| Summary: | The lakes and fjords of northern Ellesmere Island, Nunavut, harbor unique ice-dependant ecosystems. Some of these “cryo-ecosystems” are permanently stratified, with a freshwater layer overlying sea water. This extreme stratification is due in part to the limitation of wind-derived mixing because of the presence of a perennial ice-cover. The main objective of the work presented here was to evaluate the limnological and biological diversity of coastal lakes of northern Ellesmere Island in order to better understand their response to environmental change at two timescales: their long term variations over the Holocene and their recent responses to climate over the last few decades. The environments were shown to form a limnological chronosequence that reflected landscape evolution at the Holocene timescale, from stratified fjords to freshwater lakes, via phases in which the water bodies were stratified, meromictic lakes with different degrees of wind-induced mixing depending on the duration of ice cover, from perennial to seasonally open water conditions under warmer climates. Each of these phases is represented today in northern Nunavut. The sensitivity of these stratified ecosystems to environmental change at shorter timescales was then explored, by observing the limnological impacts of current climate change in the Ellesmere Island region. As a first step towards addressing the question of biodiversity and microbial community structure in these ecosystems, a molecular ecology analysis of the lake and fjord biota was made, and underscored the dominance of picocyanobacteria in their surface waters, in contrast with the low abundance of these microbes in the Arctic Ocean, from which those aquatic environments originate. The DNA analysis of the picocyanobacteria implied broad tolerances among these organisms, with the same genetic groups found in a great variety of environments, both on a local and a planetary scale. Finally, a study was undertaken of the zooplankton communities in a lake and fjord of northern ...
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