Atlantic Westerly Winds Biome
171
350–450 m, mixed-layer nitrates in the range of 2–5 M kg
−1 have been hindcast for that
period. HPLC analysis of phytoplankton accessory pigments at BATS enables characterization of the taxa that contribute seasonally to blooms and to the permanent deep
maximum. It is, of course, the prokaryotic picoplankton that dominate the autotrophic
biomass almost year-round, with some seasonal segregation between groups. Prochlorophytes are seasonally variable, being important from late spring, and through summer,
while cyanobacteria are more regular in their biomass, never falling to such low values
as prochlorophytes (DuRand et al., 2001).
Coccolith blooms occur in late winter and spring (<100 cells liter
−1 ) at OWS S, with
much lower concentrations during summer and with some species succession: Emiliana
huxleyi dominates near-surface in spring and Florisphaera profunda at the DCM in late
summer (Haidar and Thiersen, 2001). Prymnesiophytes and pelagophytes numerically
dominate the eukaryote phytoplankton and dinoflagellates are relatively less important.
Diatom blooms occur, often late in the summer but, even so, their cell numbers may
be quite high. It has been suggested that the phytoplankton community, and especially
the eukaryotes, are resistant to changes in oceanographic conditions as these change
seasonally. Perhaps this is an observation of general relevance in warm open oceans?
The DCM is a site of especially intense trophic activity in this province as it is elsewhere,
and it is a preferred depth zone for herbivorous mesozooplankton. Consumption follows
a strong diel cycle not only for diel migrant micronekton and copepods but also for
those resident in the photic zone. For the diel vertical migration of micronekton and
large copepods, consumption is lowest in late afternoon and peaks around midnight. It
is in this province that the role of small heterotrophic nanoflagellates in both “nurturing
and grazing on planktonic bacteria,” as Sieburth and Davis (1984) put it, was worked
out. As elsewhere, the bulk profiles at the BATS show that bacteroplankton dominate (if
cyanobacteria are included) the living biomass, and also that the heterotrophic bacterial
biomass is maximal in the deep chlorophyll maximum layer, though it is somewhat more
seasonal there than the autotrophic cells. Likewise, most of the biomass of heterotrophic
organisms at BATS comprises the nanozooplankton (2–5 m): acantharia, radiolaria, and
foraminifera are, as usual, the most abundant forms. As elsewhere, this was a difficult
community to assess at BATS, given the high level of production by autotrophic symbionts
within the cells of these organisms.
The seasonal biological cycle at the 1989 JGOFS station (32
N 20
W) in the NASTE subprovince reported by Jochem and Zeitzschel (1993) appears to be similar to the
weak spring bloom found at the BATS mooring. However, we must be very careful
when comparing data sets not obtained simultaneously, because the 1955–1994 data
from OWS S show strong decadal and between-year variability, principally forced by the
relative length of the winter mixing period and the depth of mixing (Michaels and Knap,
1996); these factors determine the nutrient levels in the mixed layer when stratification
is reestablished.
One may be surprised that modern ecological studies, such as those derived from the
BATS series of observations, do not appear to consider seriously the macroalgae. Here,
the existence of floating Sargassum (Fucacae) should not pass without notice, for it is
a unique feature of this province. Several species (S. natans, S. fluitans, S. hystrix, and
S. polyceratus) are involved, having differential distributions (e.g., Stoner, 1983), while
different morphological forms of at least one species are recognized. The occurrence of
masses of gulf weed dense enough to stop sailing ships were probably only the stuff of old
sailors’ yarns, and Sargassum normally occurs in yellow, disc-shaped clumps 10–50 cm
in diameter at the surface of the ocean. These may be randomly dispersed (as I have
seen them) or arranged in windrows, accumulated by the Langmuir circulation, as much
as 50 m wide, and aligned parallel to the direction of the wind. It has not been easy to
derive satisfactory estimates of biomass from surface net tows, though many have been
171
350–450 m, mixed-layer nitrates in the range of 2–5 M kg
−1 have been hindcast for that
period. HPLC analysis of phytoplankton accessory pigments at BATS enables characterization of the taxa that contribute seasonally to blooms and to the permanent deep
maximum. It is, of course, the prokaryotic picoplankton that dominate the autotrophic
biomass almost year-round, with some seasonal segregation between groups. Prochlorophytes are seasonally variable, being important from late spring, and through summer,
while cyanobacteria are more regular in their biomass, never falling to such low values
as prochlorophytes (DuRand et al., 2001).
