Cooperative Extension Fact Sheet FS1216 | December 2013
Blue-green algae are common primitive microorganisms that resemble algae, but are uniquely classified with bacteria. Blue-green algae are actually cyanobacteria, microscopic organisms that use sunlight to photosynthesize and produce their own food, but lack a cell nucleus or membrane-bound organelles like true algae.
"Cyan" means blue and cyanobacteria get their name from the blueish pigment phycocyanin. They also contain chlorophyll a similar to plants, and use both pigments to capture light for photosynthesis (Crayton, 2004). Cyanobacteria occur naturally in freshwater worldwide and can be found in oceans, ponds, lakes, streams, and moist soil.
Blue-green algae are a normal part of a healthy aquatic environment, but the population can "explode" in response to certain environmental conditions. High concentrations of blue-green algae can form "blooms" within just a few days.
There are three main factors that have shown to increase the likelihood of a cyanobacteria bloom on a body of water. First, since cyanobacteria are photosynthetic, they need direct access to sunlight for significant growth. More light and corresponding warmer water temperatures have been associated with increased growth. Second, nutrient enrichment, particularly nitrogen and phosphorous, is essential for a bloom in waters with a pH range of 6–9. Lastly, poor water circulation can facilitate growth of cyanobacteria. With mild winds or currents, large cyanobacteria colonies will accumulate on the leeward shore and expand rapidly as water becomes stagnant. Under these conditions a body of water can become very turbid with green, blue-green, or a reddish brown color, the appearance of a thin oily looking film resembling paint, or a thick floating scum on the surface.
Three genera of cyanobacteria account for the vast majority of blooms; Microcystis, Anabaena, and Aphanizomenon. A bloom can consist of one or a mix of two or more genera of cyanobacteria.
Cyanobacteria cannot maintain this abnormally high bloom population for long and will rapidly die and disappear after one or two weeks. If conditions remain favorable, another bloom can quickly replace the previous one. Successive blooms may overlap so that it appears as if one continuous bloom occurs for up to several months.
As long as nutrients remain in excess, cyanobacteria can grow until some other factor such as light or temperature limits their growth. Increased nutrients enter the water body as runoff from either point or nonpoint sources. Nutrient sources can include stormwater and agricultural runoff, runoff from fertilized lawns or recreation fields, sediments from soil erosion, improperly functioning septic systems, as well as from natural sources such as leaves, plant residues, and woody material.
Some of the negative impacts of a cyanobacteria bloom include:
Some, but not all cyanobacteria can produce toxins. Even blooms that contain known toxin-producing species may not produce toxins at detectable levels. It is not known what triggers toxin production in the cyanobacteria. These toxins are produced inside the cells and stay there as long as the cells are alive. When the cyanobacteria cells die and break down, the toxins are released into the water. Toxin concentrations may vary dramatically and are not evenly distributed. Potential toxins include nerve toxins, anatoxins, and liver toxins called microcystins. It is not possible to tell if the cyanobacteria present in the water body are producing toxins without laboratory tests. These toxins have been known to kill cattle, dogs, and other animals that drink infested waters. Because of this, waters that show signs of cyanobacteria should be treated with caution.
While rare, humans and animals that come in contact with cyanobacteria toxins may result in health problems that include:
To protect yourself and your pets:
If symptoms from contact develop, contact your physician or veterinarian.
While there are some short term treatment options for controlling cyanobacteria, the long term solution involves finding ways to reduce phosphorous and nitrogen inputs at their source, before they can runoff into a water body. Since nutrients come from a variety of sources, it is often difficult to pinpoint an exact cause.
Some management practices to include:
If source control does not manage the cyanobacteria blooms, there are other options:
Algaecides are not recommended as most chemicals cause the cells to break up and release toxins into the water that may result in fish kills.
Phosphorous can be inactivated or "bound up" using chemical additives, such as buffered alum (aluminum sulfate combined with sodium aluminate at a ratio of 2:1). Alum can be applied to the water forming a precipitate "flock" that removes phosphorous from the water column as it settles. In addition, as alum covers the bottom sediments, it binds with, and reduces or prevents, the release of phosphorous from the sediment so this nutrient is not readily available to the cyanobacteria.
Caution must be used when treating with additives as they can raise or lower the pH level. When the pH levels falls below 6.0 or rises above 8.5, it may result in some additives becoming toxic and trigger a fish kill. Due to the potential water quality changes, and to calculate the correct amount of additive necessary, it would be prudent for the lake owner to consult with a professional lake management specialist if considering using a phosphorous inactivation additive. More information can be found in Holdren et. al., (2001).
It must be noted that phosphorous inactivation only treats the phosphorous that is currently in the lake or pond. Stormwater runoff can add new amounts of phosphorous to a pond and trigger new blooms.
Liquid dyes can be used to inhibit algal growth by reducing the amount of sunlight that reaches the bacteria. These easy to apply dyes tint the water blue or black and absorb the light waves used in photosynthesis. Dyes are most effective in lakes or ponds with a low flow to prevent the dye from washing out quickly.
Underwater aeration promotes artificial circulation between the surface and bottom layers of the water body, bringing oxygen-poor water up to the surface. This can be achieved by using a small compressor, weighted airlines, and one or more air diffusers to introduce a plume of bubbles at the bottom of the water body. The rising air bubbles set up a circulation pattern within the water body that breaks down thermal stratification, horizontal water layers that form due to temperatures effect on water density; colder, denser water at the bottom and warmer less dense water at the surface.
If the waterbody stratifies, the bottom layer will not get oxygen from the surface and can develop into an anoxic (very little oxygen) layer overlying the bottom sediments. If this happens, a chemical reaction occurs that results in the release of phosphorous into the water column where it can fuel additional bacteria or algal growth. Aeration more evenly mixes temperature, oxygen, and nutrients throughout the water column. By increasing the dissolved oxygen content throughout the water column including the deeper waters at the bottom, more phosphorous should remain bound in the sediment.
The circulation also keeps cyanobacteria moving through the water so they can't maintain their optimum position in the water column. This helps to prevent them from reaching nuisance levels.
Barley straw bales are sometimes used in an attempt to control algae, but have had mixed results in the control of cyanobacteria blooms (Mangiafico and Haberland, 2011). Barley straw does not kill existing algae, but may act to prevent additional growth. The exact mechanism is not known, but appears to involve chemicals released as the straw breaks down.
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