R. Goericke, RESPONSE OF PHYTOPLANKTON COMMUNITY STRUCTURE AND TAXON-SPECIFIC GROWTH-RATES TO SEASONALLY VARYING PHYSICAL FORCING IN THE SARGASSO-SEA OFF BERMUDA, Limnology and oceanography, 43(5), 1998, pp. 921-935
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.