Larger plants also need nitrogen and may depend on spring runoff and septic systems to recharge the sediments with nitrogen. Growth of Eurasian milfoil has been correlated with such fertilization of the sediment. Because nitrogen fertilizers are often applied along with herbicides and pesticides on lawns, high nitrate levels can indicate other pollutants which are not economical to test in a diagnostic survey. Besides flagging harmful chemicals, high nitrate levels can also point to poor farm management practices, such as improper manure storage, and failing septic systems, which lead to bacterial pollution. Therefore phosphorus is an important element of the food chain within a lake.
Oligotrophic lakes are generally clear, deep and free of weeds or large algae blooms.
Though beautiful, they are low in nutrients and do not support large fish populations, However, oligotrophic lakes often develop a good food chain capable of sustaining a very desirable fishery of large game fish. They are usually either weedy or subject to frequent algae blooms, or both. Eutrophic lakes often support large fish populations, but are also susceptible to oxygen depletion. Small, shallow, eutrophic lakes are especially vulnerable to winterkill which can reduce the number and variety of fish. Rough fish are commonly found in eutrophic lakes.