SEDIMENTATION OF PHYTOPLANKTON DURING A DIATOM BLOOM - RATES AND MECHANISMS

Phytoplankton blooms are uncoupled from grazing and are normally termi nated by sedimentation. There are several potential mechanisms by whic h phytoplankton cells may settle out of the photic zone: sinking of in dividual cells or chains, coagulation of cells into aggregates with hi gh settling velocities, settling of cells attached to marine snow aggr egates formed from discarded larvacean houses or pteropod feeding webs , and packaging of cells into rapidly falling zooplankton fecal pellet s. We quantified the relative significance of these different mechanis ms during a diatom bloom in a temperate fjord, and evaluated their pot ential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the Ii-day study period, averaging ca. 0.1 d(-1), and mass sedimentation a nd bloom termination did not occur. Most cells settled attached to mar ine snow aggregates formed from discarded larvacean houses, whereas se ttling of unaggregated cells was insignificant. Formation rates of phy toplankton aggregates by physical coagulation was very low, and losses by this mechanism were much less than 0.07 d(-1); phytoplankton aggre gates were neither recorded in the water column (by divers) nor in sed iment traps. The low coagulation rates were due to a very low 'stickin ess' of suspended particles. The dominant diatom, Thalassiosira mendio lana, that accounted for up to 75% of the phytoplankton biomass, was n ot sticky at al, and did not turn sticky upon nutrient depletion in cu lture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particl es (TEP), that occurred in low concentrations throughout the study per iod. Zooplankton grazing rate did not respond to the development of th e bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 30 -50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations an d, hence, control blooms, neither of the other mechanisms examined wor ked in a density dependent manner. In the absence of significant coagu lation rates, rapid mass sedimentation of this bloom did not occur.