Phytoplankton sinking rate dynamics in the southern Benguela upwelling system

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Abstract
Sinking rates of phytoplankton assemblages with diverse and variable taxonomic composition, growing under a variety of environmental conditions, were measured in St. Helena Bay (S. Africa) using the homogenous sample method SETCOL. These measurements and flux estimates from sediment trap recoveries enabled the role of sinking phytoplankton in the flux of organic material from the euphotic zone to be determined. Sinking rates were found to be influenced by the duration of the settling experiments. Chlorophyll a sinking rates ranged from 0 to 0.91 m d-1 but were poorly correlated to phytoplankton carbon sinking rates which ranged from 0 to 0.78 m d-1. Sinking rates of phytoplankton populations were significantly correlated not to any of the environmental parameters measured but to taxonomic properties of the assemblages, which were in turn governed by prevailing environmental conditions. Phytoplankton carbon flux estimates, made from both the SETCOL measurements and the sediment trap recoveries, were in general agreement and did not exceed 6.1% of primary production. Losses from the euphotic zone during the study period resulting from the sinking of intact phytoplankton were therefore of limited importance to the time-dependent changes of phytoplankton biomass. Phytoplankton sinking was, however, considered to have an impact on phytoplankton composition due to the variable sinking rates of the different assemblage components.