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.

Stable isotope ratios for six size fractions of zooplankton (80 to .2000 mm) were analyzed in the Gulf of Lion in spring 2010 and winter 2011. Environmental and plankton community variables were also recorded. The originality of this study is the use of a Lagrangian transport modeling system to determine the origin of the water masses and the assessment of the proportion of detritus in the plankton samples. The highest d 15 N values were observed in the 1000-2000 mm fraction in January and in the 500-1000 mm fraction in May. The largest size class (.2000 mm), dominated by salps, had lower d 15 N values owing to the low isotopic signatures of these organisms. The history of the water masses resulted in two main patterns with different isotopic signatures: the water masses which resided on the shelf and the waters masses carried onto the shelf from off-shelf region by the Northern Current. The d 13 C values varied strongly between January and May, mainly owing to changes in hydrographic conditions. The d 15 N values, plankton size structure and zooplankton feeding activity varied depending on the season, revealing differences in the seasonal trophic structure of the plankton communities. The trophic structure was characterized in January by a high chlorophyl a (Chl-a) concentration, a population dominated by small organisms and herbivores, and in May by patchy Chl-a distribution, higher particulate organic matter concentration, a population dominated by large size organisms and an increase in the number of omnivores.