by Sharon Oosthoek
Two dominant and much discussed threats to the boreal forest are industrial interests and logging. Now another threat has surfaced. According to researchers from Queen’s and York universities, lakes in the forest are suffering from “aquatic osteoporosis” due to declining calcium levels.
Nearly all life forms need calcium, even tiny water fleas. Scientists have discovered that calcium levels are so low in some forest lakes that Daphnia, a species of water flea that is key to the food chain, is experiencing greatly lowered reproductive rates, jeopardizing both its own survival and that of the small fish that feed on it.
In a surprising twist, the dearth of calcium is linked to a decades-old environmental threat many people considered largely solved – acid rain. Scrubbers installed on industrial smokestacks starting in the late 1970s have helped decrease levels of sulphuric and nitric acid, the main ingredients in acid rain, allowing many lakes to regain their normal pH levels.
“This is an environmental story that’s been missed,” says Queen’s biology PhD student Adam Jeziorski, lead author of a study published in the journal Science in November. “Over the last 10, 20 years, acid rain has fallen out of the public eye. The pH level is recovering so lakes are becoming less acidic, but biological recovery, that is, plants and animals in the lakes, has been lagging.”
Because sulphuric and nitric acid are positively charged, as is calcium, the elements compete for negatively charged binding sites in the soil around the lakes. The result is that calcium leaches from the soil and drains into the water. For centuries, the soil slowly released calcium into the lakes. When the acid rain phenomenon was at its peak, an abundance of this element was released, leaving precious little calcium behind in the soil. To compound the problem, calcium is produced largely by mineral-rich rock breaking down over time and is therefore replaced very slowly.
The team looked at 200 years’ worth of sediment in three lakes: Plastic Lake near Dorset, Ontario, in the Muskoka region, Little Wiles Lake in Nova Scotia and Big Moose Lake in New York State. The researchers found that key invertebrate species were disappearing in the lakes with declining calcium levels, often starting in the 1970s. It was around the same time that the effects of acid rain on our ecosystems became apparent.
The researchers also combined existing data on 770 lakes, stretching from the Muskoka-Haliburton region to Kenora, and found a third of the lakes had calcium levels below 1.5 milligrams per litre, the threshold at which Daphnia can effectively reproduce. The levels in almost two-thirds of the lakes were at two milligrams per litre.
“It has really led to a whole slew of questions,” Jeziorski says. “It’s a jumping off point for new research: How is this affecting other species, the lake as a whole? What other changes in the lakes might we have missed?” Those levels, he adds, are much lower than levels were even 20 years ago and are likely to continue to drop.