Abstract
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.
Highlights
The biological carbon pump plays a key role in regulating the partitioning of carbon dioxide (CO2) between the ocean and atmosphere, and without it atmospheric CO2 would likely be 200 ppm higher than it is today (Parekh et al, 2006)
The lack of faecal pellets (FPs) observed in our sample at 113 m may be due to the heterogeneous distribution of FPs at a particular depth associated with patchy zooplankton distributions
We present here a unique vertical profile of particleassociated microbial respiration measured directly on sinking marine aggregates collected at depth
Summary
The biological carbon pump plays a key role in regulating the partitioning of carbon dioxide (CO2) between the ocean and atmosphere, and without it atmospheric CO2 would likely be 200 ppm higher than it is today (Parekh et al, 2006). Key to determining its effectiveness is the efficiency with which organic carbon sinks through the ocean interior (quantified as the transfer efficiency), and the depth at which material is remineralised (Francois et al, 2002; Kwon et al, 2009). Despite its importance, the processes governing the loss of organic carbon within the mesopelagic are poorly understood (Burd et al, 2010). Particulate organic carbon (POC) sinking out of the euphotic zone can be transformed within the mesopelagic in many ways, including zooplankton feeding, fragmentation. A. Belcher et al.: Depth-resolved particle-associated microbial respiration via sloppy feeding, microbial solubilisation to dissolved organic carbon (DOC) and physically driven aggregation and disaggregation processes Settling organic matter is often found to be insufficient to meet the energy demands of microbes in the dark ocean, leading to an imbalanced mesopelagic carbon budget (Baltar et al, 2009; Herndl and Reinthaler, 2013; Steinberg et al, 2008)
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