Articles | Volume 13, issue 17
Biogeosciences, 13, 4927–4943, 2016
Biogeosciences, 13, 4927–4943, 2016

Research article 07 Sep 2016

Research article | 07 Sep 2016

Depth-resolved particle-associated microbial respiration in the northeast Atlantic

Anna Belcher1,2, Morten Iversen3,4, Sarah Giering1, Virginie Riou5,6, Stephanie A. Henson1, Leo Berline5, Loic Guilloux5, and Richard Sanders1 Anna Belcher et al.
  • 1National Oceanography Centre, Southampton, SO14 3ZH, UK
  • 2University of Southampton, Southampton, SO14 3ZH, UK
  • 3Helmholtz Young Investigator Group SEAPUMP, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
  • 4MARUM, University of Bremen, Bremen, Germany
  • 5Aix Marseille Univ., Univ. Toulon, CNRS, IRD, MIO UM 110, Mediterranean Institute of Oceanography, Marseille, France
  • 6CEREGE, UM34, Aix Marseille Université, CNRS, IRD, Aix-en-Provence, Cedex, France

Abstract. Atmospheric levels of carbon dioxide are tightly linked to the depth at which sinking particulate organic carbon (POC) is remineralised in the ocean. Rapid attenuation of downward POC flux typically occurs in the upper mesopelagic (top few hundred metres of the water column), with much slower loss rates deeper in the ocean. Currently, we lack understanding of the processes that drive POC attenuation, resulting in large uncertainties in the mesopelagic carbon budget. Attempts to balance the POC supply to the mesopelagic with respiration by zooplankton and microbes rarely succeed. Where a balance has been found, depth-resolved estimates reveal large compensating imbalances in the upper and lower mesopelagic. In particular, it has been suggested that respiration by free-living microbes and zooplankton in the upper mesopelagic are too low to explain the observed flux attenuation of POC within this layer. We test the hypothesis that particle-associated microbes contribute significantly to community respiration in the mesopelagic, measuring particle-associated microbial respiration of POC in the northeast Atlantic through shipboard measurements on individual marine snow aggregates collected at depth (36–500 m). We find very low rates of both absolute and carbon-specific particle-associated microbial respiration (< 3 % d−1), suggesting that this term cannot solve imbalances in the upper mesopelagic POC budget. The relative importance of particle-associated microbial respiration increases with depth, accounting for up to 33 % of POC loss in the mid-mesopelagic (128–500 m). We suggest that POC attenuation in the upper mesopelagic (36–128 m) is driven by the transformation of large, fast-sinking particles to smaller, slow-sinking and suspended particles via processes such as zooplankton fragmentation and solubilisation, and that this shift to non-sinking POC may help to explain imbalances in the mesopelagic carbon budget.

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Short summary
We address the imbalance between the supply and loss of organic carbon to the upper layer of the ocean by measuring a previously poorly quantified term: particle-associated microbial respiration of in situ particles. We find rates that are too low to account for the missing sink of carbon and suggest instead that zooplankton drive the transformation of large fast-sinking particles into slow-sinking and suspended particles. This apparent loss may help explain imbalances in the carbon budget.
Final-revised paper