Increased concentrations of phosphorus and to a lesser extent nitrogen are blamed for larger crops of blue-green algae in Lake Winnipeg.

Dr Greg McCullough a research associate with the University of Manitoba says, while there were gradual increases in phosphorus concentrations in the lake through the last half of the 20th century, the really big change seems to have happened in the mid-1990s.

Dr Greg McCullough – University of Manitoba

The models we're working with indicate that in addition to what ever changes we'd made to the landscape and additional pressures we've put on the landscape there is a fairly sudden and dramatic change in the hydrology of the Red River itself.

The Red River in the late 1990s to the present, so in the last 10 or 15 years, has had double the average annual flows that it had in the three or four decades before that.

That's a huge increase in flows and along with that there's been a real dramatic increase as well in the frequency of flooding, not only in the Red River Valley but the frequency of flooding on the Assiniboine River has increased as well so the whole Red River Assiniboine River Valley has been changing a lot in the amount of flow that's coming in and when you increase the amount of flow and increase flooding you increase the nutrient load dramatically because you have more water carrying more nutrients in but you also have a lot of flooding which picks a lot more nutrients off the land.

A flood year produces a much higher concentration of nutrients in the water than a non-flood year simply because the water flows out, sits out on the land and dissolves various chemicals from the landscape.

Dr McCullough says we have increased the amount of nitrogen and phosphorus we have put on the land and a dramatic increase in river flows which has brought flooding has really mobilised those nutrients.