Linking climate change and watershed responses to emerging cyanobacterial blooms in lakes on forested landscapes

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Fundamental changes are occurring on Canada’s forested landscapes. The Great Lakes-St. Lawrence forest region supplies 18% of the world’s drinking water. The formerly pristine clear waters of cottage country appear to be experiencing an incipient rise in cyanobacterial blooms (cyanoblooms), including potentially harmful ones that have the capacity to produce potent neural and liver toxins. Recently, there has been a significant increase in the number of cyanobloom reports by the public to regional governments, resulting in heightened public concern for cyanoblooms. We may be at an ecological tipping point. The frequency and intensity of cyanoblooms are increasing in nutrient-rich lakes, but the majority of “new” bloom reports are in nutrient-poor lakes with no prior bloom history. Based on our current understanding these lakes should not have the capacity to support blooms. It is time to challenge our understanding. We propose that climate change is leading to fundamental changes in the hydrology and biogeochemistry of forested landscapes, and thus nutrient delivery from land to lakes, from which the rise of cyanoblooms may be a consequence. This proposal focuses on nutrient sources in forested landscapes, their potential for transformation along pathways of transport, and their contributions to receiving lakes and consequences on phytoplankton communities in lakes. Our hypothesis is that climate-induced changes in iron (Fe) and dissolved organic matter (DOM) create conditions for exceptional cyanobacterial growth compared with their eukaryotic competitors. We will test this hypothesis at multiple scales – from coarse-scale monitoring of entire landscapes with many lakes to fine-scale experimental incubations. The role of phosphorus (P) and nitrogen (N) in regulating cyanoblooms is widely acknowledged; however, the role of micronutrients such as Fe is less known, although Fe has been shown to regulate P and N use efficiency in phytoplankton. We will (1) confirm whether there exists a rising trend in phytoplankton blooms by constructing a 30-year satellite survey of 1,000 lake phytoplankton pigment concentrations combined with contemporary ground surveys of cyanobacteria and determining if the peak annual phytoplankton concentrations in lakes is on the rise across the region (or if it is restricted to a few lakes), (2) identify critical features of the land-lake system affected by either humans (generally well understood) or climate change (far less understood) that lead to changes in Fe storage or export – the purported catalytic and stimulatory element associated with the emerging cyanobloom problem, (3) characterize Fe concentration and composition as it is transported from the catchment to lake by documenting rates of binding to DOM, and (4) describe the selective impact of DOM form and Fe that enters lakes from catchments and stimulates the development and toxicity of cyanoblooms. Using a subset of lakes, the land-stream-lake processing and export of DOM-Fe will be assessed in conjunction with cyanoblooms. This biogeochemical land-lake model of cyanobloom formation will be tested in three different bioassay procedures: low-volume “grow outs”, flow-through continuous cultures (“ecostats”) that enable cells to outcompete others through small changes in growth rate, and multi-trophic within-lake mesocosm experiments. The outcome of this work will provide fundamental insights to the mechanistic relationship between climate-driven forest hydrological and biogeochemical changes, the consequences of these changes to cyanoblooms, and the resultant threat of toxin production to human and ecosystem health.