The point is that the metabolism of plants that grow on land is similar to that of plants that grow in the water.

The plant fertilizers that farmers apply to their crops eventually get carried by the rains into the rivers and streams that run into Lake Erie. Those fertilizers (primarily phosphorous) end up fertilizing the growth of algae in the lake and the result is algae plumes that can poison the drinking water for towns and cities like Toledo.

Ditto for human waste. Untreated sewage that is released into the rivers and streams flowing into Lake Erie acts exactly the same way as cow manure spread onto crop land does. In both cases, the dung acts as a fertilizer to bolster the growth of plants, and it doesn't matter if those plants take root in the ground or float in the water.

It's just another one of those problems that happen as a result of growth in the population and the need to feed the additional people.