Golden algae bloom on Little Rock Lake a good sign after drawdown

RICE — After more than a decade of absence, a springtime bloom of golden algae is good news at Little Rock Lake.

An April plankton sample on Little Rock Lake was "dominated by golden algae," according to a recent news release from the Minnesota Department of Natural Resources.

"In the spring, the golden algae is what we want," St. Cloud State University biology professor Matt Julius said Monday. Julius conducted the sampling April 17.

This is good news because golden algae is "characteristic of a spring algal bloom in a healthy aquatic system," the release said.

And those words — healthy aquatic system — have not been used for years to describe Little Rock Lake, a shallow, 1,270-acre artificial lake created after a dam was built in 1907 downstream from where the Mississippi River and Little Rock Creek meet.

The lake was first listed as impaired by the state in 2008 and underwent a drawdown late last summer. The intent of lowering the water level about 3 feet for about two months was to flush out excess nutrients, expose the lake's soil to air and sun and grow plants, all in an effort to reduce phosphorus in the lake, DNR Area Fisheries Manager Eric Altena said Tuesday. 

Some phosphorus came into the lake from runoff or non-compliant septic systems, and over time, phosphorus began to build up in lake sediment, Julius said.

Extra phosphorus in a lake causes blue-green algae blooms to happen earlier — or, at Little Rock Lake, exclusively. 

"The phosphorus balance was so out of kilter that the lake never had a chance to have greens or golden algae," Julius said. "They just started with blue-greens."

Typically, golden algae is the first of three major groups of algae to bloom in the spring. After the ice melts, there is a bloom of golden algae, and fish and small animals eat the golden algae, which is full of healthy oils, Julius said.

"That sort of promotes a healthy food base for the fish that carries through," he said.

Golden algae is dependent on a nutrient ratio that heavily favors nitrogen. When the lake warms up, some of the nutrients previously circulating through the whole lake no longer circulate through the bottom layer of the lake as it becomes cold, Julius said. Algae starts to use up the nitrogen and phosphorus in the lake.

"Typically, you get an imbalance, where the nitrogen gets used up first," he said.

The lake transitions into green algae, but late in the fall, as the nitrogen is used up, bacteria that can pull nitrogen out of the atmosphere — that's blue green algae, Julius said — take over.

When there is more phosphorus in the lake, blue-green algae blooms occur earlier, accelerating the natural succession. And when people talk about a lake turning green in the late summer, this is what's going on, Julius said.

This is why a golden algae bloom on Little Rock Lake is exciting for Julius. 

"This was a really good first event because it's exactly what a healthy system is supposed to look like in the spring," Julius said. 

It is a good indicator that the drawdown had a positive effect.

"The question is ... how long is that gonna last as we're starting to put phosphorus back into that system?" he said.

According to Altena, phosphorus coming into Little Rock Lake has been greatly reduced through work done by Benton and Morrison counties' Soil and Water Conservation districts and landowners in the watershed. Their efforts have reduced phosphorous coming into the lake by over 2,500 pounds a year since 2012. That equates to over 1 million pounds of algae in bloom, Altena said.

Water clarity measurements taken last fall as soon as the lake refilled showed transparency quintupled, Altena said. 

Many of more than 45,000 aquatic plants — of which about half were purchased and planted during the drawdown by members of the Little Rock Lake Association — look to be doing well and are sprouting, he said.

But it is still spring, and the days have been cool. 

"Is the water more clear in July, or not?" Altena said. "That's ultimately what everybody's looking for."

Standard water quality sampling comes in June, taken by the lake association and sent to a lab to measure phosphorus and water clarity, he said. He will also conduct a vegetation survey and continue to check plantings.

Julius, who is interested in Little Rock Lake and communicates with the DNR about the health of the lake, plans to continue monitoring progress. This summer, he plans to learn how much sediment left the lake and how much phosphorus went with it. He said the lake needs less phosphorus coming in than going out.

"We're very worried about that legacy phosphorus," Julius said.

Though concerns about water clarity and algae blooms have existed for decades (since at least 1990, according to Benton County Soil and Water Conservation District), a 2007 blue-green algae bloom tipped the scale toward a public health risk.

The drawdown at Little Rock Lake was postponed twice over the course of the approximately six years it was in the works. The project's total cost was $235,000.

The goal is a sustained clear-water environment with emerging vegetation to protect the shoreline and provide fish habitat, Altena said.

And while golden algae is a good sign, the lake is not "cured," he said.

"It's not just an instantaneous flip of a switch," Altena said. "... The hope is there, but the fact is Mother Nature doesn't work that way. ... It's not always right away, it might not be an instantaneous fix, but you might see portions and glimmers of hope. ... We all need to do our part to make sure the water quality gets better and continues to improve."

Sarah Kocher is the business reporter for the St. Cloud Times. Reach her at 320-255-8799 or skocher@stcloudtimes.com. Follow her on Twitter @SarahAKocher.

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