Erosion of the lake shore will deposit mud, sand or rubble on the lake bottom. Silt, in the form of muddy runoff, clouds the water and limits the amount of sunlight reaching the plants and algae. Without sufficient sunlight, photosynthesis is reduced, resulting in impaired growth.
Septic fields and sewage from cottages, homes and commercial buildings can disrupt the ecologic balance of a lake.
Decomposition of plants and algae consumes oxygen. If more oxygen is consumed than is produced, organisms, including fish, may not survive. Sensitive species such as trout may die off completely if oxygen is depleted for even a short period of time. Low levels can be critical in summer or winter, when little oxygen enters the water through the ice and plants that are still growing are not producing much oxygen.
Land Development
Land development can increase siltation, cause chemical runoff or leach freshly exposed soils. Land clearing can remove the protective surface cover of plants and increase the flow of water and eroded materials into the lake. Surface and groundwater may also be more exposed to the sun and become warmer, disrupting productivity in small lakes.
Homes, cottages and commercial buildings can disrupt the ecological balance of a lake through the release of sewage from septic fields or through runoff from fertilized lawns and gardens.
Wave-deflecting structures, such as rock piers, dikes or cribs, can change wave and current patterns. When poorly placed, they may divert wave or current energy to other parts of the lake and erode the shoreline. Wave deflecting structures can change wave patterns and cause shoreline erosion.
Agricultural Activity
Agricultural activity can release high levels of fertilizers and pesticides into a lake. Runoff from manure piles and silage pits may also make its way into a lake. Herbicides commonly used in farming operations can suppress production and growth of many aquatic plants and organisms. High levels of nitrogen and phosphorus, found in fertilizers, can cause massive blooms of plankton and aquatic plants. As these organisms die and decompose, they use oxygen in the water, reducing the oxygen available for fish and plants.
Industrial Activity
Industrial activity upstream from or on a lake may discharge toxic chemicals that can harm or kill plants and animals outright. Certain chemicals like heavy metals, and organic compounds such as PCBs, may concentrate in plant and animal tissues (bio-accumulation). High levels of some of these compounds could be hazardous to humans. Toxic chemicals from industrial activity can harm fish and plants in a lake.
Introduction of Non-native Species to a Lake
Introducing non-native (nonindigenous) fish or plants can seriously reduce populations of native species. If a newly introduced species is not controlled naturally through predation and competition, its increasing population may reduce the number of fish native to the lake. Experience shows that mixing species from different areas can also spread disease. Canal construction or the diversion of water can cause non-native species to enter a watershed.