On The Biophysical Factors That Control Under-Ice Phytoplankton Bloom Onset in the Central Canadian Archipelago

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Gale, Matthew
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Sporadic reports of significant under-ice phytoplankton production indicate a critical knowledge gap of a key component of the Arctic ecosystem. In this thesis I examine the following research objectives in an effort to improve the understanding of under-ice phytoplankton production: (1) to determine the biophysical processes controlling the timing of under-ice phytoplankton production, and (2) to compare and contrast both the timing of the under-ice bloom and the controlling processes between multiple years of data. Data for objective (1) were collected during the three-year Arctic-ICE field campaign (2010-2012) near Resolute Bay, NU in the central Canadian Arctic. Additional data from the region were collected from open source databases and peer-reviewed literature for a dataset that spanned from early 1960 to the present, supporting the analysis to meet objective (2). Two separate under-ice phytoplankton blooms were observed during the three-year Arctic-ICE campaign. It was found that phytoplankton blooms conformed well to the critical depth hypothesis in the Canadian Archipelago under first-year ice, where snow and ice melt both increased light transmission and shoaled the surface mixed layer which, in turn, placed phytoplankton within a favourable light environment for positive net production underneath the ice cover. Factors such as timing of melt water drainage and water column mixing greatly affected bloom onset. From the historical analyses, I was able to show that under-ice phytoplankton blooms have regularly occurred under landfast ice from at least the 1960’s. Significant correlations between the timing of bloom onset with melt onset related variables (i.e., air temperature reaching 0 ºC and complete snow melt) suggested a strong link to climate change. In fact, the ii analysis supported that since the mid 1990s bloom onset has been occurring earlier, and is likely related to decreasing trends in day of complete snow melt, maximum ice thickness, and snow depth. Overall, this thesis has helped improve our understanding of the under-ice spring phytoplankton bloom, showing that under-ice production has been a regular occurrence in the Canadian Arctic. The results also support that timing of the spring phytoplankton blooms could be shifting earlier in response to the warming Arctic and its changing icescape. Such a shift could also have important consequences on the Arctic marine food web, influencing the transfer of energy through the food chain. Therefore, it is of utmost importance to continue future observational programs of the under-ice pelagic environment focussed on the late spring melt period to better understand how the system could change with further perturbations.
Oceanography, Arctic, Spring, Bloom, Biophysical, Biology