1. Most studies on zooplankton responses to acidification have focused on clearwater lakes with a dramatic acidification history. The role of dissolved organic carbon (DOC) in moderating zooplankton responses to acidification in naturally acidic, dystrophic lakes is less well understood and is partially impeded by a lack of baseline data.
2. Cladocera leave identifiable remains preserved in lake sediments that can be used to provide information on pre-industrial species assemblages and their responses to environmental stressors such as acidification. Therefore, we used palaeolimnological approaches to track cladoceran assemblage responses to acidification since c.1850 (inferred from sedimentary diatom assemblages) in three acidified lakes in Kejimkujik National Park (Nova Scotia, Canada) that differ markedly in DOC content. These include two highly dystrophic lakes (Kejimkujik and Pebbleogittch lakes), and one clearwater lake (Beaverskin Lake).
3. In dystrophic Pebbleogittch Lake, an increase in the acid-tolerant, jelly-clad, pelagic taxon Holopedium glacialis occurred coincident with diatom-inferred pH (DI-pH) declines, but no other notable cladoceran assemblage shifts occurred. Similarly, Cladocera assemblages did not appear to respond to lakewater acidification in dystrophic Kejimkujik Lake.
4. In contrast, in the clearwater Beaverskin Lake, several observed shifts in cladoceran assemblage corresponded to DI-pH declines, including an increase in the proportion of littoral taxa and an increase in Hill’s N2 species diversity. This may indicate increased water clarity as a result of acidification-related decreases in DOC, which may have enhanced growth of emergent aquatic macrophytes and improved visibility for planktivorous fish, leading to increased predation on pelagic taxa. Species shifts within the littoral assemblage of Beaverskin Lake may reflect the differing tolerances of littoral taxa to low pH and aluminium toxicity.
5. Overall, our results suggest that cladoceran assemblages in naturally acidic, dystrophic lakes may be resilient against additional pH declines related to industrial emissions of acidifying agents, as dystrophic lakes are less vulnerable to increased aluminium toxicity and acidification-induced increases in water clarity and often have a pre-industrial cladoceran assemblage already adapted to acidic conditions.