A MISSING LINK? -- MESOSCALE DISTRIBUTIONS OF COLONIAL PHAEOCYSTIS ANTARCTICA AND ITS GHOST COLONIES IN THE ROSS SEA

It is well established that the haptophyte Phaeocystis antarctica is a dominant member of the Ross Sea phytoplankton, contributing more than half of the annual production. Its broad spatial distribution has been documented by ship sampling, but its mesoscale variations are poorly known. In the PRISM (Processes Regulating Iron Supply at the Mesoscale) project in 2011, we deployed a Video Plankton Recorder (VPR) for 24 h between selected locations that had been identified by satellite images to have eddy-like features. As P. antarctica colonies are far larger than the VPR’s camera resolution (2 mm vs. 40 µm), we were able to quantify colonial contributions to total fluorescence and particulate organic carbon, resolving distances of a few kms and depths of ca. 1 m. Within these surveys, we found unusual forms of colonies, which we subsequently deemed “ghost colonies”; that is, colonies that have largely lost their cells and have collapsed. While ghost colonies generally were located deeper in the water column than intact colonies, not all surface Phaeocystis blooms were associated with ghost colonies. For example, in one survey ghost colonies and intact colonial forms were present throughout the water column, but with distributions separated by depth; while in another survey, their distributions appeared to be spatially distinct in the horizontal. We hypothesize that ghost colonies are formed upon extreme micronutrient stress, which causes colonies to sink and cells to be liberated from colonies. Considerable mesoscale variability was noted in P. antarctica distributions and was related to oceanographic features. The mesoscale variations in both P. antarctica and its ghost colonies potentially have significant impacts on vertical fluxes and biogeochemical cycles in the Ross Sea.