Norris Lake, a large reservoir managed by the Tennessee Valley Authority (TVA) in East Tennessee, is known for its deep, clear waters. Visitors often notice the striking green hue, which prompts questions about the lake’s health. The color is a natural result of materials suspended in the water, a phenomenon shared by many freshwater bodies. This characteristic color involves microscopic organisms and the environmental factors that encourage their growth.
The Biological Agents Causing the Color
The green color is caused by countless microscopic organisms called phytoplankton. These floating, plant-like microbes contain the pigment chlorophyll, the same compound that makes land plants appear green. Chlorophyll absorbs red and blue light, reflecting the green wavelengths back to the observer, creating the lake’s distinctive tint.
Among the various types of phytoplankton, cyanobacteria (often called blue-green algae) frequently cause the most intense green coloration. These bacteria rapidly multiply under favorable conditions, forming dense accumulations known as blooms. While Norris Lake maintains a healthy ecological balance, any noticeable increase in the green shade suggests an elevated population of these organisms.
The Role of Nutrient Pollution
The rapid growth of biological agents is driven by an overabundance of chemical elements that act as their food source. These growth-promoting elements are primarily Phosphorus (P) and Nitrogen (N), which enter the lake through nutrient pollution. This process of excessive nutrient enrichment, which accelerates biological production, is known as eutrophication.
The Norris Lake watershed, like many others, receives these nutrients from non-point sources across a broad land area. Stormwater runoff from developed areas carries fertilizers from lawns and sediment from construction sites directly into tributaries. Failing or inadequate septic systems around the shoreline can leak household wastewater high in nitrogen and phosphorus into the groundwater that feeds the lake. Agricultural and livestock operations in the surrounding counties also contribute, as runoff carries animal waste and commercial fertilizers into the river systems feeding the reservoir. The Tennessee Valley Authority monitors indicate that Norris Reservoir’s overall ecological health rating is “fair,” suggesting that nutrient loading is a concern that affects the system’s balance.
Health and Environmental Implications
When nutrient pollution fuels the explosive growth of cyanobacteria, the resulting dense formations are categorized as Harmful Algal Blooms (HABs). Certain strains within these blooms produce potent natural poisons known as cyanotoxins, which pose risks to human and animal health. Exposure to water containing these toxins, through swimming or accidental ingestion, may cause skin irritation, gastrointestinal distress, vomiting, and diarrhea.
Pets and wildlife are particularly susceptible to severe illness or even death if they drink water concentrated with these toxins, such as from scum layers near the shore. Beyond the direct threat of toxins, large blooms have secondary impacts on the lake ecosystem. When the massive population of phytoplankton dies, the decomposition process consumes vast amounts of dissolved oxygen in the deeper layers of the reservoir. This leads to a condition called hypoxia, or low dissolved oxygen, which the TVA has identified as the most significant ecological issue in Norris Lake’s deep water during summer and autumn, threatening fish and other aquatic life.