Country diary: A story of water pollution, told in seaweed

Charmouth beach in Dorset is a popular place. Even when the weather is bad, people and their dogs walk along the shore. Those looking for fossils also search the rocks that fall from the tall cliffs.

With summer starting and school holidays near, the beach is full of visitors. The car park is also very busy. It's a nice place to swim, but the local council warns about E. coli bacteria. People are advised to stay away from the River Char and where it enters the sea, as this area is often dirty.

The seawater is considered safe for swimming. However, it gets cleaner the further west you go along the coast. This change can be seen on the rocks that appear when the tide goes out. Near the river mouth, the rocks are covered in a bright green seaweed called gutweed.

Gutweed, also known as grass kelp because of its colour, is a common seaweed found on UK beaches. It grows quickly in the summer and dies off at the end of the season. This seaweed is very strong and can survive in places that become completely dry at low tide. Its thick strands hold moisture, which helps protect tiny sea creatures from drying out.

This seaweed is an important part of the environment on the shore. However, its growth can increase unnaturally when there are extra nutrients in the water. These nutrients often come from sewage and farm waste. If you walk away from the river, the gutweed becomes less common. Closer to the area called Bar Ledges, it mixes with other types of seaweed like wrack and kelp, and also red coral weed. These other seaweeds do not grow as well with extra nutrients.

Some of the larger rocks have a chalky, coral-like seaweed growing on them. This seaweed looks a bit like stone. It reminds us that in the future, all these plants and creatures will become part of the Earth's history, like the ancient ammonite fossils found in the rocks.