Microzooplankton trophic interactions and their impact on phytoplankton production and community structure in the South Slough Arm of Coos Bay, Oregon

The role of microzooplankton as consumers of phytoplankton production was investigated in the South Slough Arm of Coos Bay, Oregon, a shallow estuarine system with a direct exchange with the coastal waters that are subject to seasonal coastal upwelling (April–September). Ciliates and heterotrophic nanoflagellates (Hflag) were observed to track <5-μm phytoplankton biomass (which contributed on average 42% to the annual total biomass) during the spring, summer and early fall when diatoms were a dominant component to >5Nm phytoplankton. Ciliates were positively correlated with both <5-μm and total phytoplankton biomass between October-March when nanoflagellates dominated the >5-μm phytoplankton assemblage. Seasonal dilution-method grazing experiments showed that microzooplankton utilized 48 to 92% of primary production in the South Slough. Grazing impacts reflected the trophic relationships intimated by the field results between ciliate and phytoplankton biomass, except when heterotrophic dinoflagellates contributed significantly to the microzooplankton and imposed significant grazing impacts on diatom production and biomass. In one dilution experiment, the suppression of autotrophic picoplankton (Apico) net-growth in low dilution treatments was observed to correspond with enhanced net-growth of Hflag. Reduction of larger protozoans (i.e., ciliates) by fractionation in a subsequent experiment eliminated these responses in comparison to unfractionated controls. The findings support the hypothesis that omnivory and trophic interactions among microzooplankton can foster non-linear relationships between apparent prey growth and dilution factor that are often reported and attributed to a functional feeding response by a single grazer group. In a second experiment, the net-growth of ciliate biomass and Hflag abundance significantly decreased and increased, respectively, with increasing copepod abundance, but the net-growth of Apico remained unchanged. These findings support the interpretation of the dilution experiment results and provide an explanation for the numerical constancy of natural picoplankton populations and the failure of mesozooplankton manipulations to promote trophic cascade effects on picoplankton abundance in field experiments. It is concluded that microzooplankton are significant grazers of phytoplankton production in the South Slough and likely play a key role in energy transfer and the stabilization of food availability for larger organisms in this ecosystem.