Impact of phytoplankton composition on carbon export in the southern Weddell Sea

Climatic changes in the Southern Ocean have strong implications for the global marine carbon cycle, for example through changes in phytoplankton community composition. These shifts, in turn, can affect the strength and efficiency of the biological carbon pump, i.e. the process by which carbon is exported from the surface ocean to the deep sea via the aggregation and sinking of phytoplankton and other organic matter. At depth, carbon can be sequestered over long periods of time, effectively “buffering” increasing atmospheric CO2 concentrations. For the Southern Ocean, specifically the Weddell Sea, we only have limited data on carbon export due to the difficulties of accessing these remote and often ice-covered regions. Based on various phytoplankton bottle incubation experiments which simulated future climatic changes a possible shift in phytoplankton community composition from large diatoms to small flagellates such as Phaeocystis sp. is indicated, with unknown consequences for nutrient cycling and carbon export. To address these unknowns, we conducted in situ measurements and roller tank experiments with contrasting diatom-to-Phaeocystis ratios during a Polarstern cruise to the Southern Weddell Sea in spring 2021 to characterize aggregate formation degradation and sinking of marine snow. The same set of parameters were also assessed in controlled laboratory experiments with well-defined diatom-to-Phaeocystis ratios. Based on our results from field and laboratory, we hypothesised that a climate-mediated shift towards Phaeocystis in the future would reduce the efficiency of the biological carbon pump due to decreased silica-ballasting and increased concentrations of positively buoyant exopolymeric substances associated with Phaeocystis colonies. To our surprise, preliminary results reveal that higher Phaeocystis cell numbers relative to diatoms do not lead to a statistically significant reduction in aggregate mass density and size-specific sinking velocity. At the same time, there was a trend towards larger particles when Phaeocystis was abundant. Our results from field and laboratory observation together reveal that Phaeocystis-dominated communities do not impede carbon export in the Weddell Sea, but may actually enhance it