Swimming in water containing common algae is generally harmless, as most aquatic plant life poses no direct threat to human health. Concern arises, however, with the rapid overgrowth, or bloom, of cyanobacteria. These microorganisms are often inaccurately called blue-green algae because of their color and ability to photosynthesize, yet they are structurally bacteria. While not all cyanobacterial blooms are toxic, certain species produce potent natural poisons called cyanotoxins, which are the source of potential health risks. Understanding the difference between benign algae and a harmful cyanobacterial bloom is the first step in ensuring water safety.
Distinguishing Harmful Blue-Green Algae
Common green algae often appear as stringy mats or clumps of filamentous material clinging to objects in the water. In contrast, a harmful cyanobacterial bloom is characterized by a dense accumulation of cells that can make the water look like spilled, thick paint or pea soup. Blooms vary widely in color, ranging from green and blue-green to white, brown, or red. Dense blooms frequently form a noticeable scum layer on the water’s surface or concentrate along the shoreline, often accompanied by an unpleasant odor. Since it is impossible to visually determine if a bloom is producing toxins, any water displaying these characteristics should be avoided for swimming, fishing, or boating.
Health Risks from Toxin Exposure
Exposure to cyanobacteria becomes a health concern when the organisms produce cyanotoxins. These toxins can affect the body through ingestion, skin contact, or inhalation. The potential for illness is directly related to the specific toxins present and the route of exposure.
Gastrointestinal and Liver Effects
The most frequently encountered toxins are hepatotoxins, which target the liver, and neurotoxins, which affect the nervous system. Microcystins and cylindrospermopsin are common hepatotoxins. If ingested by swallowing contaminated water, they can cause acute gastrointestinal distress. Symptoms often begin with nausea, vomiting, diarrhea, and abdominal pain, along with headache or fever. In severe cases, these toxins can lead to liver damage, resulting in acute hepatitis or jaundice.
Neurological Effects
Toxins like anatoxins and saxitoxins are potent neurotoxins that can cause immediate and concerning symptoms, though this is rare during recreational exposure. Swallowing water containing neurotoxins can trigger symptoms like tingling, numbness, a burning sensation, and muscle twitches. Very high doses can lead to neurological effects such as speech disturbances, drowsiness, and progressive muscle paralysis.
Skin and Respiratory Irritation
Physical contact with the bloom can cause localized health issues, as some cyanobacteria produce dermatoxins. Direct skin contact during swimming can result in allergic dermatitis, manifesting as a red, raised rash, itching, or blisters. Eye exposure from splashing water may cause conjunctivitis. As blooms decay, they can release gases or aerosols. Inhaling these can lead to respiratory irritation, sometimes presenting as asthma-like symptoms.
Factors That Encourage Algal Growth
Harmful algal blooms are driven by environmental conditions that allow cyanobacteria to out-compete other aquatic organisms. A major contributing factor is the presence of excess nutrients, specifically nitrogen and phosphorus, in the waterbody. These nutrients often originate from non-point sources, such as agricultural runoff or wastewater discharge. The influx of these compounds provides the fuel needed for the rapid proliferation of cyanobacteria.
Temperature and Stratification
Warm water temperatures accelerate the growth rate of cyanobacteria, making blooms most common during the summer and early autumn. These microorganisms thrive in warmer conditions better than competing algae species, giving them a distinct advantage. Warm water also tends to stratify, preventing mixing, which allows buoyant cyanobacteria to float to the surface and form dense scums.
Water Movement
Water movement plays a significant role in bloom formation and persistence. Harmful algal blooms are more likely to occur and concentrate in stagnant or slow-moving waterbodies. Extended periods of low water flow or drought allow concentrated nutrients and warm water to remain in place, supporting prolonged cyanobacteria growth. Changes in climate, such as intense storms followed by drought, intensify nutrient runoff and create ideal conditions for these blooms.
Actions to Take After Water Contact
If you have accidentally been swimming in water suspected of having a cyanobacterial bloom, immediately rinse off your body thoroughly with clean, fresh water. This removes any residual toxins or cyanobacterial cells clinging to your skin or swimwear. You should also wash any clothing, towels, or equipment that contacted the affected water.
It is equally important to wash any pets that entered the water, as animals are highly susceptible to cyanotoxins and can become severely ill quickly. Rinse them completely with fresh water before they can lick or groom their fur, which could lead to ingesting the toxins. If you or a family member experience symptoms such as a rash, stomach upset, or respiratory irritation after contact, seek medical advice promptly.
To protect the wider community, report the suspected bloom to local environmental or public health authorities. Reporting allows officials to test the water, confirm the presence of harmful cyanobacteria, and issue public health warnings. Always follow any posted warnings or advisories to avoid further exposure to contaminated water.