Abstract
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.
Highlights
bacterial carbon demand (BCD) is the carbon required for respiration and growth, while zooplankton carbon demand (ZCD) is carbon ingested and subsequently assimilated for use in respiration, excretion, growth, and reproduction, plus unassimilated carbon egested as feces
The profiles of sinking particle flux at ALOHA were nearly identical between the two deployments, with 75% of the 150 m particulate organic carbon (POC) flux removed by 500 m (Fig. 4)
Previous studies have noted that sinking POC flux as measured by sediment traps was insufficient to fuel mesopelagic C demand in the subarctic Pacific (Boyd et al 1999; Simon et al 1992) and the Arabian Sea (Ducklow 1993)
Summary
Study sites—Samples were collected and experiments conducted aboard the RV Kilo Moana at the HOT station ALOHA (22u459N, 158uW) from 22 June 2004 to 09 July 2004 and aboard the RV Roger Revelle at K2 (47uN, 160uE) from 22 July 2005 to 18 August 2005. All zooplankton respiration and carbon demand calculations were made using a combination of day (13.5 h for ALOHA or 14.5 h for K2) + night (10.5 h for ALOHA or 9.5 h for K2) biomass data (mean day and night length at each site during our study) This method includes C requirements of diel migrators residing at depth during the day, which may (Lampitt et al 1993) or may not consume sinking particles. Active flux of CO2 and DOC by zooplankton vertical migrators—Downward active flux of CO2 by migrant zooplankton (mg C m22 d21) was calculated as in AlMutairi and Landry (2001) for the 0–150 m depth intervals, assuming migrants reside below the mixed layer 13.5 h and 14.5 h during the day at ALOHA and K2, respectively (see above), with the remainder of time spent in the surface waters at night, and applying the average temperature experienced by migrants at depth during the day at each site (Al-Mutairi and Landry 2001; Steinberg et al 2000). Downward active flux of DOC by migrant zooplankton (mg C m22 d21) was calculated as 31% of downward active flux of CO2 (Steinberg et al 2000)
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