| The Cladocera (Crustacea: Branchiopoda) are a key group of aquatic invertebrates in freshwater ecosystems. Despite their important ecological role, cladocerans have been considerably underutilized in paleolimnological research in high-latitude regions of North America. In this thesis, I attempt to improve upon the limited knowledge of the occurrence, distribution and ecology of cladocerans within northwestern Subarctic North America. First, I develop an illustrated guide to the identification of subfossil cladoceran remains preserved in lake sediments from Alaska. 28 different taxa, the majority from the family Chydoridae, but also including remains from the Bosminidae, Daphniidae, Sididae, and Macrothricidae are identified, and descriptions of the most abundant remains are provided, accompanied by high-resolution images illustrating their main diagnostic features.; Secondly, I evaluate the importance of environmental factors in influencing the distribution of cladocerans in Subarctic lakes. I show that the composition of cladoceran assemblages in lakes across a north-south transect in Alaska are significantly related to lake depth; total phosphorus (TP), and altitude. In these lakes, I report a distinct shift in dominance in planktonic taxa from the Bosminidae in lakes in the southern boreal forest to Daphniidae in lakes in the northern boreal forest, which I relate to differences in nutrients, specific conductivity, and calcium concentrations among the lakes. The relative contributions of environmental and spatial variables in explaining the distribution patterns of cladocerans were also examined in a set of lakes in the central Canadian Arctic treeline region. In these lakes, three environmental variables were identified as significantly influencing cladocerans: surface water temperature, dissolved organic carbon (DOC), and total phosphorus (TP). Variance partitioning attributed 14% of the variance in the distribution of Cladocera within the Central Canadian treeline region to spatial factors, an additional 10% to spatially-structured environmental variables, and 8% to environmental factors that were not spatially-structured.; In the final section, I demonstrate the potential of cladocerans as paleolimnological indicators, by examining their response to recent environmental change in Central Canadian treeline lakes. I show that significant changes in cladoceran species composition have occurred from pre-industrial times to the present in these lakes. However, these changes are considerably muted compared to the more substantial changes observed in the diatom record in these lakes. The response of cladocerans is not strongly coupled to the changes in diatoms. These results highlight the usefulness of cladocerans in paleolimnological studies. |
