| Summary: | Given their great abundance throughout the circumpolar North, and their intense production of greenhouse gases, thermokarst lakes (shallow lakes and ponds caused by thawing permafrost) have been identified as a globally important class of freshwater ecosystems. The objective of this doctoral study was to characterize the autotrophic plankton of this lake type, with emphasis on two ecological groups of cyanobacteria (bloom-formers and picoplankton) and their responsiveness to climate change. Thermokarst lakes were sampled across a range of landscapes in northern Quebec. Phytoplankton community structure and relationships with environmental variables were assessed with a combination of methods including limnological profiling, pigment analysis by high performance liquid chromatography, picoplankton analysis by flow cytometry, molecular assays of protist community structure, and analysis of lake water samples by inverse microscopy. Additionally, an incubation experiment with thermokarst lake water was performed to evaluate the potential direct (warming) and indirect (phosphorus enrichment) effects of climate change. Finally, a laboratory system was designed and applied to test the effects of temperature on herbivore-cyanobacteria feeding relationships using subarctic and temperate clones of the keystone species Daphnia pulex, and a high latitude strain of picocyanobacteria. The results showed that thermokarst lakes as well as a set of reference rock-basin lakes contained diverse pigments originating from autotrophic plankton, including some pigments specific for cyanobacteria and green photosynthetic sulfur bacteria. Indicators of trophic status (chlorophyll a and total phosphorus concentrations) showed that the thermokarst lakes were more enriched than the oligotrophic reference lakes. The phytoplankton communities of both groups contained low concentrations of bloom-forming cyanobacteria and picocyanobacteria, but in highly variable proportions of their total phototrophic biovolume. The experimental results ...
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