Sources of Nutrients
Generally, the higher the concentration of nutrients in lakes, particularly the limiting nutrients (e.g., typically total phosphorous in freshwater lakes) the worse the water quality. Therefore, efforts to protect and enhance Wolfeboro Waters depend upon identifying and reducing the sources of nutrients to them. These sources typically include:
- Surface water runoff: (largest contributor) Nutrients can enter lakes in runoff in many ways, (e.g., storm water that contains sediment, often containing phosphorous), picks up fertilizer off of lawns, and carries leaves and debris to our waters.
- Point sources: Point sources are ones coming from a well-defined location, such as factory outlets, tributaries flowing into a lake, or treatment plant outfalls.
- Atmospheric: Air pollution can carry some nutrients, such as sulfur and nitrogen oxides, into lakes. Dust can also carry phosphorous into lakes.
- Waterfowl: Excrement from ducks, geese, and other waterfowl, as well food thrown to them can contribute nutrients (as well as “duck itch” organisms) to lakes.
- Septic system leakage: Septic systems that are too close to lake shores, are improperly installed, are poorly maintained, or that fail can contribute nutrients as well as pathogens to lakes. Phosphorous gets bound up in the soil around such septic systems to a greater degree than nitrogen and pathogens, but can be released if a system fails.
- Groundwater: Water needs to be added to our lakes to maintain their levels. Much comes from precipitation directly to the lakes, some comes from surface runoff, and the rest comes via groundwater springs. Of these routes, typically groundwater contains the least amount of nutrients, while surface water runoff contains the most.
- Internal lake: Once nutrients are in a lake, their concentrations in the water can be reduced in several ways. For example, they may precipitate as insoluble compounds in the sediment, they can be carried away by flowing water, or, like nitrogen, converted from soluble ammonia or nitrates biologically into nitrogen gas that escapes into the air. On the other hand, nutrients in the sediment may build up over time and be released to the water under certain conditions. In particular, insoluble phosphorous in lake bottom sediments may be converted into a soluble form under low-oxygen conditions, such as can occur in stratified deep waters. So, internal nutrients in lakes can become significant sources.