Coccolith blooms occur in late winter and spring (<100 cells liter
−1 ) at OWS S, with
much lower concentrations during summer and with some species succession: Emiliana
huxleyi dominates near-surface in spring and Florisphaera profunda at the DCM in late
summer (Haidar and Thiersen, 2001). Prymnesiophytes and pelagophytes numerically
dominate the eukaryote phytoplankton and dinoflagellates are relatively less important.
Diatom blooms occur, often late in the summer but, even so, their cell numbers may
be quite high. It has been suggested that the phytoplankton community, and especially
the eukaryotes, are resistant to changes in oceanographic conditions as these change
seasonally. Perhaps this is an observation of general relevance in warm open oceans?
The DCM is a site of especially intense trophic activity in this province as it is elsewhere,
and it is a preferred depth zone for herbivorous mesozooplankton. Consumption follows
a strong diel cycle not only for diel migrant micronekton and copepods but also for
those resident in the photic zone. For the diel vertical migration of micronekton and
large copepods, consumption is lowest in late afternoon and peaks around midnight. It
is in this province that the role of small heterotrophic nanoflagellates in both “nurturing
and grazing on planktonic bacteria,” as Sieburth and Davis (1984) put it, was worked
out. As elsewhere, the bulk profiles at the BATS show that bacteroplankton dominate (if
cyanobacteria are included) the living biomass, and also that the heterotrophic bacterial
biomass is maximal in the deep chlorophyll maximum layer, though it is somewhat more
seasonal there than the autotrophic cells. Likewise, most of the biomass of heterotrophic
organisms at BATS comprises the nanozooplankton (2–5 m): acantharia, radiolaria, and
foraminifera are, as usual, the most abundant forms. As elsewhere, this was a difficult
community to assess at BATS, given the high level of production by autotrophic symbionts
within the cells of these organisms.
The seasonal biological cycle at the 1989 JGOFS station (32
N 20
W) in the NASTE subprovince reported by Jochem and Zeitzschel (1993) appears to be similar to the
weak spring bloom found at the BATS mooring. However, we must be very careful
when comparing data sets not obtained simultaneously, because the 1955–1994 data
from OWS S show strong decadal and between-year variability, principally forced by the
relative length of the winter mixing period and the depth of mixing (Michaels and Knap,
1996); these factors determine the nutrient levels in the mixed layer when stratification
is reestablished.
One may be surprised that modern ecological studies, such as those derived from the
BATS series of observations, do not appear to consider seriously the macroalgae. Here,
the existence of floating Sargassum (Fucacae) should not pass without notice, for it is
a unique feature of this province. Several species (S. natans, S. fluitans, S. hystrix, and
S. polyceratus) are involved, having differential distributions (e.g., Stoner, 1983), while
different morphological forms of at least one species are recognized. The occurrence of
masses of gulf weed dense enough to stop sailing ships were probably only the stuff of old
sailors’ yarns, and Sargassum normally occurs in yellow, disc-shaped clumps 10–50 cm
in diameter at the surface of the ocean. These may be randomly dispersed (as I have
seen them) or arranged in windrows, accumulated by the Langmuir circulation, as much
as 50 m wide, and aligned parallel to the direction of the wind. It has not been easy to
derive satisfactory estimates of biomass from surface net tows, though many have been