Primary productivity and biogeochemical carbon cycling in the Beaufort and Chukchi Seas, Arctic Ocean [electronic resource]

The Arctic Ocean has undergone unprecedented changes in sea ice extent and thickness in recent years, including record-setting sea ice minimums in 2007 and 2012. These changes are predicted to affect Arctic marine primary productivity (the photosynthetic fixation of carbon dioxide by tiny algae called phytoplankton) because the timing and intensity of the summer phytoplankton bloom are strongly controlled by the dynamics of sea ice and water column stabilization. Satellite-based estimates indicate that primary production in ice-free waters has increased dramatically over the last few decades as a result of the increases in open water and length of the growing season associated with the thinning ice cover. In addition, climate models predict that the Arctic will experience greater and more rapid warming than other areas of the planet over the next century, suggesting that these changes may become even more prevalent in the future. The thinning sea ice has already had a dramatic impact on regional biogeochemistry: in 2011, we observed one of the most massive phytoplankton blooms ever recorded under the sea ice in the Chukchi Sea, an area traditionally thought of as too dark and too cold for massive blooms to occur. In the Chukchi, melt-ponds on the ice surface have proliferated to an extent that, in combination with the thinning ice cover, light penetration through the ice to surface waters is now sufficient for net photosynthesis to occur. The bloom we witnessed in 2011 extended for over 100 km into the > 1 m thick ice pack, and was characterized by extraordinarily high diatom biomass and rates of production. These changes represent a marked shift in our conception of Arctic marine ecosystems and have potential global-scale implications due to feedbacks relating to sea ice albedo, global atmospheric and ocean circulation patterns, and natural greenhouse gas exchanges between the atmosphere and ocean. Chapter 1 presents an overall introduction to the Arctic and discusses the causes and consequences of this changing seasonal cycle of productivity. Chapter 2 presents results from field work performed in the Beaufort Sea in the summer of 2008 exploring the spatial and temporal variability of phytoplankton photosynthesis in the ice-associated region of the flaw-lead polynya (area of perennially open water that rings the Arctic Ocean between land-fast ice and the central Arctic ice pack; it can be used somewhat as an analog for future open-water and ice-edged based productivity). Continuing with this theme of exploring primary productivity and biogeochemical cycles in the changing Arctic, Chapter 3 details the results from photophysiological experiments performed during the summer of 2010-2011 that highlight the unique features allowing Arctic phytoplankton to reach high levels of biomass in the extreme environment under the ice. In Chapter 4, I present data from recent 1-D modeling efforts that utilize the light and nutrient-controlled responses of phytoplankton growing under the ice to explore the consequences and implications of this shifting bloom cycle on regional biogeochemical processes.

Keywords: Environmental Earth System Science.
Advisor: Arrigo Kevin R. primary advisor. Thesis advisor
Dunbar Robert advisor. Thesis advisor
Thomas Leif N. advisor. Thesis advisor