Assessing the Role of Declining Calcium in Biological Recovery on Zooplankton in Historically Acidified Lakes
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Throughout much of the 20th century, industrial emissions caused the widespread acidification of thousands of lakes in North America and Europe. Acidification has led to unprecedented damage on zooplankton communities, with reductions in species richness, and in extreme cases, local extirpations. Although lake pH is recovering due to emission control policies, many acid-sensitive species (e.g. large Daphnia) have failed to re-establish in lakes, despite their occasional detection in plankton records. Coincidently, calcium, a critical component of cladoceran exoskeletons, has declined precipitously in many historically acidified lakes over the last few decades, and may be approaching ecologically concerning concentrations. Studies suggest that crustaceous zooplankton are likely vulnerable to calcium decline because of their fully aquatic lifecycle, calcified exoskeleton, and inherent reliance on available aqueous calcium. However, disentangling biological impacts of low calcium in acidic, and chemically recovered (pH>6.0) lakes is difficult because of a strong correlation between pH and calcium concentration. To investigate the impact of low calcium concentration on zooplankton recovery in acid-damaged lakes, lake mesocosms were stocked with zooplankton from the regional species pool and four treatments were applied in a factorial design: pH (5.7 and 6.3) and calcium (0.9mg L-1 and 2.3mg L-1). A comparison between the effects of calcium and pH indicates that impacts from declining calcium are likely to be similar to those of acidification. Total community metrics (i.e. abundance, richness, evenness, size-structure) remained similar across treatments; however, low calcium and low pH each resulted in lower abundance of Daphnia pulex/pulicaria – a large acid-sensitive, and heavily calcified cladoceran. In contrast, the abundance of small cladocerans, including smaller, acid-tolerant daphniids (Daphnia catawba, Daphnia ambigua), was higher in low calcium treatments. When both stressors were combined, changes largely reflected the effect of the most severe independent stressor. By prohibiting recovery of acid-sensitive species, and promoting establishment of zooplankton already common to acid-structured communities, our results suggest that declining calcium is likely a barrier to biological recovery in historically acidified lakes.