Under-Ice Ecology of Aquatic Insects in North-Temperate Lentic Ecosystems

Winter in north-temperate lentic ecosystems is characterized by cold temperatures, limited light availability, and critically in aquatic ecosystems, periods of ice cover and hypoxia. Despite these challenging conditions, many aquatic invertebrates remain active throughout winter. Climate change is decreasing ice-coverage duration, with ice forming later and melting earlier. Current research on winter aquatic ecology is lacking, particularly on aquatic macroinvertebrates, a diverse group important to ecosystem function. My thesis examines how winter conditions shape aquatic invertebrate communities and the potential consequences of milder winters. My first set of experiments investigated why Sympetrum vicinum dragonflies exhibit clutch-splitting behaviour, ovipositing eggs in both terrestrial and aquatic environments where the eggs then spend the winter. I concluded that the dragonflies engage in risk-spreading in response to unpredictable levels of danger present in each environment; in the aquatic environments egg-depredation by winter-active detritivores such as caddisflies, and in the terrestrial environment the risk that eggs will not be inundated by spring flooding and fail to hatch. Next I examined patterns of overwinter decline in dragonfly community samples from Ontario and Michigan, then tested hypoxia as a possible driver of decline. I concluded that a large-bodied top-invertebrate-predator species, Anax junius, is likely experiencing hypoxia-driven winterkill in some of its overwintering habitats. My third set of experiments examined the effects of a prolonged warm pre-winter period (late onset winter) on overwinter survival as well as spring dispersal ability and reproductive success in Notonecta undulata. I found that late-onset winter conditions increase male survivorship over winter, as well as advanced the timing of egg laying and dispersal in females, although total egg production and dispersal attempts were not affected. Finally, I used open and closed greenhouses to manipulate ice coverage duration on in-ground pond mesocosms. I found that closed greenhouses successfully advanced the date of ice-off as well as decreased ice thickness in comparison to the open greenhouses. My thesis provides new insight into how winter affects ecological processes such as behaviour, dispersal, reproduction, survival, and community structure, as well as predictions of how these processes might change in response to milder winters.

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