Controls on the attenuation of sinking particulate organic carbon in the mesopelagic

Abstract

The biological carbon pump plays a key role in regulating the ocean-atmosphere balance of CO2, without it atmospheric CO2 would likely be 200ppm higher than it is today. The most rapid attenuation of downward particulate organic carbon (POC) flux typically occurs in the upper few hundred meters of the water column, yet the practical difficulties of making measurements in this dynamic region of the ocean mean that the processes controlling POC flux attenuation are still poorly understood.

In this thesis, Marine Snow Catchers were deployed in the Scotia Sea, Antarctica and the northeast Atlantic to obtain intact sinking particles and investigate the relationship between particle type and attenuation rate. Faecal pellets (FP) were a major component of the flux at all stations, yet total POC attenuation varied between sites in relation to zooplankton composition and bloom timing. A novel method was employed to characterise particle-associated microbial respiration on FP, which is currently a poorly understood term. Oxygen microsensors were used to measure small scale oxygen gradients through the boundary layer at the interface of FP. Rates of particle-associated microbial respiration were too low to account for the observed large decreases in FP flux over the upper 200 m, and evidence suggests that losses via zooplankton grazing and fragmentation are more important.

The importance of Antarctic krill for setting the export efficiency of POC in the marginal ice zone (MIZ) of the Scotia Sea is highlighted through unique comparisons between observed mesopelagic krill FP fluxes and predicted surface FP production. Krill FP are transferred through the upper mesopelagic much more efficiently than values of POC attenuation typically used in global biogeochemical models. I conclude that improved, regionally specific knowledge of the zooplankton community composition is vital to understand global variations in POC flux attenuation, and hence allow better predictions of ocean carbon sequestration.

Ultimately carbon is lost from the organic carbon pool as CO2 via respiration, and hence in theory, at steady state, the attenuation of POC should be balanced by community respiration. In reality this balance is difficult to achieve, suggesting that our understanding is incomplete. Very low rates of both absolute and carbon specific particle-associated microbial respiration (a term missing from previous budget studies) were measured on marine snow particles collected in the northeast Atlantic and hence cannot resolve imbalances in the upper mesopelagic POC budget. Microbial disaggregation and solubilisation of POC as well as fragmentation of large particles into slowly sinking and non-sinking POC by zooplankton, may help to explain imbalances in the carbon budget, highlighting the need to measure respiration losses on both fast, slow and non-sinking pools of POC.

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