Insights into biogeochemical transformations and fluxes from high-frequency ocean time-series data.

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The research will examine ocean cycling of carbon, nitrogen, oxygen and iodine. Oxygen and carbon will be studied in the Labrador Sea, where wintertime deep mixing creates potential for rapid, climate-sensitive transfer of oxygen, anthropogenic CO2 and organic carbon to the ocean interior. The cycling of nitrogen and iodine will be studied in Bedford Basin, Nova Scotia, a 70m deep coastal fjord, adjacent to Halifax. The research will be based on time-series of chemical, physical and biological parameters from: a) an autonomous, multidisciplinary observatory in the deepwater formation region of the Labrador Sea which is a key region for delivery of O2 and anthropogenic CO2 to the deep ocean; b) a multidisciplinary, weekly time-series from the seasonally-stratified Bedford Basin, whose nitrogen cycle is impacted by humankind and in which both nitrogen and iodine undergo major seasonal transformation. Deployment of the SeaCycler profiling winch and sensor package in the Labrador Sea (summer 2021), will provide high-resolution profiles (0-150m; 1-4 times per day for a year) of bio-optical properties, dissolved gases (CO2 and O2), nitrate, phosphate, temperature, salinity and turbulence, as well as estimates of zooplankton biomass. The SeaCycler will be operated in parallel with fixed-depth measurement of pH, alkalinity, pCO2 at a nearby mooring which will also be equipped (by other investigators) with a phytoplankton sampler allowing species succession to be related to chemical/physical data. HQP will use the new data as well as data from an earlier deployment to examine: 1) the fraction of net community production exported to the deep ocean by winter mixing (compared to settling via the biological carbon pump); 2) factors that explain the timing of the spring bloom; 3) the organic carbon export efficiency after seasonal stratification, based on nutrient, carbon and oxygen budgets above and below the euphotic zone. Given constrained exchanges and manageable size, Bedford Basin has characteristics of a large natural mesocosm so that time-series allow detailed examination of processes underlying speciation changes as well as fluxes of nitrogen and iodine. The time-series will provide context for the HQP projects which will involve repeated, short-term experimental manipulations to test and refine understanding and parameterization of chemical and microbially-mediated transformations (e.g. nitrification; iodate reduction). In both locations, the time-series provide an environment for multidisciplinary teams of HQP to work on complementary projects. The research will deliver: (a) tests, with comprehensive in-situ data, of diverse hypotheses for initiation of spring phytoplankton blooms, debated since the 1950's; (b) understanding of the sensitivity to climate change of organic carbon and oxygen delivery to the deep ocean via deep-mixing; (c) understanding of controls of nitrogen and iodine speciation and fluxes in the coastal ocean.