Local Factors and Colonist Dispersal Influence Crustacean Zooplankton Recovery from Cultural Acidification

We investigated the roles of local environmental conditions and dispersal limitation in zooplankton recovery from acidification in Swan Lake, Ontario, a historically acidified, metal-contaminated lake. We hypothesized that local environmental conditions (pH and the presence of resident, acid-tolerant zooplankton) would limit the establishment of several zooplankton colonist species. We tested this in a factorial mesocosm experiment that ran for 32 days during the mid summer. Ambient pH (5.6) reduced the abundance of two acid-sensitive cladoceran taxa, Daphnia spp. and Ceriodaphnia lacustris compared to elevated pH (6.5) but increased the abundance of cyclopoid copepod juveniles. The resident community suppressed Skistodiaptomus oregonensis and Diacyclops bicuspidatus thomasi, and to a lesser extent Mesocyclops edax, but slightly enhanced Daphnia spp. We also hypothesized that conditions in the sediments of acidified Swan Lake would limit zoo-plankton recruitment from diapausing eggs. We tested this by reciprocally transferring sediments containing eggs between Swan Lake and a nearby recovered lake, and incubating them for 15 weeks in 20-L emergence traps. Most zooplankton emerged from diapause in both lakes indicating that this mechanism contributes to the recolonization of acidified lakes once pH returns to normal. Some species, however, emerged in only one lake or the other, indicating that hatching cues such as light, temperature, oxygen, or appropriate pH may have been missing. Our experiments demonstrate that both local lake conditions and diapausing eggs can influence zooplankton recovery. Continued recovery may require additional management efforts to reduce and control regional acid emissions and active intervention in the form of food web manipulations.