The causes of over fertilization are relatively easy to identify. They are almost exclusively due to human activity. Waste water is top on the list because, if left untreated, it contains enough phosphates to rapidly over fertilize a lake. The second villain on the list is erosion. Deforestation, degradation of shoreline, agriculture, pasture, removal of topsoil, etc. are all activities that fertilize the soil and make it vulnerable to erosion.
Erosion impoverishes the soil while enriching the lake.
This list is far from complete. Nevertheless, it is essential to be able to identify the sources of phosphates due to human activity because they are the only ones that we can control. The control methods vary depending on the source of the phosphates. Agriculture, pasture and lawns are sources that are difficult to control. In certain cases, phosphate control would require profound changes in agricultural practices and personal life styles.