Assessing interacting stressors in lakes

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Lakes are among Canada's most defining landscapes and most our sensitive sentinels of environmental change. Stressors of these systems include cultural eutrophication, habitat loss, species introductions, and climate change. Identifying the impact of an individual stressor on a lake's plankton community is often challenging because stressors commonly occur in combination. Previous work identified that interacting stressors may encompass everything from additive effects to more complex situations in which they enhance (synergism) or damp one another (antagonistic effects). In this proposal I will use a combination of lake sampling over space and time, experimental mesocosms, and modeling to create a landscape of expected responses to major interacting stressors. The main field site for development of the model will be western Lake Erie. This basin can experience gradients in cyanobacteria (mainly Microcystis aeruginosa) blooms near major inflowing rivers that foster blooms while other sections of the basin are bloom-free. At the same time, the basin experiences annual development of a key introduced predator, the waterflea Bythotrephes longimanus, whose abundance and impact vary seasonally and spatially. Major zooplankton groups vary with respect to sensitivity to presence of cyanobacteria colonies, with specialist feeders (e.g. rotifers) little affected, while others, like the major herbivore Daphnia spp., are strongly and negatively impacted. These same zooplankton groups also vary with respect to vulnerability to ingestion by Bythotrephes, with rotifers not affected and Daphnia heavily impacted. Interestingly, then, the same two co-occurring stressors that harm populations of grazers like Daphnia could simultaneously either neutrally or positively affect rotifers (owing to insensitivity and competitive release). The complexity of these responses indicates that defining the impact of interacting stressors will be a challenging problem. In this proposal we will assess zooplankton community composition and abundance spatially and seasonally in conjunction with development of cyanobacteria blooms and Bythotrephes populations, with the expectation that shifts in community composition and abundance will reflect sensitivity to the interacting stressors. Mesocosm experiments will be conducted across a gradient of Microcystis and Bythotrephes densities with lab-reared mixed community of a key cladoceran (Daphnia) and rotifers (Keratella). Modeling exercises will explore the nature of the interactions between this mixed community in the absence of stressors, and with one or both stressors present. Finally, we will validate our models using inland lakes and other areas of the Great Lakes where both Bythotrephes populations and Microcystis blooms co-occur. Our projects will shed light on the complexity of interacting stressors that are expected as co-occurring Bythotrephes and Microcystis blooms spread to more lakes across Canada.