Too much biodegradable material, though, can cause the serious problem of oxygen depletion in receiving waters. But, oxygen is not very soluble in water. Add in the dead fish, and you see why we don't want large amounts of biodegradable materials entering lakes and streams. The elements phosphorus and nitrogen are necessary for plant growth, and are plentiful in untreated wastwater. This can cause several problems.
Weeds can make a lake unsuitable for swimming and boating.
If the water is used as a drinking water source, algae can clog filters and impart unpleasant tastes and odors to the finished water. Although they may be kept in suspension by turbulence, once in the receiving water, they will eventually settle out and form silt or mud at the bottom. These sediments can decrease the depth of the body of water. If there is a lot of biodegradable organic material in the sediment, it will become anaerobic and contribute to problems mentioned above. Toxic materials can also accumulate in the sediment and affect the organisms which live there and can build up in fish that feed on them, and so be passed up the food chain, causing problems all along the way .