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

With the discovery of the existence and importance of the picophytoplankton and the microbial loop, a new paradigm of planktonic communities has arisen: the autotrophic picoplankton can generally outcompete the larger phytoplankton for nutrients. The picoplankton, however, are grazed by protists, which have short response time scales—similar to the generation times of the picoplankton. The rapid response of herbivorous protists to changes in picoplankton growth rate limits the net population growth rate of the picoplankton to close to zero. The larger ‘netphytoplankton’ (>10 μm), however, tend to be grazed by metazoan zooplankton, which have relatively long generation times and therefore long response times to changes in phytoplankton growth rate. As a result, the biomass of small phytoplankton and the microbial loop represent a relatively constant background in the ocean, and larger phytoplankton account for variability associated with physical forcing and nutrient inputs. This paradigm has been articulated in many forms in the literature, perhaps most concisely as the ‘ecumenical iron hypothesis’ (Morel et al., 1991; Cullen, 1995). There is a great deal of evidence supporting the constancy of the picoplankton biomass and the variability of netphytoplankton biomass under fluctuating environmental forcing. A particularly vivid example was given by the IronEx II experiment (Coale et al., 1996), in which iron enrichment of a high-nutrient, low-chlorophyll (HNLC) environment led to about a doubling of the picoplankton biomass, but up to an 85-fold increase in some species of diatom. Chisholm has shown that increases in chlorophyll over the constant picoplankton ‘background’ tend to be confined to increasingly large size classes of phytoplankton, particularly diatoms and dinoflagellates (Chisholm, 1992). Phytoplankton blooms in coastal upwelling regions and fronts tend to be dominated by diatoms and other large phytoplankton [e.g. review in (Kiørboe, 1993)]. Similarly, the spring bloom in temperate waters is largely a product of increases in diatoms rather than picoplankton. While the data support the tenor of the paradigm presented above, there has been some ambiguity in the use of