The widespread decline in aqueous calcium (Ca) is emerging as a newly recognised stressor in freshwater ecosystems in regions with historically high acid deposition, especially when coupled with multiple logging cycles. Currently, Ca and other base cations are being depleted in the soils of acid-sensitive watersheds of eastern North America and western Europe, resulting in falling Ca levels in streams and lakes. Freshwater crustaceans have high Ca demands due to their heavily calcified exoskeleton and regular moult cycle. Because they rely on Ca in the external environment for the majority of their Ca uptake, aquatic crustaceans are restricted to waters at or above the Ca concentration needed to satisfy their demands. The zoogeographic distributions along ambient Ca gradients, and Ca requirements and metabolism of three groups of freshwater crustaceans — daphniids, crayfish and gammarids — have been relatively well studied. Here we have four objectives: (1) We briefly review biological features of the above three taxa that relate to Ca metabolism. (2) We review the literature regarding minimum Ca thresholds permitting survival, Ca saturation points, and the individual and population scale costs of existence at suboptimal Ca concentrations. (3) Using Daphnia as an example, we explore the ecological consequences of falling environmental Ca concentrations. (4) We identify gaps and weaknesses in the literature that may inhibit the development of environmental risk assessments for Ca decline for these three crustacean groups. We conclude that crayfish are especially vulnerable to ongoing Ca decline, and that all three taxa are probably living under suboptimal conditions in areas of Ca decline, and are therefore likely under chronic metabolic stress.

Calcium (Ca) concentrations are decreasing in softwater lakes across eastern North America and western Europe. Using long-term contemporary and palaeo-environmental field data, we show that this is precipitating a dramatic change in Canadian lakes: the replacement of previously dominant pelagic herbivores (Ca-rich Daphnia species) by Holopedium glacialis, a jelly-clad, Ca-poor competitor. In some lakes, this transformation is being facilitated by increases in macro-invertebrate predation, both from native (Chaoborus spp.) and introduced (Bythotrephes longimanus) zooplanktivores, to which Holopedium, with its jelly coat, is relatively invulnerable. Greater representation by Holopedium within cladoceran zooplankton communities will reduce nutrient transfer through food webs, given their lower phosphorus content relative to daphniids, and greater absolute abundances may pose long-term problems to water users. The dominance of jelly-clad zooplankton will likely persist while lakewater Ca levels remain low.

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