Everything and everyone ages…plants, animals, earth, humans, and even lakes. Like humans, some lakes age faster than others depending on how long and to what extent they have been abused.
The aging of a lake is a natural process known as eutrophication. It involves increased aquatic weed and algae growth. The rate at which a lake receives nutrients
and sediments from its watershed determines the rate of eutrophication. Under natural conditions eutrophication occurs over a long period. A mountain lake in a wilderness area, for example, lives a very long time. However, in the case of Lake Mamanasco, which is an urban lake in a developed region, the aging process speeds up considerably due to increased nutrients and sediments entering the lake from its watershed. When eutrophication is accelerated by man-made conditions it is known as cultural eutrophication.
According to a 1990 study, Lake Mamanasco is in a state of advanced cultural eutrophication due to nutrients entering the lake from its mostly residential 537-acre watershed. These nutrients are the cause of the excessive aquatic weed growth which contribute to its premature aging and, at times, make it unfit for recreational activity. The limiting element, or the element in the shortest supply and, therefore, easiest to control, is phosphorus. Studies have shown that this element is coming from both stormwater (50%) and groundwater sources (25%). In order to slow the aging process, the amount of phosphorus entering the lake annually from the watershed must be reduced to within acceptable limits.
The phosphorus entering the lake via stormwater runoff is from oil and gas, road sand, salt and lawn fertilizers. New construction and poorly or unpaved roads can result in erosion and the movement of sand, silt and organic matter into the lake. These conditions all exist in the Mamanasco watershed.
Studies have also shown that failing septic systems in the Mamanasco watershed are a major source of the pollutants entering the lake. These failures are of two types:
Outright failure is the surfacing of effluence on property. If the property is in the immediate area of the lake, or discharging into a tributary that flows into the lake, this sewage can cause enteroccocal bacteria colonies to form, which are a health hazard and unsafe for swimming conditions. These types of failures, when detected, are in violation of local health laws which require that the failed system be brought into conformance with local codes. Unfortunately, there are no routine surveys of properties in the area being done by the local health department. So, such violations could go, and have gone, undetected for long periods of time which are a contributing factor to the condition of the lake.
The second type of problem is resource overloading. This occurs when the density of housing units and the condition of the soils are such that the nutrients emanating from the septic systems are not properly filtered and result in contaminated groundwater that eventually seeps into the lake causing excessive weed growth. In the Mamanasco watershed there are over 60 dwellings placed on land areas as small as .09 acres, with typical lots running roughly .25 acres. (A minimum lot size of .5 acres is required to insure adequate filtering of nutrients from a well designed septic system.) Tests have shown that 25% of the phosphorus, the limiting element contributing to excessive weed growth, enters Mamanasco through ground water seepage emanating from failing septic systems.
For many years Lake Mamanasco was plagued by heavy growth of native weeds which were controlled by harvesting. However, in 1995, the lake was invaded by the weed Eurasian Watermilfoil, which is an introduced species from eastern Europe and Asia.
It was unknown in Lake Mamanasco prior to 1995, but it is an ongoing problem now and was probably transported into the lake by contaminated boats coming into Lake Mamanasco from other lakes. It is very aggressive, often growing at the rate of up to two feet per week. In the winter, it collapses and lies dormant on the bottom allowing it to get an early start in the spring. It produces a canopy that blocks the sun from the native weeds, thereby killing them. Once milfoil is introduced into a lake, it can never be completely eradicated, only controlled by the ongoing use of chemicals.
Eurasian Watermilfoil (myriophyllum spicatum)
Ninety-five percent of Lake Mamanasco was covered by Eurasian milfoil in 1995. It produces flowery spikes