Calcium levels integral to the reproduction and survival of many aquatic organisms in Europe and eastern North American freshwater lakes are declining towards critically low levels. A new global study has found this is due in part to recovery efforts against acid rain.

The study, Widespread diminishing anthropogenic effects on calcium in freshwaters, published recently in the Nature International Journal of Science, reveals that low calcium levels affect parts of the food web, such as freshwater mussels and zooplankton. Governmental and industry action to reduce sulphate deposition associated with acid rain means lakes are now subject to less calcium leaching from surrounding terrestrial areas, researchers found.

“Paradoxically, therefore, successful actions taken to address the harmful impacts of acid rain may have led a decline towards critically low levels of calcium for many aquatic organisms,” said Uppsala University professor Gesa Weyhenmeyer, lead researcher on the study, in a statement.

Low calcium concentrations can also be exacerbated by anthropogenic drivers, such as timber harvesting, the study notes, and can often be naturally low because of regional geology, highly-weathered soils and the effect of a cooler climate.

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A variety of freshwater lakes, particularly those in Eastern Canada and the Canadian Shield, as well as Fennoscandia, were found to have notably low calcium levels.

Researchers discovered a global median calcium concentration of 4 milligrams per litre, but 20.7% of their water samples showed calcium concentrations less than 1.5 milligrams per litre, the threshold critical for the survival of many organisms.

“It remains unknown for how long the process of declining Ca [calcium] concentrations will continue into the future, and how many freshwaters might reach critically low Ca levels for the survival and reproduction of the most sensitive organisms,” the study states.

The study drew on more than 440,000 water samples from 43,184 inland water sites across 57 countries. Researchers analyzed decades of trends in over 200 water bodies since the 1980s.

Canada’s IISD Experimental Lakes Area — the world’s freshwater laboratory — contributed a significant amount of expertise and data to the calcium study from its long-term monitoring dataset of over 50 years.