Giving a damn about DOM in the subtropical North Pacific: Quantifying the role of dissolved organic matter in the cycling of carbon, oxygen and nutrients in the upper ocean

The primary goal of this thesis was to establish a basin-wide DOM dataset with which to quantitatively assess DOM's role in the nutrient cycling of the subtropical North Pacific. Surface DOM distributions between high nutrient regions and the subtropics indicated that DOM advection is strongly non-Redfield. DON advection can support a small, but significant fraction (∼20%) of the nitrogen required for new production, whereas DOP advection results in an increase within the subtropics that exacerbates an already present phosphorus deficiency. Closer inspection of soluble reactive phosphorus concentrations revealed a net flux into the gyre, which can support the observed DOP increase as well as 40% of new phosphorus production. Nitrogen fixers likely utilize this SRP as a supplemental phosphorus source and, in turn, release a significant fraction of their productivity as DOC (>25%). Analysis of upper thermocline DOM and oxygen distributions suggested that the produced DOC is highly labile and contributes at least 75% to total organic matter remineralization in the upper thermocline. This is in contrast to DON, which contributes negligibly to the regeneration of nitrate. An along isopycnal model was developed that quantitatively links this DOM remineralization to a negative preformed nitrate anomaly in the region. This remineralization has strong implications for the processes that control nitracline depth and, ultimately, the degree to which inorganic nitrate can be supplied to the euphotic zone. In addition, an analysis of historical trends in preformed nitrate suggested that this DOM remineralization has increased within the last two decades, likely in response to shifts in phytoplankton community structure. In an effort to develop a tracer for DON sources, this study also tested novel methods for determining the stable isotopic composition of bulk marine DON. Preliminary δ15N results for a subtropical gyre sample suggested that nitrogen fixation might be the source for nearly 80% of the bulk DON in the surface ocean. Taken together, the results presented here suggested a positive feedback in the upper ocean DOM cycle that may enhance the ecological niche of nitrogen fixers in the surface and ultimately increase the importance of DOM production and remineralization in the upper ocean.