Presenter: Jessica Gray (Ocean Process Analysis Laboratory, University of New Hampshire)

Description:
Past studies have explored how the vertical partitioning of heterotrophic microbial communities affects the remineralization rates of dissolved organic matter (DOM) including dissolved organic nitrogen (DON) and carbon (DOC) in the subtropical North Atlantic and North Pacific. For both DON and DOC, it was observed that surface accumulated DOM is recalcitrant to degradation at weekly to seasonal timescales when exposed to surface-dwelling microbial communities; in turn this material is preferentially consumed at shallow mesopelagic depths. A similar assessment of dissolved organic phosphorus (DOP) heterotrophic lability is currently lacking. To address this, we performed bioassay incubation experiments to quantify the magnitude and rate of heterotrophic DOP remineralization in surface waters (5m) and the shallow mesopelagic (125m) at two stations in the subtropical North Pacific while also quantifying the surface ocean DOP concentration gradient from Station ALOHA (22°45’) to 31° N on a line north of Oahu, HI. Experimental results found evidence for the existence of a measurable pool of labile DOP present in surface waters on the order of 25 – 60 nM that was consumed in ~5 days near the southern edge of the gyre at Station ALOHA. This consumption was ~1/3 of the latitudinal gradient in surface waters to 31°N, the core of the gyre. However, both approaches found little evidence for labile DOP present at 31°N. DOP consumption was greatest in surface waters and in treatments where the heterotrophic bacteria were released from grazer control, with some evidence of DOP consumption by the mesopelagic microbial community present at Station ALOHA. The latitudinal gradients observed in the size of the surface ocean DOP pool and its vertical lability to heterotrophic microbes found in this study largely affirms the importance of upper-ocean lateral organic nutrient transport on supplying North Pacific subtropical gyre surface waters with bioavailable phosphorus.