Phosphorous, algae blooms follow flooding

IF predictions of a large spring flood pan out, beach goers on Lake Winnipeg this summer should brace themselves for a scummy mess of blue-green algae.

The culprit is the pile of phosphorus that will wash into the lake with the draining of this spring's looming Red Sea.

Greg McCullough, a research associate at the University of Manitoba's Centre for Earth Observation Science, said any time there is a major flood, the Red River and the lake take on a bigger load of the nutrient.

As the water sits for weeks on flat farm fields along the Red from south of Fargo to the city of Winnipeg, it dissolves phosphorus from the soil, manure, fertilizer, rotting crop residues and other organic material. All of this nutrient finds its way into the lake.

On years where the river stays within its banks, the phosphorus issue is much less severe, said McCullough, who tracks nutrient readings in the Red and other streams. There are 200 to 300 micrograms of phosphorus per litre of water in the Red River during a normal summer. But in spring when the river exceeds its banks, the concentrations rise to 400 micrograms. And the amount of water flowing northward to the lake is dramatically higher.

"You'll get a large injection of phosphorus into Lake Winnipeg if you have a flood like '97," McCullough said in a recent interview.

Some people think the melting snow dilutes the nutrients entering the lake. Not so, said McCullough.

"Roughly speaking, if you had 10 times as much flow come in (to the lake) during the year, you would have 15 to 20 times as much phosphorus come in because you have bumped up the concentration as well as bumping up the amount of water," he said.

Nitrogen concentrations don't increase to the same degree, but there is still more nitrogen entering the lake in a big flood year, he added.

And the Red is the big nutrient villain among the rivers that drain into the lake, producing 70 per cent of its phosphorus, said McCullough. Its summertime flows contain eight to 10 times more phosphorus than the Winnipeg River or the Saskatchewan River. The Assiniboine River is also a smaller contributor because of its relatively low flows and the fact that it drains more rolling land.

Once the Red Sea drains and reverts to farmland once again, Lake Winnipeg becomes a breeding ground for algae, first in the south basin and then in subsequent years in the north basin.

"The phosphorus is critical" to the production of algae, McCullough said. And high levels of phosphorus blended with lesser concentrations of nitrogen actually create conditions that allow blue-green algae to compete well against other forms of algae. That's because the blue-green species are also able to feed off the nitrogen in the air.

larry.kusch@freepress.mb.ca

Republished from the Winnipeg Free Press print edition March 19, 2011 J16