Is biomanipulation the best way to save our lakes? Well, for a while scientists thought bass and other carnivorous fish at the top of the watery food chain could safeguard our waters from nuisance algae. Algae growth is caused by agricultural and urban run off, especially in the form of fertilizers.

    The biomanipulation idea was to stock deep lakes with big fish (like bass) that would eat the smaller ones. Decreasing the smaller fish population would allow zooplankton (small fishes' favourite food) to proliferate. The zooplankton would, in turn, feed on the algae. The result would be a nice clean lake without the use of chemical controls or expensive sewage treatment plants.

    Unfortunately, it didn't work out, says York biology professor Don McQueen. His team of researchers began a biomanipulation study in 1991 at two lakes that were as similar as possible with respect to physical and chemical characteristics, but very different in terms of food web structure.

    After three years, Ranger Lake, which was stocked with bass, showed substantial decreases in smaller fish and increases in large zooplankton. In Mouse Lake, which was not stocked, the absence of top carnivores meant lots of small fish and small zooplankton levels. So far so good. The hitch was, says McQueen, that in both lakes the algal populations were similar, apparently completely unaffected by differences in fish or zooplankton levels.

    So McQueen reversed the food web structures on both lakes to determine if other factors, unrelated to the food web, could be responsible for the algal similarity. Zooplankton proliferated where there were bass, and decreased where there were none but, again, algae at the bottom of both food webs remained unaffected.

    Says McQueen, "Unfortunately, results suggest nuisance algae caused by human beings can only be removed by sewage treatment."