Phytoplankton blooms in a fluctuating environment: the roles of plankton response time scales and grazing

Our new view of planktonic ecosystems states that the picoplanktoncan outcompete the netplankton for nutrients, but are held toa relatively constant biomass because of the short responsetime scales of their protist grazers. The long response timeof the mesozooplankton grazing the netphytoplankton allows theselarger phytoplankton to respond to environmental fluctuationswith large changes in biomass. There is some ambiguity in theliterature, however, over the relative importance of ‘responsetime scales’ versus grazing in controlling the phytoplanktonbiomass. To address this issue, a simple model including explicitresponse time scales and grazing was formulated. The model wasused to explore the influence of these two dynamics in controllingthe response of phytoplankton to sudden changes in the carryingcapacity of the environment. It was applied to the IronEx IIdata to explore the implications of the two types of control.The model supports the hypothesis that the short response timescale of the protists limits the picoplankton biomass. However,it also shows that the zooplankton grazing rate (here representedby a clearance rate a) has a much stronger effect in determiningthe phytoplankton biomass than the response time scale.