forests—not of wood and leaves, but of swaying green blades rooted in the seabed. These are seagrass meadows, often described as “underwater forests,” and across the United Kingdom, they are now under mounting strain from pollution.

A Fragile Ecosystem Seagrass meadows form one of the most important marine habitats on Earth. Though they cover only a small fraction of the ocean floor, they support biodiversity, stabilize sediments, and act as powerful carbon sinks—absorbing and storing significant amounts of carbon dioxide.

In the UK, however, these ecosystems have already suffered dramatic losses. Estimates suggest that more than 90% of historic seagrass coverage has disappeared over the past century, leaving remaining habitats increasingly vulnerable.

Pollution from Land to Sea Recent findings highlight what scientists describe as an “alarming” impact from two major sources: sewage discharge and agricultural runoff. These pollutants carry high levels of nutrients such as nitrogen and phosphorus into coastal waters.

At first glance, nutrients might seem beneficial. But in excess, they trigger a process known as eutrophication—fueling rapid algae growth that blocks sunlight. Without sufficient light, seagrass struggles to photosynthesize, weakening and eventually dying off.

Agriculture contributes through fertilizers and manure washing into rivers and eventually the sea, while sewage systems release nutrient-rich waste directly into coastal environments. Together, they create conditions that overwhelm these delicate ecosystems.

From Carbon Sink to Carbon Source One of the more troubling consequences lies beneath the surface of climate science. Healthy seagrass meadows store carbon efficiently, helping mitigate climate change. But when degraded by pollution, they can reverse this role—releasing stored carbon back into the environment.

In some UK sites, elevated nutrient levels have already been linked to ecosystem stress, fragmentation, and declining capacity to absorb carbon. What was once a natural buffer against climate change risks becoming part of the problem.

A System Under Pressure The issue is not confined to isolated locations. Studies suggest that a significant portion of UK coastal waters and protected areas are affected by wastewater pollution, raising questions about how effectively these environments are being safeguarded.

The challenge, then, is systemic. Pollution does not begin in the sea—it flows from land, through rivers, and into coastal ecosystems, connecting urban infrastructure, agriculture, and marine health in a single chain.

The term “underwater forest” captures both the beauty and the vulnerability of seagrass meadows. Like forests on land, they depend on balance—light, nutrients, stability. When that balance is disrupted, decline can follow quietly, out of sight. What is unfolding in UK waters is not sudden, but cumulative: a gradual erosion of a hidden landscape. And as with many environmental changes, the most important question is not only what has been lost, but how much can still be restored.

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