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 taxonspecific 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 Frn) contributed 79% to total Chl 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-l, declining to values <O.l d-l at the 1.6% light level. Growth rates of cyanobacteria in the surface layer were highest during the winter (0.6 d-l) and lowest during the summer (0.3-0.4 d-l). 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. Open-ocean phytoplankton communities respond dramatically to seasonally varying physical forcing in most temperate and polar environments. Phytoplankton biomass and productivity can change by orders of magnitude over the seasons, and the composition of the community often changes from a large-celled, diatom-dominated community during the spring to a small-celled, flagellate-dominated community during the summer (Raymont 1983). In contrast, dramatic seasonal changes are thought to be absent in the central subtropical gyres (Blackburn 1981). An exception is the Sargasso Sea, whose northern part is subject to severe physical forcing during the winter when the upper water column is mixed to a depth of up to 400 m and the euphotic zone is enriched with nutrients (Riley 1957; Hulburt 1990). As a consequence, diatom-dominated spring blooms, characterized by high levels of phytoplankton biomass and productivity, are regularly observed (Riley 1957). The southern Sargasso Sea, in contrast, does not experience wintertime mixing and nutrient enrichments and is highly oligotrophic throughout the year (Ryther and Menzel 1960, 1961), similar