 Algal blooms like this occur for a variety of really complex overlapping reasons, which Simon lays out for me. Upland soils have been compacted by sheep hooves. There's unseasonal flooding. There's the death of a keystone species of aquatic plant called ranunculus. But for the sake of time, I'll keep it simple. The river is suffering because there's just too much phosphorus in it. Phosphorus that's come from poo. Human poo, sheep poo and chicken poo.