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 lmol L 21 over three days, 1.5-5.7 lmol L 21 over one week, and 1.8-10.8 lmol L 21 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 lmol L 21 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. V C 2015 Association for the Sciences of Limnology and Oceanography

Carbon uptake and partitioning of two globally abundant diatom species, Thalassiosira weissflogii and Dactyliosolen fragilissimus, was investigated in batch culture experiments under four conditions: ambient (15°C, 400 μatm), high CO2 (15°C, 1000 μatm), high temperature (20°C, 400 μatm), and combined (20°C, 1000 μatm). The experiments were run from exponential growth into the stationary phase (six days after nitrogen depletion), allowing us to track biogeochemical dynamics analogous to bloom situations in the ocean. Elevated CO2 had a fertilizing effect and enhanced uptake of dissolved inorganic carbon (DIC) by about 8% for T. weissflogii and by up to 39% for D. fragilissimus. This was also reflected in higher cell numbers, build‐up of particulate and dissolved organic matter, and transparent exopolymer particles. The CO2 effects were most prominent in the stationary phase when nitrogen was depleted and CO2(aq) concentrations were low. This indicates that diatoms in the high CO2 treatments could take up more DIC until CO2 concentrations in seawater became so low that carbon limitation occurs. These results suggest that, contrary to common assumptions, diatoms could be highly sensitive to ongoing changes in oceanic carbonate chemistry, particularly under nutrient limitation. Warming from 15 to 20 °C had a stimulating effect on one species but acted as a stressor on the other species, highlighting the importance of species‐specific physiological optima and temperature ranges in the response to ocean warming. Overall, these sensitivities to CO2 and temperature could have profound impacts on diatoms blooms and the biological pump.

Many environmental factors are thought to control the bioavailability of marine dissolved organic matter (DOM) for marine microbes including its composition, the microbial community structure, and nutrient availability, yet which factors dominate at the ocean basin scale remains uncertain. Understanding the controls on DOM lability is an important goal given the role of DOM in the marine carbon cycle. We performed DOM lability experiments at two contrasting stations, one oligotrophic and one mesotrophic, in the eastern tropical South Pacific (ETSP) to investigate the controls on microbial remineralization of surface ocean DOM. Surface layer dissolved organic carbon (DOC) and nitrogen (DON) were recalcitrant to remineralization over 9 to 14 days when exposed to the microbial communities from the surface mixed layer, however exposure to microbial communities from the upper mesopelagic (twilight zone) allowed consumption of DOC but not DON. The DOC remineralization response differed between the mesotrophic site (~21 μM consumed), likely experiencing allochthonous inputs of DOM from the adjacent eastern boundary upwelling system, versus the oligotrophic station (~3 μM consumed) further offshore in the South Pacific gyre. DNA fingerprinting of the microbial communities across the ETSP with terminal restriction fragment length polymorphism (T-RFLP) analyses revealed greater differences between microbial communities in surface vs. subsurface (e.g., 100 m) waters at the same station than between surface water microbial communities separated by 1000s of kilometers. The subsurface microbial community at the mesotrophic station responsible for the greatest observed DOC remineralization, with a concomitant consumption of nitrate, consumed DOC to concentrations below that observed in situ (at 100 m), suggesting a potential role for co-metabolism of relatively labile with more recalcitrant DOC or relief from micronutrient limitation, in driving the additional DOC consumption. DOC remineralization by the mesopelagic (200 m) microbial community was much less at the oligotrophic station and similar to previously published results from the Sargasso Sea. Both microbial community composition and nutrient availability contribute to DOM persistence over weekly timescales in the surface mixed layer with varying degrees of DOC lability in the subsurface waters of the ETSP.

We sought to resolve the extent, variability, and magnitude of productivity enrichment around a high tropical island consistent with the phenomenon of an Island Mass Effect (IME). Key biogeochemical constituents and physical oceanographic parameters were measured offshore over the upper 500 m from July 27 to August 7, 2014 around the Society Island of Mo'orea in French Polynesia in association with the nearshore measurements of the Mo'orea Coral Reef Long Term Ecological Research program. High-resolution synoptic sampling in a rectangular grid around the island revealed vertical and horizontal patterns in hydrographic conditions, inorganic nutrients, rates of productivity, and concentrations of organic matter that are characteristic of oligotrophic gyre ecosystems. Within the upper euphotic zone (0-75 m), levels of net primary productivity (NPP), chlorophyll a (Chl), heterotrophic bacterioplankton productivity (BP), and particulate organic carbon (POC) exhibited concurrent enhancement at stations located within 5-15 km of shore, relative to stations farther offshore. These observations of enhanced productivity near an island are consistent with an IME. Particulate organic matter nitrogen isotopes (POM-δ 15 N) were significantly lower near the island than at stations farther offshore, further emphasizing spatial differences in water column biogeochemistry consistent with an IME. Vertical profiles suggest thermocline shoaling and mixing associated with the pycnocline impinging on the island's submerged flanks and coral reef slope may have contributed to the decreasing depth and increasing intensity of chlorophyll-a concentration in the DCM at nearshore stations relative to farther offshore. Shipboard measurements of an anticyclonic flow within the upper 75 m of the water column in the vicinity of Mo'orea suggest that retention of inorganic nutrients and organic matter near Mo'orea may also have contributed to the patterns in NPP, Chl, BP, POC, and POM-δ 15 N, providing a potential mechanistic understanding of the processes driving an IME.

Factors that affect the removal of organic carbon by heterotrophic bacterioplankton can impact the rate and magnitude of organic carbon loss in the ocean through the conversion of a portion of consumed organic carbon to CO2. Through enhanced rates of consumption, surface bacterioplankton communities can also reduce the amount of dissolved organic carbon (DOC) available for export from the surface ocean. The present study investigated the direct effects of elevated pCO2 on bacterioplankton removal of several forms of DOC ranging from glucose to complex phytoplankton exudate and lysate, and naturally occurring DOC. Elevated pCO2 (1000–1500 ppm) enhanced both the rate and magnitude of organic carbon removal by bacterioplankton communities compared to low (pre-industrial and ambient) pCO2 (250 –~400 ppm). The increased removal was largely due to enhanced respiration, rather than enhanced production of bacterioplankton biomass. The results suggest that elevated pCO2 can increase DOC consumption and decrease bacterioplankton growth efficiency, ultimately decreasing the amount of DOC available for vertical export and increasing the production of CO2 in the surface ocean.