Assessing swimming safety in water bodies with visible algae is complex because not all green growth is the same. While most true green algae are harmless and part of the natural ecosystem, a specific group of microscopic organisms poses a serious public health risk. These organisms, often misidentified as “blue-green algae,” can multiply rapidly to form toxic blooms, making the water unsafe for recreation. Understanding the difference between these blooms is crucial for safety.
The Critical Difference Between Algae and Cyanobacteria
The danger in water comes not from true algae, which are complex organisms classified as eukaryotes, but from cyanobacteria, which are actually bacteria. Cyanobacteria are prokaryotes, meaning they lack a nucleus and other complex cell structures. They are frequently mislabeled as blue-green algae because they perform photosynthesis and bloom in water.
True green algae are generally non-toxic, though they can create dense mats. However, certain species of cyanobacteria produce powerful natural poisons known as cyanotoxins. This ability to create toxins is the most important distinction when assessing water safety. Since a toxic cyanobacteria bloom can be visually indistinguishable from a non-toxic bloom, you cannot determine if water is safe just by looking at it.
Specific Health Risks Associated with Toxic Blooms
Exposure to toxic cyanobacterial blooms (CyanoHABs) occurs through three primary routes: swallowing contaminated water, direct skin contact, and inhaling water droplets or aerosols. Ingestion is the most concerning route, as it can lead to severe systemic illness. Children and pets are at greater risk because they are more likely to accidentally swallow water or ingest the bacteria after swimming.
The two most serious types of cyanotoxins are neurotoxins and hepatotoxins, which target the nervous system and the liver. Neurotoxins, such as anatoxin-a, can cause rapid effects like tingling, numbness, dizziness, and muscle tremors. Hepatotoxins, like microcystins, can lead to liver damage, with symptoms that may take hours or days to appear. Both toxin types can cause severe illness.
More common symptoms include gastrointestinal distress such as stomach pain, vomiting, and diarrhea. Direct contact with the bloom material can cause skin and eye irritation, including rashes and hives. Inhaling water spray may also cause respiratory irritation or asthma-like symptoms.
Practical Steps for Identifying and Avoiding Contamination
The most effective way to protect yourself is to avoid water that shows signs of a bloom, following the principle: “When in doubt, stay out.” A harmful algal bloom may look like thick, paint-like material, pea soup, or a dense scum on the surface. The color can vary widely, appearing blue-green, bright green, brown, or red, and it often accumulates along the shoreline.
Some cyanobacteria blooms produce a distinctive odor, often described as earthy, musty, or fishy. However, the absence of a strong odor does not guarantee safety. Always check for public health advisories, which are often posted online by health departments or on warning signs near the water.
If you or a pet accidentally contact water suspected of containing a toxic bloom, rinse off immediately and thoroughly with fresh water. Remove any contaminated clothing. If symptoms like skin irritation persist, or if more serious symptoms like vomiting, diarrhea, or neurological issues appear, seek medical attention promptly.