Most algae living in the surface waters of oceans and lakes are pretty tame. But some algae produce nasty toxins. This becomes a bigger problem when runoff carries nutrients like nitrogen and phosphorus into waterways and triggers population explosions known as blooms. Harmful algal blooms can poison shellfish and contaminate regional drinking water. Swimmers can inhale algal toxins through water vapor, swallow them or absorb them through the skin.
In freshwater, the biggest concern is blue-green algae, or cyanobacteria, which likely caused 11 of the disease outbreaks reported to the CDC between 2009 and 2010. Some strains of cyanobacteria produce toxins that can damage your liver, while others target the nervous system. According to the CDC report, just touching the algae produced rashes, swelling and skin sores. Swimmers who ingested or inhaled contaminated water got a range of symptoms such as diarrhea, nausea, cramps, coughing, wheezing, congestion and ear infections.
In saltwater ecosystems, dinoflagellate algae like Karenia brevis are the main culprits behind harmful blooms called red tides. K. brevis produces brevitoxins, chemicals that disrupt nerve cell signaling. For swimmers this may translate to trouble breathing, sore throats and irritated eyes.