How is phosphorus getting into lakes and rivers?
Phosphorus &amp; phosphates are a key component of agricultural and garden fertilizers. Most plants need nitrogen (N), phosphorus (P) and potassium (K). They are generally combined into one product called NPK fertilizers.

Nitrogen is needed for leaf growth. Phosphorus is needed for roots, flowers, seeds and fruit and potassium is needed for stem growth and water movement.

The trouble with the use of fertilizers is they get dumped onto plants periodically and the plants cannot use all of the elements in the fertilizer at that time. What isn't absorbed by plants eventually is leached from soil via rain or irrigation water and it ends up in rivers, streams and sometimes lakes.

In Australia, phosphorus can  be toxic to some native plants because over millennia Australia's, so called, soil, have been depleted of phosphorus and plants evolved to live with either no or very little phosphorus,