The heat wave baking Nebraska and Iowa is mutating some lakes and ponds into toxic algae-growing, fish-killing stews.
Water in stricken lakes will have a greenish or reddish-brown color caused by blue-green algae blooms along shallow and stagnant shorelines.
Oxygen-deprived fish will surface and gulp for air. Dead fish may wash ashore in waves.
"If I had a little lake that was pea green, I'd be nervous,'' Dave Tunink, a Nebraska Game and Parks Commission fisheries administrator in Lincoln, said Monday.
The culprit is blue-green algae that can dominate a pond under the right conditions. They deplete water of its oxygen content and thrive in warm, shallow water rich in nutrients from wastewater discharges and runoff from farmland and towns.
The algae also release a toxin that can sicken people. Pets and farm animals have died from drinking contaminated water or licking wet fur or paws.
Last week the Nebraska Department of Environmental Quality issued its first blue-green algae health alert for 2011. Potentially toxic concentrations of blue-green algae were discovered at Willow Creek Lake near Pierce in northeast Nebraska.
In Iowa, officials last week declared Big Lake north of Des Moines safe for swimming after an algae bloom cleared. Sun Valley Lake in southwest Iowa's Taylor County sustained a small fish kill after a blue-green algae bloom in June.
Samples taken at 700-acre Willow Creek Lake were above Nebraska's health-alert threshold of 20 parts per billion of total microcystins, a toxin released by certain strains of blue-green algae. The toxins were found at a number of Nebraska lakes from 2004 through 2010.
The new alert will continue at Willow Creek until at least late July. Lakes must have two consecutive weeks of readings below the threshold before an alert is discontinued.
Signs advising people to use caution were posted, and the swimming beach was closed.
Recreational boating and fishing are permitted, but people are advised to avoid activities that could involve accidental ingesting of water, and to avoid full immersion in the water.
Public areas for camping, picnics and other outdoor activities remained open.
Algae thrive in hot, sunny weather. Tunink said they grow quickly, producing supersaturated oxygen in the water during the day. At night, fish, thriving plants and decaying material create too much demand for the oxygen, he said.
"You get some wild swings in oxygen levels. By morning, you have fish surfacing and you may have fish kills,'' Tunink said. "That's the problem we'll have this week.''
A minor fish kill was reported at a private lake south of Bellevue last week, he said.
Calm, hot and cloudy days are bad, too, because water plants don't produce as much oxygen, Tunink said. Breezy and windy days are good because wave action adds a little oxygen to the water.
Weekly sampling has been conducted at 47 public lakes in Nebraska since the beginning of May. The lakes will continue to be monitored weekly throughout the recreational season.
Sampling results for toxic algae and bacteria are updated Fridays and posted on the environmental quality department's website, www.deq.state.ne.us.
Tunink said the best advice is for people to avoid beaches where there is a buildup of blue-green algae and the wind is blowing into the shoreline.
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