Synchronous shifts in dissolved organic carbon bioavailability and bacterial community responses over the course of an upwelling-driven phytoplankton bloom

Interactions between dissolved organic carbon (DOC) and bacterioplankton were examined during a diatom and Phaeocystis bloom in the Santa Barbara Channel (SBC) over five days following an upwelling event. The SBC was heterogeneous in physical state (recently upwelled vs. more stratified), nutrient concentration, and productivity, encompassing phytoplankton physiological states from a healthy bloom through the onset of silicon stress. DOC accumulated in the upper 10 m over the bloom, with compositional shifts indicated by chromophoric dissolved organic matter (CDOM) parameters. DOC bioavailability and bacterial growth and community composition responses were assessed with dilution batch-culture bioassays. In these experiments, bacterioplankton DOC usage increased over the bloom, with uptake of 1.5-5.3 mu mol L-1 over three days, 1.5-5.7 mu mol L-1 over one week, and 1.8-10.8 mu mol L-1 over 10 weeks. DOC removal was poorly correlated with traditional proxies of bioavailability (chlorophyll a concentration, elemental ratios of dissolved organic matter, and CDOM). However, bacterial growth efficiency (BGE) was strongly related to in situ conditions, with higher BGEs on fresher, late-bloom DOC. After 10 weeks, 1.6-15.7 mu mol L-1 of the DOC that accumulated during the bloom remained unutilized in the bioassays, with higher concentrations of persistent DOC in experiments from senescent bloom physiological states, supporting the putative relationship between phytoplankton blooms and seasonal DOC accumulation in the field. These experiments demonstrate that DOC released by the plankton community during a bloom fuels increased short-term and long-term bacterial activity, enhances presumed trophic transfer via increased BGEs, and leads to the accumulation of persistent, potentially exportable, DOC. (C) 2015 Association for the Sciences of Limnology and Oceanography