Late Season Productivity, Carbon, and Nutrient Dynamics in a Changing Arctic

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Satellite measurements have documented significant reductions in sea ice cover, thickness, and duration throughout the Arctic Ocean in recent decades. Because of the complex interactions and feedbacks among physical, biogeochemical and ecological processes in the Arctic, there is significant uncertainty regarding how these changes in sea ice will impact the marine biological pump, which fuels the rich bottom communities on Arctic ocean shelves and is key to the regional carbon cycle. While responses of the biological pump to increased light availability in spring are currently being pursued, a considerable gap in knowledge exists regarding biological processes in the late summer and early fall. Recent studies indicate a several-decade trend of intensifying wind forcing over the Alaskan margin along the Chukchi and Beaufort Seas in the late, ice-free season (September-November). These events could lead to mixing of nutrients into surface waters, fueling increased biological production and sequestering additional carbon into shelf bottom waters prior to the winter ice advance. However, the lack of direct, concurrent observations of primary and net community productivity rates, and nutrient and carbon distributions during this poorly-sampled time of year hinders efforts to assess the ecosystem implications of such changes. This project proposes to address the question of increasing biological production in the late season through intensive field sampling in two successive years.The investigators will use an innovative approach to better characterize how primary productivity (PP), net community productivity (NCP), and particulate and dissolved organic and inorganic carbon stocks (POC, DOC, DIC) are influenced by episodic nutrient inputs to the surface ocean in late summer and early fall. Over two field campaigns, they will collect high resolution surface measurements of dissolved gases, particulate and dissolved organic matter stocks and dissolved inorganic nutrients and carbon using a ship of opportunity (year 1) and dedicated research cruises (year 2). The use of geochemical rate tracers (oxygen/argon-based NCP and oxygen isotope-based PP; inventory changes of POC, DOC, nutrients, and DIC) offers a distinct advantage over incubation-based approaches by integrating processes on longer timescales (weekly to monthly). This approach is well-suited to capture the effects of episodic upwelling and subsequent ecosystem processing. On the dedicated regional research cruises in year 2 the investigators will also use a towed pumping sled to capture high-resolution vertical and cross-shelf observations. This towed sled will be equipped with standard CTD and optical sensors and interfaced to fast shipboard analyzers for measurement of nutrients and inorganic carbon (pCO2, DIC), as well as to automated samplers to collect water for oxygen/argon ratios, oxygen isotope, POC and DOC measurements. High-resolution chemical and biological distributions will be combined with ship-based ADCP measurements of water column velocities and continuity-based mass-balance calculations to constrain rates of NCP as well as the extent of vertical and lateral material fluxes. Outreach and education elements include a pilot program to engage pre-service secondary STEM teachers in educational planning and field work portions of the work. The program will provide field experience and on the job science training to teachers and result in short media outreach pieces and targeted curriculum addressing the three dimensions of science from the next generation science standards. The proposal will support one graduate student and several undergraduate students, who will participate in all aspects of the field and analytical work. The project will contribute to the early career development of a junior investigator.