What can we learn about carbon export from optically determined mesopelagic particle abundance?

Whether optical surveys of the ocean help us understand and even predict carbon fluxes depends on the premise that the relative standing stocks of particles in the deep ocean reflect sinking fluxes. This proposition entails that deep-sea production of particles and isopycnal lateral transport of resuspended sedimentary material from slopes, as alternative sources of observed particle abundance, are negligible. To test hypotheses relevant to the usefulness of deep-sea particle inventories for estimating fluxes, we explored two environments: the open North Atlantic and the Ross Sea shelf system. We found that there were significant correlations between total particle volumes in the upper ocean (50-60 m) and those found at 200-300 m in both regions, albeit at high variability and low shared variance. Some of the observed variability in the relationship between shallow and mesopelagic particle volume abundances can be explained by the relative strength of pycnoclines, and in the Ross Sea, deep mixing and accelerated fluxes of large particles due to strong katabatic winds. There, estimated carbon export, based on upper water column carbon mass balance, was significantly correlated with 1) total particle volumes from images, 2) particulate organic carbon, and 3) chlorophyll fluorescence, all recorded at 200 – 300 m. Measuring total particle volume below the primary pycnocline may thus be a useful approach to estimate carbon export at least in regions characterized by seasonally high particle export.