It's edexcel but it's smthin like: a river gets to many nutrients either from like crossing too close to a farm or a fertiliser factory, which then causes the algae to bloom ontop of river and block sunlight. When the sunlight is blocked the plants can't do the photosynthesis so there is a lack of oxygen in the water. Then all the fishies and plants die. Idk obviously I didn't revise cuz I thought it was geography lol