Response of phytoplankton community structure and taxon-specific growth rates to seasonally varying physical forcing in the Sargasso Sea off Bermuda

The response of phytoplankton community structure and dynamics to varying physical forcing was studied at a station southeast of Bermuda over a period of 18 months. The dominant groups of algae, as delineated by taxon-specific carotenoids, were prymnesiophytes, pelagophytes, and cyanobacteria. Other groups such as diatoms, dinoflagellates, prasinophytes, chlorophytes, and cryptophytes were never abundant. Although phytoplankton biomass varied over the year with maxima in the winter, likely in response to elevated nutrient concentrations, the composition of the eucaryotic phytoplankton community and the relative biomass of cyanobacteria in the surface layer did not change. The distribution of phytoplankton biomass and primary production over five size classes was virtually constant over the year as well: picoplankton (<3 mu m) contributed 79% to total Chi a and primary production. Taxon-specific phytoplankton growth rates were measured using the pigment-labeling method. Growth rates of prymnesiophytes and pelagophytes did not vary systematically over the year; rates in the surface layer ranged from 0.2 to 0.6 d(-1), declining to values <0.1 d(-1) at the 1.6% light level. Growth rates of cyanobacteria in the surface layer were highest during the winter (0.6 d(-1)) and lowest during the summer (0.3-0.4 d(-1)). These results are surprising since it was expected that the increased physical forcing during the winter period would affect growth rates of all phytoplankton, not only cyanobacteria, and shift the composition of the phytoplankton community from picoplankton toward larger phytoplankton, possibly diatoms. These results suggest that the structure of the phytoplankton community, particularly the eucaryotic component, is extremely resilient to change. Similar conclusions have been drawn from studies of the distribution and abundance of phytoplankton and zooplankton in the North Pacific Central Gyres. The properties that convey these characteristics to the communities are likely the physiological attributes of the organisms and the parameters governing their interactions with grazers.