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Stukel, MR, Aluwihare LI, Barbeau KA, Chekalyuk AM, Goericke R, Miller AJ, Ohman MD, Ruacho A, Song H, Stephens BM, Landry MR.  2017.  Mesoscale ocean fronts enhance carbon export due to gravitational sinking and subduction. Proceedings of the National Academy of Sciences of the United States of America. 114:1252-1257.   10.1073/pnas.1609435114   AbstractWebsite

Enhanced vertical carbon transport (gravitational sinking and subduction) at mesoscale ocean fronts may explain the demonstrated imbalance of new production and sinking particle export in coastal upwelling ecosystems. Based on flux assessments from U-238:Th-234 disequilibrium and sediment traps, we found 2 to 3 times higher rates of gravitational particle export near a deep-water front (305 mg C.m(-2).d(-1)) compared with adjacent water or to mean (nonfrontal) regional conditions. Elevated particle flux at the front wasmechanistically linked to Fe-stressed diatoms and high-mesozooplankton fecal pellet production. Using a data assimilative regional ocean model fit to measured conditions, we estimate that an additional similar to 225 mg C.m(-2).d(-1) was exported as subduction of particle-rich water at the front, highlighting a transport mechanism that is not captured by sediment traps and is poorly quantified by most models and in situ measurements. Mesoscale fronts may be responsible for over a quarter of total organic carbon sequestration in the California Current and other coastal upwelling ecosystems.

Samo, TJ, Pedler BE, Ball GI, Pasulka AL, Taylor AG, Aluwihare LI, Azam F, Goericke R, Landry MR.  2012.  Microbial distribution and activity across a water mass frontal zone in the California Current Ecosystem. Journal of Plankton Research. 34:802-814.   10.1093/plankt/fbs048   AbstractWebsite

Ocean fronts with accumulated biomass and organic matter may be significant sites of enhanced microbial activity. We sampled a frontal region (the A-Front) separating oligotrophic and mesotrophic water masses within the California Current Ecosystem (CCE) to assess the influence of frontal hydrography on several microbial parameters. Samples for heterotrophic bacterial, viral and flagellate abundance, dissolved and particulate carbon and nitrogen, transparent particles and bacterial carbon production were collected at 6 depths from the surface to 100 m with 59 conductivity/temperature/depth casts along a 26-km northerly transect across the front. Relative to adjacent oligotrophic and mesotrophic waters, the frontal transition displayed peaks in the mean estimates of cell-specific bacterial carbon and bulk bacterial production, particulate organic carbon and particulate organic nitrogen concentrations, and the abundance and size of transparent particles. Bacterial carbon production increased approximate to 5-fold northward from oligotrophic waters to the frontal zone, in agreement with an increase in the frequency of dividing cells, but bacterial abundance was lower than at adjacent stations. This may be partially explained by high chlorophyll, elevated virus:bacteria ratios and low nanoflagellate grazer abundance at the front. Our data suggest that CCE fronts can facilitate intense biological transformation and physical transport of organic matter, in sharp contrast to adjacent low productivity waters, and harbor dynamic microbial populations that influence nutrient cycling.

Hansman, RL, Griffin S, Watson JT, Druffel ERM, Ingalls AE, Pearson A, Aluwihare LI.  2009.  The radiocarbon signature of microorganisms in the mesopelagic ocean. Proceedings of the National Academy of Sciences of the United States of America. 106:6513-6518.   10.1073/pnas.0810871106   AbstractWebsite

Several lines of evidence indicate that microorganisms in the meso-and bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (Delta(14)C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 mu m and >0.5 mu m) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 mu m size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, (14)C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over > 50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.