Lakes often appear green, a common sight that sparks curiosity about its cause. This visual transformation indicates significant biological activity within the ecosystem. While sometimes aesthetically pleasing, the green color can also signal environmental shifts impacting the lake’s health.
Algae as the Primary Cause
The most frequent reason for a lake’s green appearance is the abundant presence of microscopic organisms known as algae. Algae are simple, plant-like organisms that thrive in aquatic environments. Like terrestrial plants, algae perform photosynthesis, converting sunlight into energy. This process relies on chlorophyll, a green pigment that captures light, which is why large concentrations of algae impart a green color to the water.
There are various types of algae that contribute to this green tint. Common green algae are eukaryotic organisms. In contrast, cyanobacteria, often referred to as blue-green algae, are prokaryotic bacteria. Despite this biological difference, both green algae and cyanobacteria utilize photosynthesis and can proliferate to levels that visibly color a lake green or even blue-green. Cyanobacteria are naturally present in most water bodies but can multiply rapidly under specific conditions.
The Role of Nutrient Enrichment
Algae and cyanobacteria flourish with an ample supply of nutrients in the water. The primary nutrients that fuel their excessive growth are phosphorus and nitrogen. These elements are naturally occurring in aquatic ecosystems, but human activities frequently introduce them in excessive quantities, leading to a process called eutrophication. Eutrophication describes the enrichment of a water body with nutrients, resulting in an overgrowth of plants and algae.
Sources of this excess nutrient runoff are diverse, often originating from land-based activities. Agricultural fertilizers, rich in nitrogen and phosphorus, wash into waterways during rain events. Wastewater discharge, including improperly treated sewage and effluent from septic systems, also contributes significant amounts of these nutrients. Additionally, stormwater runoff from urban areas carries pollutants like pet waste and lawn fertilizers into lakes and rivers. This influx creates an imbalance, providing an abundant food source that allows algal populations to expand rapidly, turning the water green.
Understanding Harmful Algal Blooms
While some lake greening is harmless, certain types of algal growth, particularly dense cyanobacteria blooms, are problematic and are termed Harmful Algal Blooms (HABs). These blooms are not merely an aesthetic concern; they produce toxins, known as cyanotoxins, which pose risks to humans, pets, and wildlife. Exposure to these toxins, whether through ingestion, skin contact, or inhalation, can lead to health issues like gastrointestinal illness, liver damage, and neurological symptoms. Dogs and livestock are particularly vulnerable to severe reactions, including death, from drinking contaminated water.
Beyond toxin production, dense algal blooms severely impact the aquatic ecosystem. When large quantities of algae die, bacteria decompose the organic matter, consuming significant dissolved oxygen. This oxygen depletion creates low oxygen conditions, making it difficult for fish and other aquatic organisms to survive, leading to widespread fish kills. Furthermore, thick surface mats of algae block sunlight from reaching submerged aquatic plants, inhibiting their growth and disrupting the entire food web.