Keyingham Drain is entirely green on some days.

The algal bloom is a problem in many lakes and rivers. In the wrong weather conditions, a body once teeming with life can quickly become an water graveyard if certain organisms cannot be controlled.

An algal bloom is a rapid increase in the population of algae in an aquatic system. There is no fixed benchmark for when an algal growth becomes a bloom – some say the concentration should be in the hundreds of cells per millilitre, some say it should be in the thousands. A bloom occurs when a body of shallow, slow-moving water has an excess of phosphorus and nitrogen nutrients, usually caused by fertiliser leakage or waste-water. This leads to green plants and algae growing at an increased rate at the expense of other organisms. In particular the algae can clump together to form a gelatinous blanket on the surface of the water, which blocks out the light of the sun. The then-permanent darkness means that the plants beneath the surface can no longer photosynthesise, with the inevitable result that they perish. Their corpses are devoured by decomposers. The sudden abundance of food allows these organisms to grow and multiply rapidly, and they consume the oxygen in the water which – in the absence of photosynthesising plants – cannot be replaced. Once the oxygen is exhausted the fish and aquatic insects within the water body die off and the internal ecosystem collapses. Beneath the garish top layer, the water is devoid of life.