Something that’s often called “algae” really isn’t. But it can be deadly.
So-called “blue-green” algae are actually cyanobacteria; unlike algae, their cells do not contain nucleii. However, like algae and other plants, they obtain energy through photosynthesis.
The two have something else in common, “Cyanobacteria cause a film or scum on the pond, like other algae that would normally be harmless would do, ”according to University of Arkansas Extension veterinarian Heidi Ward. “Although they look like an algae bloom, it’s actually more of a biofilm. Those (blue-green) algae are actually beneficial to the pond, feeding the fish.”
Standards forms of algae can be damaging to ponds; when they proliferate to the point where the water is green or brown, it’s called an algae bloom. Ward said that typically happens in warmer months, when the algae multiply due to the presence of nutrients in the water like fertlizer runoff or livestock waste. “It’s going to start taking oxygen out of the water for fish,” she said. “Some of them may also produce specific toxins when they are in bloom.”
A 2012 University of Missouri Extension publication, Managing Missouri Fish Ponds During an Extended Drought, said drought combined with high temperatures can have devastating effects on fish ponds. The authors, Robert A. Pierce II of the MU School of Natural Resources and Aquaculture Specialist Charles E. Hicks of Lincoln University, said as pond levels decrease through seepage, evaporation and lack of runoff, nutrient levels become more concentrated. As algae bloom die and decay, they deplete oxygen levels and compromise other pond life. “Watch for any changes in pond water color,” they wrote. “A change from green to brown, grey or black may indicate that the algae (have) died, and oxygen depletion will occur in as little as 24 hours.”
Ward noted algae can be controlled with filtration; you can test for the bloom density with a circular, flat disk called the Secchi disk. It’s lowered by pole or line into the water until it is no longer visible; that depth is measured to determine the water’s level of transparency, and its degree of algae population.
Since livestock waste can provide the nutrients that cause algae blooms, Ward recommended cattle be prevented from hanging around ponds during times when algae form. Copper sulfide can also be administered to the water as an algaecide.
When it comes to toxicity, people are mainly concerned with the blue-green algae, which can kill cattle, humans or any other animal if ingested. Ward said it works similarly to cyanide poisoning, which prevents cells from using oxygen, and animals may die after drinking heavily contaminated water. University of Missouri Extension Livestock Specialist Dr. Patrick Davis told OFN there are several species of cyanobacteria; some produce toxins that influence the nervous system, while others affect the liver. Davis said the organism starts out green, turns blue after it dies and ends up on the surface or shoreline. “It may be visible to the naked eye as very fine grains of green sand, or green blobs on the water surface,” he said.
Watch for muscle tremors, decreased movement and difficulty breathing on the part of infected animals. They can also collapse and go into convulsions. In the case of liver toxicity, the animals will appear weak with pale color mucous membranes; they can also display mental derangement, bloody diarrhea and death. “Even those that survive they may lose weight and become chronic poor doers,” Davis said.


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