[TheMercenary]

Originally Posted by King

It's called eutrophication. Too much fertiliser is spread on fields, and when it rains, the nutrients of the fertiliser are washed away (leaching). They are carried into rivers and lakes and promote algal growth. It's usually nitrogen that's the main problem, I think. This means that light can't reach the plants at the bottom of the river/lake, and so they die. Also, there is so much algae growing that it has to compete for space, and so some dies and falls to the bottom. All the dead plant matter means lots of food for bacteria, which thrive, and then take up all the oxygen in the water, and eventually the rest of the life in the water dies, along with animals that rely on it for food. It's a huge ecological problem, and it can take years to redress the balance.

We have two large ponds in our neighborhood. We had a similar problem. We ended up hiring a group called The Pond Doctors who came up and sprayed some EPA approved stuff that killed off the alge and now are ponds are much healthier. The fish are thriving.