Lake Characterization and Trophic Status
All lakes undergo a natural "aging" process called eutrophication. Lakes formed from glaciers are often very deep, with cold water, low levels of nutrients (phosphorus and nitrogen), and low biological productivity (relatively little aquatic life). As a lake "ages", nutrient accumulation in the lake water and bottom sediments leads to greater biological productivity. Greater biological productivity leads to increased deposition of organic matter, which, combined with sediments that wash in during rain storms or snowmelt, cause the lake to become shallower and water temperatures to rise. In a natural setting this process occurs very slowly with little or no apparent change over the course of a human lifetime.
Human activities in a watershed often greatly accelerate the natural aging process of lakes by contributing additional nutrients and sediment. The acceleration of the process is called 'cultural eutrophication'. When excessive nutrients are added to a lake, aquatic plants and algae thrive, resulting in excessive growth and large blooms. Excessive plant growth and large algae blooms can make swimming undesirable, make boating difficult and lead to the formation of 'dead zones'. Dead zones are areas in a water body devoid of aquatic life due to dissolved oxygen depletion. Although aquatic plants and algae contribute oxygen to the ecosystem during day-time photosynthetic activities, they consume oxygen during the night while respiring, which (with excessive plant growth) can result in oxygen depletion. Increased sediment contributions from erosion, construction activities, and other human activity also accelerate the aging process.
Data collected by volunteers in the Volunteer Lake Monitoring program are used by Watershed Council staff to determine the current level of productivity or the "trophic status" of a lake. Lakes are classified according to their trophic status, which ranges from oligotrophic (low productivity) to eutrophic (high productivity). In general, oligotrophic lakes are considered to have high water-quality, mesotrophic lakes have moderate water quality and eutrophic lakes have poor water quality. However, keep in mind that eutrophic lakes occur naturally during the aging process and do not necessarily reflect poor water quality.