Seasonal Variability of Deep Chlorophyll Maxima in an Oligotrophic Regime

Abstract:

Deep chlorophyll maxima (DCM) have been observed in oligotrophic regimes around the world, and have been attributed to a wide range of biotic and abiotic factors. These include nutrient-light co-limitation, photoacclimation to low light levels, particle sinking, and light-dependent grazing. While the debate over DCM formation and maintenance mechanisms continues, the seasonal evolution of DCMs is even less understood due to sparse in situ observations, particularly in historically undersampled regions like the subtropical South Pacific. Using fluorescence and backscatter data from seven Biogeochemical Argo floats spread across the Subtropical Zone of the South Pacific, we characterize the seasonal cycle and vertical structure of chlorophyll and particulate organic carbon (POC), and we explore their relationship to mixed-layer depth, euphotic zone depth, and nitrate concentration. We find that DCMs are persistent throughout most of the year right at the base of the euphotic zone, and are associated with deep biomass maxima inferred from POC. During the winter months, the mixed layer deepens, which brings the already existing chlorophyll up to the surface where it can be detected by satellites. Consistent with this, satellite data shows peak surface chlorophyll in the region in winter. However, this signal is due to the mixing up of a DCM, rather than nutrient entrainment fueling new production, as has been suggested in past studies.