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Winter aeration is often used to prevent fish winter-kill among lakes supporting recreational fisheries and is often used in conjunction with trout stocking as trout are sensitive to hypoxia.We collected limnological data as well as pelagic microcrustacea and rotifers one year before and one year after aeration was initiated in Birch Lake, located in the boreal foothills ecozone of Alberta, Canada. Using a Before-After-Control-Impact design, we compared changes in Birch Lake to those in two nearby and similarly mesotrophic, stocked control lakes (one aerated for more than six years, one unaerated). During winter, surface aeration increased the depth of well-mixed water in Birch Lake, maintaining levels of dissolved oxygen suitable for supporting stocked trout. Linear mixed model analyses indicated that among MayÀAugust water quality parameters, only phosphorus concentrations showed a marginally significant treatment £ year interaction, increasing only in the unaerated control lake. Aeration did not affect the springÀsummer abundance, biomass, or sizes of Birch Lake's zooplankton community. Once aerated, the microcrustacean community in Birch Lake showed partial overlap with the community in the aerated control, although this could not be statistically linked to changes in specific taxa. Ours is the first study to investigate impacts of winter surface aeration on zooplankton communities despite the numerous lakes managed under this strategy.

Stocking lakes with trout can have strong effects on native communities; however, the nature of impacts is not universal across receiving ecosystems. To assess effects of non-native trout, relative to native small-bodied fish, on microcrustacean zooplankton, we compared stocked, unstocked (but fish-bearing), and fishless lakes in the boreal foothills of Alberta, Canada. Relative to unstocked lakes, stocked lakes had greater richness, but otherwise showed few additional effects on microcrustacean communities. In contrast, fishless lakes supported lower abundances of Cladocera, Calanoida, and Cyclopoida, but were dominated by larger cladoceran and calanoid taxa, compared with fish-bearing lakes (stocked and unstocked). Vertical distributions also differed significantly among lake types; microcrustaceans had far higher relative abundances at 1 m than at 2 m in fishless lakes compared with fish-bearing lakes (distributions in stocked and unstocked lakes were similar). Microcrustacean communities in fishless lakes were likely shaped by the invertebrate planktivore Chaoborus, which was abundant in these systems, whereas native fishes likely structured microcrustacean communities prior to trout introductions, with planktivory by trout causing few additional effects.

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