Eutrophication is the process of increased nutrient input to a lake over the natural supply. This increased lake fertilization usually results in an increase in the biological production that occurs in the lake. Since eutrophication is increased nutrient input, any activity in the watershed of a lake that increases nutrient input causes eutrophication. Land use changes can result in significant changes in nutrient runoff. Activities that often contribute to eutrophication are agriculture, lawn and garden fertilizers, faulty septic systems, washing in or near the lake, erosion into the lake, dumping or burning leaves in or near a lake, and feeding ducks (NHDES, 1995, http://www.des.state.nh.us/factsheets/bb/bb-3.htm). 

Trophic state not only refers to the nutrient status of the water, but also to the biological production that occurs in the water and to the morphological characteristics of the lake basin itself. Lakes are divided into three trophic categories: oligotrophic, mesotrophic, and eutrophic. An oligotrophic lake is typically a large deep lake with crystal clear waters and a rocky or sandy shoreline.  Both planktonic and rooted plant growth are sparse, and the lake can support a coldwater fishery. A eutrophic lake, on the other hand, is typically shallow with a soft, mucky bottom. Rooted plant growth is abundant along the shores and out into the lake, and algal blooms are not unusual. Water clarity is not good and the water often has a tea color. If deep enough to thermally stratify, the bottom waters are devoid of oxygen. Mesotrophic is an intermediate trophic state with characteristics between the other two (NHDES, 1995, http://www.des.state.nh.us/factsheets/bb/bb-3.htm). 

Lakes generally change trophic state very slowly, gradually becoming more eutrophic over time, where time is measured in thousands of years. This process is called natural eutrophication. Often the process is greatly accelerated due to human activity (called cultural eutrophication). Cultural eutrophication can be controlled by managing human activity within the watershed and on the lake (NHDES, 1997, http://www.des.state.nh.us/factsheets/bb/bb-27.htm).