Atlantic Coastal Biome
253
clearly shown by between-year comparisons of satellite images to respond to the mesoscale
pattern in the cyclonic circulation around the Gulf. The response of phytoplankton growth
rate to dynamic physical events at the mesoscale is self-evident: a quick comparison of a
few pairs of matched SST and surface chlorophyll images is worth a thousand words.
The regional and between-year variability that we can now observe makes it all the
more satisfactory that the early serial data obtained across this region by the Continuous
Plankton Recorder (CPR) does yield apparently correct seasonal patterns. The CPR
database (Colebrook, 1979) differentiates seasonal cycles for (i) the Grand Banks (strong
spring blooms, weak autumn blooms), (ii) the Scotian Shelf (weaker and more similar
spring and autumn blooms), and (iii) the Cape Cod region (algal abundance barely higher
in summer than winter). The CPR algal data are based only on large phytoplankton cells
but, even so, they suggest that the physical processes discussed previously do determine
the regional pattern of algal growth dynamics in the province and that the pattern of
chlorophyll enhancement observed in the SeaWiFS images can mostly be related to frontal
dynamics.
A spring bloom occurs over the whole western Gulf of St. Lawrence as soon as ice
cover clears in spring, and algal growth is no longer light-limited; it has particularly high
values (<200 mg C m
−2 hr
−1 ) in the region of upwelling and also further down the Gulf
where it is reinforced by strong tidal mixing over the Magdalen shallows, and also in the
region of the tidal front between Anticosti Island and the Gaspé peninsula. Knowledge
of the pelagic ecology of this region was advanced by the Canadian JGOFS process study
performed there in 1992–94 (Roy et al., 2000a). This revealed yet another example of the
now-familiar pattern of seasonality in the large and small autotrophic cell fractions; the
small fraction forms a “background” of autotrophy that has a relatively small seasonal
cycle of relative abundance, because the growth of protistan herbivores is so closely
matched to autotrophic cell-division rate. Against this background, episodic blooms of
larger (>5 m) cells are responsible for the much more variable total chlorophyll biomass.
At least in part, the strength of the chlorophyll enhancement here is a consequence of the
climatological strength of wind mixing. Strong mixing induces strong nutrient flux from
below, but limits the access of autotrophic cells to surface light, whereas weak mixing
restricts the cells to a shallow, well-lit mixed layer with weak nutrient flux: moderate
mixing, says Doyon, et al (2000), is responsible for strong phytoplankton growth here.
The pelagic ecosystem of the Gulf of St. Lawrence is dominated during winter and
spring by large phytoplankton cells that are heavily grazed by mesozooplankton, whereas
the heterotrophic loop dominates in summer when nanophytoplankton and heterotrophic
dinoflagellates and ciliates replace the diatoms and copepods of winter. Despite this, the
flux of sinking organic material is approximately similar summer and winter (Savenkoff
et al., 2000). Of the total POC flux, ∼50% comprised mesozooplankton fecal pellets—with
some seasonality—while <10% comprised intact phytoplankton cells. The composition
of this flux responds to the taxonomic composition of the superjacent zooplankton
population, so that where the smaller copepods (e.g., Temora) were more relatively
numerous, the fecal pellet fraction was relatively larger (Roy et al., 2000b).
In the Gulf of Maine (see Townsend et al., 1992) upwelling occurs at many small
estuarine fronts and is thought to be a major factor in overall productivity, because
the water in the deep basins is constantly renewed by episodic pumping from deep
slope-water sources. However, the effects of tidal mixing dominate the distribution and
strength of algal blooms, and the principles discussed already for the northeast Atlantic
continental shelf (see Northeast Atlantic Shelves Province) apply here. Blooms are initially
light limited in tidally mixed areas, then persist longer because of the constant renewal of
nutrients due to benthic regeneration. In stratified areas, an oligotrophic profile rapidly
forms when the initial nutrient charge is utilized. In winter, very dense aggregations of C.
finmarchicus and the euphausiid M. norvegica occur in the deep troughs and basins, both
253
clearly shown by between-year comparisons of satellite images to respond to the mesoscale
pattern in the cyclonic circulation around the Gulf. The response of phytoplankton growth
rate to dynamic physical events at the mesoscale is self-evident: a quick comparison of a
few pairs of matched SST and surface chlorophyll images is worth a thousand words.
The regional and between-year variability that we can now observe makes it all the
more satisfactory that the early serial data obtained across this region by the Continuous
Plankton Recorder (CPR) does yield apparently correct seasonal patterns. The CPR
database (Colebrook, 1979) differentiates seasonal cycles for (i) the Grand Banks (strong
spring blooms, weak autumn blooms), (ii) the Scotian Shelf (weaker and more similar
spring and autumn blooms), and (iii) the Cape Cod region (algal abundance barely higher
in summer than winter). The CPR algal data are based only on large phytoplankton cells
but, even so, they suggest that the physical processes discussed previously do determine
the regional pattern of algal growth dynamics in the province and that the pattern of
chlorophyll enhancement observed in the SeaWiFS images can mostly be related to frontal
dynamics.
A spring bloom occurs over the whole western Gulf of St. