59 research outputs found

    Bacterial vs. zooplankton control of sinking particle flux in the ocean\u27s twilight zone

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    The downward flux of particulate organic carbon (POC) decreases significantly in the oceanÂs mesopelagic or ‘twilight’ zone due both to abiotic processes and metabolism by resident biota. Bacteria and zooplankton solubilize and consume POC to support their metabolism, but the relative importance of bacteria vs. zooplankton in the consumption of sinking particles in the twilight zone is unknown. We compared losses of sinking POC, using differences in export flux measured by neutrally buoyant sediment traps at a range of depths, with bacteria and zooplankton metabolic requirements at the Hawaii Ocean Time‐series station ALOHA in the subtropical Pacific and the Japanese times‐series site K2 in the subarctic Pacific. Integrated (150‐1,000 m) mesopelagic bacterial C demand exceeded that of zooplankton by up to 3‐fold at ALOHA, while bacteria and zooplankton required relatively equal amounts of POC at K2. However, sinking POC flux was inadequate to meet metabolic demands at either site. Mesopelagic bacterial C demand was 3‐ to 4‐fold (ALOHA), and 10‐fold (K2) greater than the loss of sinking POC flux, while zooplankton C demand was 1‐ to 2‐fold (ALOHA), and 3‐ to 9‐fold (K2) greater (using our ‘middle’ estimate conversion factors to calculate C demand). Assuming the particle flux estimates are accurate, we posit that this additional C demand must be met by diel vertical migration of zooplankton feeding at the surface and by carnivory at depth—with both processes ultimately supplying organic C to mesopelagic bacteria. These pathways need to be incorporated into biogeochemical models that predict global C sequestration in the deep sea

    Phenology in large grazing copepods in the Oyashio region, western subarctic Pacific

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     Seasonal sequence of population structure (=copepodid stage composition) of large grazing copepods (Metridia pacifica, Eucalanus bungii and Neocalanus spp.) was analyzed based on seasonal samples collected with 100 μm mesh nets from 0-500 m stratum at Site H in the Oyashio region, western subarctic Pacific, during 1996-1997 and 2002-2007. On the premise that there are little year-to-year differences, the composite data were arranged to the date of samplings of each year to yield seasonal developmental patterns of each copepod. Seasonal developmental pattern estimated by tracing the sequence of mean copepodid stages of the population at each sampling date revealed that the recruitment season of the population was January for N. cristatus, March for N. flemingeri and May for N. plumchrus and E. bungii. In contrast to these copepods with single recruitment seasons in the year, M. pacifica exhibited two recruitment seasons (mid-May and August) in a year. Phenology in reproduction and development of these copepods reflects their species-specific differences in energy utilization pattern ; M. pacifica and E. bungii spawn in phytoplankton-rich surface layer in spring (females need to feed for spawning) while Neocalanus spp. spawn in deep layer in winter (females do not feed). Development from C1 to C5 of N. cristatus, N. flemingeri and N. plumchrus was in January to June, March to June and May to August, respectively, thus the three sympatric Neocalanus spp. showed a clear temporal separation in the developmental timing in the western subarctic Pacific. This temporal separation in utilizing the surface layer is considered to be a mechanism to reduce inter-specific food competition. Regional comparison of phenology in copepods within the entire subarctic Pacific and its adjacent waters revealed that reproduction timing of the surface spawning M. pacifica and E. bungii was highly variable, while this was not the case for deep spawning Neocalanus spp
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