At Charmouth beach in Dorset, a vivid display of bright green gutweed (Ulva intestinalis) near the River Char's mouth serves as a natural indicator of water pollution, according to a recent Country Diary entry. The fast-growing seaweed thrives on nutrients found in sewage and agricultural runoff, mapping out contamination levels along the shoreline.
Gutweed Dominance at River Mouth
At low tide, rocks closest to the river mouth are covered with a rippling mono-pelt of gutweed, also known as grass kelp. This harmless, common seaweed is found around all UK shores and is highly resilient, surviving in exposed places that dry out completely during low tide. Its ability to trap moisture in dense filaments provides a valuable refuge for tiny marine creatures, such as hundreds of copepods—microscopic crustaceans that scavenge on bacteria and algae—which can shelter inside a single translucent frond.
Nutrient Pollution Drives Growth
While gutweed is a useful part of seashore ecology, its growth is artificially boosted by nutrients from sewage and agricultural runoff. As one walks west from the river, the seaweed becomes less dominant. At the flat Blue Lias pavements of Bar Ledges, gutweed intermingles with a brown, rubbery mix of wrack and kelp, tufted with red coral weed—species that do not respond as prolifically to extra nitrogen. Some larger boulders are encrusted with brittle frills of pink plates (Mesophyllum lichenoides), a chalky, coralline species that seems halfway to becoming stone.
Bathing Water Safety
Charmouth beach is always busy, especially with summer and school holidays approaching. The seawater is designated as safe for bathing, but council signs warn of E. coli and advise keeping away from the River Char and its immediate outflow, which is often contaminated. The seaweed distribution offers an organic map of water quality, with cleaner water supporting a more diverse mix of seaweeds to the west.
Geological Perspective
The fixity of the coralline algae echoes the spiral bosses of ammonites in the rock underfoot, a reminder that all these weeds and creatures will one day become another sedimentary layer in Earth's history. The observation highlights the interconnectedness of pollution, ecology, and geology along this dynamic coastline.
