The influence of short-term events on the hydrographic and biological structure of the southwestern Ross Sea

Abstract

The Ross Sea continental shelf supports very high productivity and phytoplankton biomass. Conventional methods, including ship-based sampling, instrumented moorings, satellite imagery, and modelling, have illustrated the typical patterns of seasonal progression of the phytoplankton blooms. While variability in the Ross Sea is substantial over relatively large scales, our understanding of smaller scales of variability (on the order of a few hours or several kilometers) is limited. Utilizing data from an autonomous glider, we examined the mechanisms driving both the transitions between stages of the phytoplankton bloom and the short-term perturbations in chlorophyll concentrations. Three phases within the bloom were defined based on chlorophyll changes: an accumulation phase, a dissipation period, and a post-dissipation phase. Short-term perturbations in chlorophyll were repeatedly observed and correlated with wind speed. These chlorophyll perturbations were strongly influenced by the degree of temporal coupling between wind events and the depth of mixing, which varied among phases. Delays of 12–24h between wind events and chlorophyll changes were observed during the accumulation phase, but shortened to 2–12h in the dissipation phase. Furthermore, while physical factors contributed to the observed short-term reductions in biomass and the appearance of chlorophyll at depth throughout the study, we hypothesize that turbulence during the period of maximum biomass induced aggregate formation and led to rapid vertical flux. These results suggest that the small-scale, short-term physical perturbations may induce substantial vertical redistribution of biogenic material, which in turn can have significant biogeochemical impacts.