Sometimes, when farmers put fertilisers on their fields it can cause problems because when it rains the rain washes substances such as nitrates and phosphates which are in the fertilisers into the rivers and lakes. These nutrients help the plants to grow quicker, and you get an algal bloom, which makes the water look green! The excess plants blocks the sunlight from getting to the plants deeper in the water. That means that they can’t photosynthesise, and less oxygen gets into the water. That means that all the fish and animals in the water don’t have enough oxygen to breathe, and so they can’t live! So it’s very bad for the ecosystem!