As temperatures start to rise and rainfall becomes less, summer is a great time to cool off at your local water body, be it swimming, water skiing, or fishing. However, summer also marks the season of cyanobacteria. Cyanobacteria are often the dominant primary producers in ponds void of aquatic vegetation. They have the ability to fix nitrogen and regulate buoyancy, providing a clear advantage over desired species that fuel the food-chain such as diatoms and green algae.
In addition to their ability to fix nitrogen and rise above (literally) their competition, many species of cyanobacteria have the ability to produce toxic compounds known as cyanotoxins. There are three main types of cyanotoxins: microcystins, anatoxins, and cylindrospermopsin. Many species of cyanobacteria are capable of producing variations of all three types of cyanotoxins, ranging from hepatoxins (liver toxins) to neurotoxins. These toxins affect all types of organisms, both aquatic and terrestrial. Fish exposure typically leads to reduced growth, off-flavoring, etc. in addition to the inherent risks of lowered oxygen levels due to drastic changes in pH and oxygen content. On the terrestrial front, livestock are at an especially high risk of exposure due to use of small ponds for drinking water. Clinical signs of cyanotoxicity in cattle include weakness, weight loss, excessive salivation, bloody stool, sudden collapse, and death. Other organisms, including dogs and humans, are at great risk of accidental ingestion by playing or swimming in an active bloom.
This is not to say that you should not or cannot use your pond in the summer, there are some warning signs that help predict your pond’s potential of turning into a toxic “cesspool”. Blooms are often found in ponds with little to no riparian buffers adjacent to fields with a legacy of fertilization regimes. Although this perfectly describes typical farm ponds used for watering of livestock, many fail to realize that residential, community ponds also fit this description due to the abundance of well-groomed lawns. Many of us fertilize to improve the appearance of our yard without checking the nutrient levels (N:P ratio) of our soil. You should have your soil tested through your local extension office to optimize the efficiency of fertilization and reduce adding excess phosphorous that eventually finds its way into a local body of water fueling summer cyanobacteria blooms. Also, blooms usually occur after the first rain following a prolonged dry period (~2 weeks) as excess nutrients from the watershed, such as excess phosphorous, are washed away as runoff. Cyanobacteria maintain their ability to thrive while other plants and algae struggle in ponds limited by nitrogen because of their ability to obtain nitrogen from the atmosphere (nitrogen-fixation).
The three most common types of freshwater cyanobacteria found in America belong to the genera Anabaena, Aphanizomenon, and Microcystis (easily remembered as Annie – Fannie – & Mike). Species in all three of these genera are capable of producing toxins and although microscopic identification by a trained biologist is recommended, each genera have distinct bloom characteristics that can help one macroscopically identify the species at-a-glance. Anabaena blooms often have a darker, slightly bluish tinge than the other species. Aphanizomenon blooms are easily distinguished by the appearance of tiny blades of grass floating throughout the water column. Microcystis booms are similar to Anabaena but are often a much brighter green in color and form odorous lime-green paint like scums. If your pond begins to look more like a paint spill than water, you should restrict or cease use until the species (and any potential toxins) can be properly identified by a biologist.
The most common approach to the acute management of cyanoblooms is the application of copper based herbicides. However treatments should be made with caution as high concentrations of copper can harm fish and the rupturing of toxin-producing cells may lead to an increase in toxicity following the application. The water should be tested for toxins prior to return of livestock or use for recreation. Other management options include the re-establishment of riparian buffers or the establishment of native, shoreline emergents such as water-willow to help intercept excess nutrients, mainly phosphorus, found in runoff. Aeration systems are a preventative management options as they improve lake circulation and reduce stratification (development of a thermocline) making it more difficult for cyanobacteria to establish dominance over beneficial phytoplankton such as diatoms and green algae.
For more information on the management of cyanobacteria and other algal species, visit the Aquatic Ecosystem Restoration Foundation Best Management Practices Handbook – Chapters 13 and 14.
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Article developed by Jamie Morgan