Policy News - September 29, 2004

Climate altering nutrient dynamics in lakes

As the warming climate increases evaporation and cuts stream inflow to lakes, resource managers may have to lower point-source nutrient inputs even more to avoid nuisance algal blooms, according to new research presented at the Ecological Society of America (ESA) meeting held in Portland, Ore., in August.

Climate warming is altering not only the dynamics of the carbon cycle but also the coupling of the cycles of phosphorus, nitrogen, and silica—critical nutrients for algal growth in lakes, says Dave Schindler, an aquatic ecologist at the University of Alberta in Edmonton (Canada).

The province of Alberta has already experienced aspects of climate warming: Evaporation has increased by 12% since 1970, and midsummer flows in some rivers have been reduced to about 20% of historic levels, Schindler says. The warming and drying trend in western Canada has reduced inputs of silica, nitrogen, and phosphorus to lakes; this trend has led to clearer, more algae-free lakes in undisturbed areas.

However, Lake Winnipeg, nestled in a 953,250 square kilometer (km²) watershed spanning from western Alberta to northern Ontario, still receives a big dose of phosphorus and nitrogen from sewage and agricultural runoff that is no longer diluted by inflows, Schindler says. With natural inflows down by 80%, diatoms, a kind of algae that depends on silica brought in by streams, have been elbowed out by the more noxious blue-green algae, which have spread in a 6000-km² bloom for the past several summers.

“Because there is less silica coming into lake ecosystems and because there is less dilution of point sources to lakes, we may need to cut back phosphorus and nitrogen inputs more than we did years ago to maintain the same water quality,” Schindler says. —JANET PELLEY