(M. Vernet, PI)
Phytoplankton population growth is a function of photosynthesis minus respiration, grazing, sedimentation, advection, excretion and lysis. We focus first on three loss terms that have ties to other components within the project: grazing (both by micro- and macrozooplankton), sedimentation and DOC excretion. Carbon excretion will be measured as part of our daily primary production experiments (Vernet et al., 1998). Sedimentation of phytoplankton cells and fecal pellets will be measured in chambers (Heiskanen and Keck, 1996) in collaboration with Ross and Quetin, both at Palmer Station and on the cruises, by collecting fresh fecal pellets from recently caught krill, salps and copepods and measuring their sinking rates. Sediment trap data suggest that between 0.3 and 7% of the annual primary production on the grid sinks to 300 m depth. Assuming an 85% assimilation efficiency by macrozooplankton, and assuming that all carbon in the traps is the result of fecal pellets, between 2% and 47% of the annual primary production is grazed. The rest of the carbon must be recycled in the upper water column (Carrillo 2001). Uneaten phytoplankton cells can also sink, in particular when under stress (Waite et al., 1992) transporting vegetative cells and resting spores to depth (Ferrario et al., 1998). We measure sedimentation rates of phytoplankton during the austral summer and fall when the sediment traps indicate maximum sedimentation. Microzooplankton grazing will be carried out by the dilution method both in the light and in the dark (Caron et al., 2000).