Relations of Selected Types of Micro‐organisms with the Vernal Phytoplankton Bloom in the Tvärminne Area, Southern Coast of Finland

Abstract

The fluctuations of selected types of micro‐organisms were considered in relation to the vernal phytoplankton bloom, consisting mainly of diatoms, in the Tvärminne area in March‐June 1978. Water samples were taken almost daily from the surface layer during the bloom, which comprised two biomass (chlorophyll a) peaks.Viable bacteria corresponded closely with the course of the bloom, the bacterial maxima occurring some days after the phytoplankton maximum. The bacteria apparently multiplied by mainly utilizing organic compounds released from decaying phytoplankton cells.Proteolytic bacteria had their maximum below the ice in the low‐salinity surface layer, but showed a positive correlation with the bloom. They evidently utilized proteins supplied by domestic effluents as well as those released from phytoplankton cells. Yeasts showed no distinct connection with the bloom; they were probably unable to compete for nutrients with bacteria. The low numbers of yeasts during the bloom can mainly be ascribed to the mixing occurring after the break‐up of the ice, which decreased the yeasts of terrestrial origin in the surface layer.