Algae are microscopic organisms that form the base of most aquatic food webs and are a natural component of aquatic environments. Under certain conditions, their growth can accelerate uncontrollably, leading to dense concentrations known as an algal bloom. When this excessive growth involves species that produce toxins or creates conditions harmful to human health and the environment, it is designated a Harmful Algal Bloom (HAB). This proliferation becomes a public safety issue in lakes, rivers, and coastal waters.
Distinguishing Harmful Algal Blooms
Not all visible algae are harmful, but the distinction between a harmless bloom and a dangerous one is often impossible to make visually. The most common organisms responsible for HABs in freshwater are cyanobacteria, often called blue-green algae. These blooms frequently appear as thick, vibrant green, blue-green, or reddish-brown surface scums.
A toxic bloom may resemble spilled paint or pea soup. These scums tend to congregate along shorelines due to wind action, posing an increased risk to people and pets. The bloom can also cause a strong, unpleasant odor.
While toxin production is the primary concern, a bloom can be harmful simply due to its sheer mass. When large quantities of algae die, the decomposition process rapidly consumes dissolved oxygen. This depletion creates hypoxic zones, which suffocates fish and other aquatic life. Some HABs are not visible at the surface, growing deeper in the water column, making visual detection unreliable.
Quantifying “Too Much” Algae
Determining “how much” algae constitutes a threat relies on thresholds measured using three primary metrics: cell counts, toxin concentrations, and chlorophyll levels. For recreational waters, the U.S. Environmental Protection Agency (EPA) recommends issuing a swimming advisory if the concentration of the common cyanotoxin microcystin reaches \(8\) micrograms per liter (\(\mu g/L\)), which is equal to \(8\) parts per billion (ppb).
Many state and international agencies also use the density of cyanobacteria cells to trigger advisories. Public warnings are often recommended when the total cell count of cyanobacteria exceeds \(20,000\) cells per milliliter of water. More stringent advisories are set for specific, highly toxic genera, like Microcystis and Planktothrix, at thresholds such as \(40,000\) cells/mL.
Monitoring agencies also use chlorophyll-a concentration as an indicator. High chlorophyll-a levels are a proxy for high algal biomass. The World Health Organization (WHO) proposes risk levels based on chlorophyll-a, suggesting a low probability of acute health effects below \(10 \mu g/L\) and a moderate probability in the range of \(10\) to \(50 \mu g/L\).
Environmental Triggers for Overgrowth
The transition from a natural algal presence to a harmful bloom is primarily driven by three environmental factors: nutrient loading, warm temperatures, and water stagnation. Excessive nutrient loading, specifically an overabundance of nitrogen and phosphorus, provides the fuel for rapid algal proliferation. These nutrients enter waterways primarily through nonpoint sources like agricultural runoff, which carries chemical fertilizers and animal manure from farmlands.
Municipal and urban sources also contribute to nutrient pollution. Wastewater treatment plants may not fully remove all nutrients, and their effluent acts as a consistent point source. Stormwater runoff from urban areas carries pet waste and excess lawn fertilizers directly into surface waters.
Warm water temperatures provide optimal conditions for cyanobacteria to outcompete other algae. Most cyanobacteria species thrive at temperatures above \(20\) degrees Celsius and demonstrate faster growth rates than other phytoplankton. Warmer surface water increases thermal stratification, where a warm layer sits stably atop a cooler layer, which benefits buoyant cyanobacteria. This stratification allows them to regulate their position in the water column, staying near the surface to maximize light exposure while accessing nutrients from deeper layers.
Safe Practices During Algal Events
Avoid any water that appears discolored or has visible scums, mats, or foam. If a bloom is suspected, individuals should not swim, water ski, or allow pets to enter the affected area. Pets are highly susceptible to cyanotoxins because they may ingest the water or lick concentrated algae off their fur after swimming.
If contact with a suspected bloom occurs, immediately rinse off thoroughly with clean, fresh water. This action helps prevent skin irritation and reduces the chance of ingesting toxins. Fish caught in bloom-affected waters should be cleaned of all internal organs, which can accumulate high levels of toxins.
Reporting a suspected bloom to local environmental or health agencies is important. Many areas have dedicated hotlines or online reporting tools, such as the EPA’s BloomWatch app. Individuals can help prevent future blooms by reducing nutrient runoff from their property through simple actions like using phosphate-free detergents, maintaining septic systems, and limiting fertilizer use on lawns.