Someone in the comments explained it really well, but plants need nitrogen and phosphorus to grow. Usually those are in limited quantities, so growth is limited by the amount available. Fertilizer is designed to provide an abundance of nitrogen and phosphorus to plants, so if those high quantities end up in the water, algae growth isn’t limited and can grow more rapidly.

I’m in college right now to (hopefully) double major in political science and environmental studies, so that’s exactly what I want to be doing in the future!

I’m not sure if there is some animal that can “fix” it, but I know one of the biggest problems is that the lack of oxygen kills fish. If there’s an abundance of food, the fish will reproduce at rates that the oxygen in the water can’t support. It’s a good idea though!

Oh! I can actually answer this!

Fertilizers have a lot of nitrogen and phosphorus. When they enter a waterway, that causes algae to grow really rapidly and form “algae blooms” that cover the surface of the water. This blocks sunlight and makes it harder for plants to photosynthesize, which reduces the amount of oxygen in the water. As the plants and algae die, they’re also eaten by bacteria that require oxygen which limits the amount of oxygen in the water even further. The whole process is called eutrophication.

EDIT: Thanks for the awards!! This is my first comment that’s gotten more than like ten upvotes, how fitting for it to be about something nerdy.