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

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.

Eukaryotic influence on the oceanic biological carbon pump in the Scotia Sea as revealed by 18S rRNA gene sequencing of suspended and sinking particles

“…Due to their relatively fast sinking, POC export dominated by mesozooplankton fecal pellets generally leads to limited connectivity between sinking and suspended particle pools in the mesopelagic. Fecal pellets are indeed more resistant to remineralization compared to phytoplanktondominated particles and conferred low microbial respiration rates (Belcher et al 2016 and2016b), as they have higher densities and are surrounded by a protective membrane Duret et al…”

Slow‐sinking particulate organic carbon in the Atlantic Ocean: Magnitude, flux, and potential controls

“…Originally, MSCs were designed to capture large rare particles with minimal disturbance from turbulence [Lampitt et al, 1993] and often the main focus of MSC sampling is still fast-sinking particles. The carbon content of fast-sinking particles collected from MSCs has, until recently [Belcher et al, 2016b], always been calculated using imaging and Alldredge [1998] conversion factors. However, Bishop et al [2016] recently reported that Alldredge [1998] conversion factors underestimate the carbon content of aggregates considerably.…”

“…However, Bishop et al [2016] recently reported that Alldredge [1998] conversion factors underestimate the carbon content of aggregates considerably. In the future we would recommend measuring POC of the fast-sinking particles directly as in Belcher et al [2016b] but as this study is a retrospective data collation, using conversion factors is unavoidable. To investigate whether conversion factors affected the results found in the Southern Ocean, we also calculate the fastsinking POC fluxes using two conversion factors for FP [Alldredge, 1998;Manno et al, 2015] which demonstrated differences in magnitude but minimal change in observed trends (see section 3.4).…”

“…Microbial respiration of fast-sinking POC flux is often found to be too slow to explain observed decreases in flux [Belcher et al, 2016b] which suggests that zooplankton processing may have significant interactions with fast-sinking particles. Several recent papers have concluded that zooplankton sloppy feeding , microbial gardening [Mayor et al, 2014], and zooplankton fragmentation [Cavan et al, 2015;Belcher et al, 2016a;Cavan et al, 2017a] may lead to the in situ production of slow-sinking particles from fast-sinking particles.…”

Drivers of Carbon Export Efficiency in the Global Ocean

“…Only a fraction of PP leaves the upper ocean, known as the export efficiency (export flux/PP). The export efficiency is affected by multiple factors such as phytoplankton community structure (which may affect formation of aggregates or sinking by ballasting; Buesseler, 1998;Boyd & Newton, 1999;Francois et al, 2002), zooplankton (which package phytoplankton into fast-sinking fecal pellets or may transfer carbon directly to depth via diel vertical migration; Cavan et al, 2015;Dagg et al, 2014;Steinberg et al, 2002), and bacterial remineralization (Belcher et al, 2016;Buchan et al, 2014;Buesseler et al, 2007;Le Moigne et al, 2016).…”