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

Our new view of planktonic ecosystems states that the picoplankton can outcompete the netplankton for nutrients, but are held to a relatively constant biomass because of the short response time scales of their protist grazers. The long response time of the mesozooplankton grazing the netphytoplankton allows these larger phytoplankton to respond to environmental fluctuations with large changes in biomass. There is some ambiguity in the literature, however, over the relative importance of 'response time scales' versus grazing in controlling the phytoplankton biomass. To address this issue, a simple model including explicit response time scales and grazing was formulated. The model was used to explore the influence of these two dynamics in controlling the response of phytoplankton to sudden changes in the carrying capacity of the environment. It was applied to the IronEx II data to explore the implications of the two types of control. The model supports the hypothesis that the short response time scale of the protists limits the picoplankton biomass. However, it also shows that the zooplankton grazing rate (here represented by a clearance rate a) has a much stronger effect in determining the phytoplankton biomass than the response time scale.