Evaluating the effect of lake calcium concentration on the acquisition of carapace calcium by freshwater crayfish

Aquatic systems in many parts of the world, including those on the Canadian Shield in central Ontario, Canada, are facing severe declines in aquatic calcium (Ca). The relationship between the Ca content of the carapace of freshwater crayfish, Orconectes virilis, O. rusticus and Cambarus bartonii, and lake Ca concentration was statistically evaluated across a broad [Ca] range in south-central Ontario. Carapace Ca content was significantly positively related to lake [Ca] in O. virilis but not C. bartonii or O. rusticus, probably because the latter two were only sampled over a comparatively small range of lake [Ca]. We evaluated additional variation in carapace Ca content potentially explained by the characteristics of O. virilis individuals, including size (carapace length), sex, and hardness (rigidity), using multiple linear regression. The carapace Ca content of O. virilis was significantly positively related to the hardness of the carapace, and to a lesser extent the size of the individual. For O. virilis, lake [Ca] appears to limit carapace Ca acquisition below approximately 8 mg l(-1). O. virilis is therefore experiencing stress due to low [Ca] in the majority of soft-water boreal lakes where it occurs because they are undergoing [Ca] decline.