Vertical Flux of Particulate Matter by Sedimentation and Zooplankton Movements in St. Georges Bay, the southern Gulf of St. Lawrence

Particulate sedimentation and upward movement by zooplankton were measured simultaneously as dry matter, organic carbon, and nitrogen in two field experiments in St. Georges Bay, N.S. The size of the metazooplankton populations and the seasonal stability of the water column determined the relative magnitude of vertical particle flux. The interaction of zooplankton with their food supply appears to be of critical importance in influencing their vertical movements. Food produced and consumed in the euphotic zone is redistributed throughout the water column by both mortality and defecation of daily migrating zooplankters. In summer, when production and consumption of phytoplankton was high and in approximate balance, the rate of organic carbon and nitrogen transferred vertically as swimming zooplankters exceeded that by gravitational sedimentation. Near isothermal conditions in spring allowed storms to resuspend bottom materials, which gradually settle out of the water column. During our spring experiment, a diatom bloom had depleted its nutrients, and cells had begun to settle to the bottom. Metazoans were considerably scarcer in the spring plankton and aggregated near a submerged diatom bloom. All these factors contributed to the greater amount of material settling daily in spring compared to that being actively transported vertically as zooplankton.