In situ imaging of large colonial Phaeocystis quantifies inefficient carbon export

Phaeocystis is a cosmopolitan phytoplankter found across diverse marine environments, often forming extensive colonial blooms. Despite their global ecological significance, quantifying the distribution of large colonies and their contribution to carbon export remains challenging due to their fragile structure, limiting the understanding of the biological carbon pump. Here, we show the first in situ quantification of Phaeocystis colonies >600 µm in high spatiotemporal resolution using the Underwater Vision Profiler 6, a quantitative and non-destructive camera system, allowing a mechanistic understanding of flux attenuation. We document a massive colonial Phaeocystis bloom in the subarctic Labrador Sea, and a sedimentation event in the Antarctic Weddell Sea. The resulting carbon inventories indicate that large Phaeocystis colonies contribute little to vertical carbon fluxes below the productive mixed layer. Despite comprising an important fraction of large particle biovolume, they contributed at most 15% to vertical carbon fluxes in the surface Labrador Sea and 0.25% in the Weddell Sea upper mesopelagic. Their slow sinking velocity likely promoted grazing and remineralization in the upper water column, therefore reducing export efficiency into mesopelagic waters. As Phaeocystis blooms are expected to increase worldwide, integrating in situ high-resolution spatiotemporal data is essential to understand ecological dynamics and biogeochemical impacts of this cosmopolitan phytoplankton taxon.