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 vary ing physical forcing was studied at a station southeast of Bermuda ove r a period of 18 months. The dominant groups of algae, as delineated b y taxon-specific carotenoids, were prymnesiophytes, pelagophytes, and cyanobacteria. Other groups such as diatoms, dinoflagellates, prasinop hytes, chlorophytes, and cryptophytes were never abundant. Although ph ytoplankton biomass varied over the year with maxima in the winter, li kely 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 w as virtually constant over the year as well: picoplankton (<3 mu m) co ntributed 79% to total Chi a and primary production. Taxon-specific ph ytoplankton growth rates were measured using the pigment-labeling meth od. Growth rates of prymnesiophytes and pelagophytes did not vary syst ematically over the year; rates in the surface layer ranged from 0.2 t o 0.6 d(-1), declining to values <0.1 d(-1) at the 1.6% light level. G rowth 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)). T hese results are surprising since it was expected that the increased p hysical forcing during the winter period would affect growth rates of all phytoplankton, not only cyanobacteria, and shift the composition o f the phytoplankton community from picoplankton toward larger phytopla nkton, possibly diatoms. These results suggest that the structure of t he phytoplankton community, particularly the eucaryotic component, is extremely resilient to change. Similar conclusions have been drawn fro m studies of the distribution and abundance of phytoplankton and zoopl ankton 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 interac tions with grazers.