Lake Michigan’s appearance can sometimes shift to a greenish hue, a sight that often prompts questions about the lake’s health. This phenomenon is a common occurrence in many freshwater bodies, and understanding its various causes provides insight into the complex dynamics of the Great Lakes ecosystem.
What Makes Lake Michigan Appear Green?
The primary reason Lake Michigan sometimes looks green is the presence and growth of microscopic organisms, specifically algae and cyanobacteria. These organisms contain chlorophyll, the same pigment found in plants, which gives them their green coloration. Their rapid growth often leads to visible “blooms” in the water.
These blooms are frequently fueled by excess nutrients, particularly phosphorus and nitrogen, entering the lake. Sources of these nutrients include agricultural runoff, wastewater discharges, and stormwater runoff. Warm water temperatures, ample sunlight, and calm water conditions provide an ideal environment for these microscopic organisms to multiply rapidly. Invasive species, such as zebra and quagga mussels, also play a role; these mussels filter the water, increasing its clarity and allowing more sunlight to penetrate deeper, which can fuel algal growth or alter the types of algae that thrive.
Different Types of Green Water
Not all green water in Lake Michigan signifies the same condition, as various factors can contribute to its hue. A certain level of greenness from natural phytoplankton is typical and indicates a healthy aquatic ecosystem, forming the base of the food web. These natural green algae grow on submerged surfaces and do not produce toxins.
However, intense green appearances can stem from harmful algal blooms (HABs), which are often caused by cyanobacteria. These blooms can appear as a scummy film, foam, or even resemble spilled paint on the water’s surface, sometimes accompanied by an earthy or musty smell. Unlike benign green algae, cyanobacteria can produce harmful toxins. In addition to algal growth, green-brown or murky green water can also be attributed to suspended sediments, especially after strong winds or storms churn the lakebed and bring silt to the surface.
Implications of Green Water
Extensive greening, particularly from harmful algal blooms, can have several negative consequences for Lake Michigan and its users. Ecologically, dense algal blooms can deplete oxygen levels in the water when they decompose, creating “dead zones” that harm fish and other aquatic life. These blooms can also block sunlight, negatively affecting submerged aquatic vegetation that relies on light for photosynthesis.
From a human health perspective, contact with or ingestion of water affected by harmful algal blooms can pose risks. Symptoms can include skin irritation, rashes, respiratory issues, and gastrointestinal problems. Some toxins produced by cyanobacteria can affect the liver or nervous system. Public health advisories often recommend avoiding swimming or allowing pets into visibly green, scummy, or discolored water. Beyond health, widespread greening can negatively impact recreational activities like swimming, boating, and fishing, leading to economic losses for local communities dependent on lake tourism.