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What Are Harmful Algal Blooms, HABs Explained

A lake that looks perfect in midsummer can suddenly develop a thick, green, paint like surface scum. This is not harmless pond growth. It is the visible sign of a serious and growing problem called a Harmful Algal Bloom, or HAB.

A HAB occurs when a specific organism grows rapidly and accumulates at the lake surface. These blooms can discolor the water, form dense scums, and release toxins that threaten drinking water, wildlife, pets, and human health.

Cyanobacteria, The True Cause of Toxic Blooms

Harmful Algal Blooms are not caused by all algae. In most freshwater lakes, the primary cause is cyanobacteria, often called blue green algae. Despite the name, cyanobacteria are not true algae. They are ancient photosynthetic bacteria with traits that allow them to dominate stressed lakes.

The Three Conditions That Trigger HABs

HABs form when three conditions occur together.

First, high nutrient levels, especially phosphorus and nitrogen. These nutrients enter lakes from fertilizer runoff, wastewater, septic systems, and stormwater. Over time, nutrients also accumulate in bottom sediments. When deep water loses oxygen, these stored nutrients are released back into the water, a process known as internal nutrient loading.

Second, low oxygen in deep water. When algae die and sink, bacteria decompose the organic material and consume oxygen. This creates hypoxic conditions that further accelerate nutrient release from sediments. Low oxygen fuels nutrient release, which fuels new blooms, creating a self reinforcing cycle.

Third, warm and calm water. Cyanobacteria thrive in warm temperatures and stable, stratified lakes where surface and deep water do not mix. These calm conditions allow cyanobacteria to concentrate near the surface.

Buoyancy Control, A Competitive Advantage

Cyanobacteria possess gas filled structures called vesicles that allow them to control buoyancy. They can sink into deeper water to access nutrients released from sediments, then rise back to the surface to capture sunlight. This ability gives them access to both nutrients and light, allowing them to outcompete true algae.

How HABs Become Dangerous

Once triggered, cyanobacteria multiply rapidly at the surface, forming thick scums that resemble spilled paint. Wind and currents push these blooms toward shorelines, concentrating toxins in areas where people, pets, and wildlife are most likely to be exposed.

Cyanobacteria produce toxins called cyanotoxins. Microcystins attack the liver. Anatoxins affect the nervous system. Other toxins damage kidneys or cause paralysis. Surface scums are especially dangerous, as toxin concentrations there can be hundreds or thousands of times higher than in open water.

Exposure can cause skin irritation, illness in pets and livestock, contamination of drinking water, fish kills, and wildlife die offs. Toxins can also become airborne through waves and wind, increasing exposure beyond the shoreline.

Why HABs Are Increasing Worldwide

HABs are becoming more frequent and severe due to several global trends. Nutrient pollution continues to increase from agriculture and urban development. Climate warming raises water temperatures and lengthens bloom seasons. Extreme weather delivers large nutrient pulses during storms and reduces flushing during droughts.

Invasive species also alter food webs. Filter feeding mussels remove beneficial algae while rejecting cyanobacteria, giving toxic blooms an advantage. Longer stratification periods caused by warming temperatures extend the time when blooms and oxygen depletion can occur.

HABs Summary

Harmful Algal Blooms are not simply excess algae. They are the result of cyanobacteria exploiting nutrient pollution, warm calm water, low oxygen conditions, and internal nutrient recycling. These blooms pose serious risks to ecosystems, drinking water, and public health. As climate change and nutrient loading continue, HABs represent one of the most pressing challenges facing freshwater lakes today.