Indian Ocean Coastal Biome
301
hence below the effective depth of satellite remote sensing. At this period, Prochlorococcus
is abundant in the pico fraction, although during upwelling periods this organism occurs
only at offshore, oligotrophic situations (Campbell, 1998). The pico fraction forms >90%
of phytoplankton biomass at oceanic stations, but only 35% in the diatom-dominated
coastal zone during monsoon periods, at which times Prochlorococcus is effectively absent,
and Synechococcus dominates the picoplankton (Brown, 1999).
Larger cells (1–2 m), on the whole, lie deeper than smaller cells (07 m) so that in
the deep chlorophyll maxima, small diatoms have relatively greater importance, as they
do in the near-surface chlorophyll maxima of upwelling cells close to the coast. In such
situations, production rates of around 1–2 gC m
−2 day
−1 are commonly observed, of
which about 50% is generated by the pico fraction, about 10% by the nano fraction, and
about 40% by larger cells. Offshore, in oligotrophic situations this is reversed: from 60 to
75% of the daily production of 0.4 to 05 gC m
−2 d
−1 is generated by picoplankton, principally phycoerythrin-rich chlorococcoid cyanobacteria and prochlorophytes that occur
at concentrations of ∼110 cells liter
−1 .
Dilution experiments (Landry et al., 1998) showed that phytoplankton growth in the
upwelling areas during the Southwest Monsoon is based on a division rate 09–22 d
−1
compared with < 05 d
−1 in the oligotrophic region offshore; enhanced growth rates
inshore during the Northeast Monsoon were significantly lower, at 03–13 d
−1 . Microzooplankton grazing rate was of order 45–50% of phytoplankton growth rate inshore during
both monsoons, compared with parity in the oligotrophic condition. The relationship
between phytoplankton growth rate and NO 3 concentration in these same experiments
show a discontinuity at ∼02 M NO 3 , below which growth rates fall very rapidly to very
small numbers, and above which high growth rates are sustained. The highest growth
rate observed (27 d
−1 ), at the inshore station off Ras al Hadd in August-September represents, as Landry et al. point out, almost four doublings per day. This is the highest rate
observed anywhere by the dilution technique and approaches the maximum potentially
observed in cultures: it is ascribable to “the unique combination of high temperature
(>22
C), high nutrients (>19 M NO 3 ) and the almost complete dominance of a single
fast-growing diatom species, Chaetoceros curvisetus.” The imbalance in such situations
between herbivore consumption and production is such that the equivalent of one doubling per day is left unconsumed. Smith et al. (1998b) suggest that no more than 70% of
daily primary production is likely to be consumed by mesozooplankton in the upwelling
regions.
This analysis reflects the established view of the response of the autotrophic cells
to upwelling in this province, that of a “classical” upwelling process in which nutrient limitation during the oligotrophic, nonupwelling season is relieved by the vertical
entrainment of a sufficient supply of micro- and macronutrients to induce a change of
state in the phytoplankton cells whose rate of increase, to some extent, then escapes from
the balance previously struck with herbivore consumption. However, the JGOFS studies
suggested a novel mechanism to Marra and Barber (2005), who participated in the work
at sea and who suggest (to use their own words) that “Vertical mixing dilutes both
phytoplankton and micrograzers alike, but since phytoplankton are not mixed to greater
than their critical depth, they continue to grow but experience less grazing pressure”; this,
Marra and Barber suggest, is a sufficient explanation of the observed changes in phytoplankton biomass associated with regional changes in mixed-layer depth. Differential
growth rates of autotrophs and consumers had been evoked earlier by Goericke (2002)
to support a suggestion that top-down effects were responsible for the seasonal changes
in phytoplankton biomass in the Arabian Sea. This mechanism must form part of the
complex processes that control the dynamics of production and consumption after a
deepening of the mixed layer here as elsewhere, but I believe that the relative importance
301
hence below the effective depth of satellite remote sensing. At this period, Prochlorococcus
is abundant in the pico fraction, although during upwelling periods this organism occurs
only at offshore, oligotrophic situations (Campbell, 1998). The pico fraction forms >90%
of phytoplankton biomass at oceanic stations, but only 35% in the diatom-dominated
coastal zone during monsoon periods, at which times Prochlorococcus is effectively absent,
and Synechococcus dominates the picoplankton (Brown, 1999).
Larger cells (1–2 m), on the whole, lie deeper than smaller cells (07 m) so that in
the deep chlorophyll maxima, small diatoms have relatively greater importance, as they
do in the near-surface chlorophyll maxima of upwelling cells close to the coast. In such
situations, production rates of around 1–2 gC m
−2 day
−1 are commonly observed, of
which about 50% is generated by the pico fraction, about 10% by the nano fraction, and
about 40% by larger cells. Offshore, in oligotrophic situations this is reversed: from 60 to
75% of the daily production of 0.4 to 05 gC m
−2 d
−1 is generated by picoplankton, principally phycoerythrin-rich chlorococcoid cyanobacteria and prochlorophytes that occur
at concentrations of ∼110 cells liter
−1 .
Dilution experiments (Landry et al., 1998) showed that phytoplankton growth in the
upwelling areas during the Southwest Monsoon is based on a division rate 09–22 d
−1
compared with < 05 d
−1 in the oligotrophic region offshore; enhanced growth rates
inshore during the Northeast Monsoon were significantly lower, at 03–13 d
−1 . Microzooplankton grazing rate was of order 45–50% of phytoplankton growth rate inshore during
both monsoons, compared with parity in the oligotrophic condition. The relationship
between phytoplankton growth rate and NO 3 concentration in these same experiments
show a discontinuity at ∼02 M NO 3 , below which growth rates fall very rapidly to very
small numbers, and above which high growth rates are sustained. The highest growth
rate observed (27 d
−1 ), at the inshore station off Ras al Hadd in August-September represents, as Landry et al. point out, almost four doublings per day. This is the highest rate
observed anywhere by the dilution technique and approaches the maximum potentially
observed in cultures: it is ascribable to “the unique combination of high temperature
(>22
C), high nutrients (>19 M NO 3 ) and the almost complete dominance of a single
fast-growing diatom species, Chaetoceros curvisetus.” The imbalance in such situations
between herbivore consumption and production is such that the equivalent of one doubling per day is left unconsumed. Smith et al. (1998b) suggest that no more than 70% of
daily primary production is likely to be consumed by mesozooplankton in the upwelling
regions.
This analysis reflects the established view of the response of the autotrophic cells
to upwelling in this province, that of a “classical” upwelling process in which nutrient limitation during the oligotrophic, nonupwelling season is relieved by the vertical
entrainment of a sufficient supply of micro- and macronutrients to induce a change of
state in the phytoplankton cells whose rate of increase, to some extent, then escapes from
the balance previously struck with herbivore consumption. However, the JGOFS studies
suggested a novel mechanism to Marra and Barber (2005), who participated in the work
at sea and who suggest (to use their own words) that “Vertical mixing dilutes both
phytoplankton and micrograzers alike, but since phytoplankton are not mixed to greater
than their critical depth, they continue to grow but experience less grazing pressure”; this,
Marra and Barber suggest, is a sufficient explanation of the observed changes in phytoplankton biomass associated with regional changes in mixed-layer depth. Differential
growth rates of autotrophs and consumers had been evoked earlier by Goericke (2002)
to support a suggestion that top-down effects were responsible for the seasonal changes
in phytoplankton biomass in the Arabian Sea. This mechanism must form part of the
complex processes that control the dynamics of production and consumption after a
deepening of the mixed layer here as elsewhere, but I believe that the relative importance