Acid rain has lingering impact on Canadian lakes: study

Declining levels of calcium in North American lakes, caused in part by acid rain, are having a lasting impact on the food web of these aquatic ecosystems, scientists said Thursday.

Calling the phenomenon "aquatic osteoporosis," Queen's University PhD candidate Adam Jeziorski and research colleagues at Queen's, York University and the federal and provincial environment ministries found a direct relationship between levels of calcium and the abundance of water fleas, tiny crustaceans that are one of the foundations of aquatic food webs.

A lack of calcium in a lake is in part based on its surrounding environment. Lakes in the Canadian Shield, for example, are naturally low in calcium because the surrounding granite bedrock is poor in the mineral.

But external, man-made, activities such as pollution leading to acid rain and logging activities can also lower a lake's calcium levels, said Queen's biology professor John Smol, one of Jeziorski's contributing authors in a paper to be published Friday in the journal Science.

While scientists have known from lab experiments that many organisms need calcium as a nutrient to thrive and reproduce, to date no study had been done to measure the impact of decreased calcium levels on a particular species in the lakes themselves.

Lab results have shown that the water flea, or Daphnia, in particular requires calcium to be present in the water in concentrations of at least 1.5 mg per litre to reproduce effectively.

This made it the ideal creature to study, said Jeziorski, since many lakes in the Canadian Shield were already known to have calcium levels below that threshold. If the declining calcium levels were affecting the population of water fleas, then the scientists should be able to see the declining number of water fleas in the fossil records contained in the lakebeds.

The researchers looked at the lake sediment cores of three lakes: Plastic Lake in south-central Ontario, Little Wiles Lake in Nova Scotia and Big Moose Lake in the state of New York, and in all cases found that a rise in lake acidity led to a decline in calcium and water flea remains.

They also conducted a survey of 770 lakes in north America and found a third of the lakes had calcium levels below 1.5 mg per litre and almost two-thirds at 2.0 mg per litre, suggesting lake acidity may be having a profound impact on the ecosystems.

Even more alarming, says Smol, is the case of Big Moose Lake. While the lake's acidity levels have recovered from a pH of about 4.6 in the 1950s to its current levels of greater than 5.5, the lake has not seen a subsequent return to calcium or water flea levels.

"You've had a chemical recovery but not a biological recovery," he said. "This suggests that things are more complicated and that the problem may be worse than we think."

Jeziorski said the results are an "alarm bell" to the complex and fragile state of our aquatic ecosystems.

"I think acid rain as a topic with the general public has fallen on the backburner, as if it was a problem we had solved 20 years ago," said Jeziorski.

"Our results are a reminder that we are still feeling its legacy," he said.