Charmouth beach is always busy. Even on grey and stormy winter days, walkers and their dogs patrol the hissing waves, and fossil hunters pick over rubble newly fallen from the black cliffs.
With summer here and school holidays approaching, the sands are strewn with visitors and the car park packed with glittering windscreens. It’s a lovely place to swim, as long as you heed the council signs warning of E coli and keep away from the River Char and its immediate outflow, which is often contaminated.
While the seawater is designated as safe for bathing, it also becomes cleaner as it progresses west, a process that is mapped out organically on the rocks that appear at low tide. Those near the river mouth are covered with a rippling mono-pelt of bright-green gutweed (Ulva intestinalis).
Also known as grass kelp because of its colour, this harmless common seaweed is found all around UK shores. It’s a fast-growing summer plant that dies off at the end of the season, leaving bleached and deflated white hanks that disintegrate as the autumn progresses. Highly resilient, in its prime it can survive in exposed places that dry out completely every low tide.
Its ability to trap moisture in its dense filaments means that it provides a valuable refuge for tiny marine creatures that would otherwise perish. Hundreds of copepods – microscopic crustaceans that scavenge on bacteria and algae – can shelter in the hollow space inside a single translucent frond.
Gutweed is a useful part of seashore ecology, but its growth can be boosted artificially by nutrients found in sewage and agricultural runoff. Walk away from the river and it becomes less dominant. By the time you reach the flat Blue Lias pavements of Bar Ledges, it intermingles with a brown, rubbery mix of wrack and kelp, tufted with red coral weed, none of which respond so prolifically to extra nitrogen.
Some of the larger boulders are encrusted with brittle frills of pink plates (Mesophyllum lichenoides), a chalky, coralline species that seems halfway to becoming stone. Their fixity echoes the spiral bosses of ammonites in the rock underfoot, a reminder that one day all these weeds and creatures will be another sedimentary layer in Earth’s history.
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024, is available now at guardianbookshop.com
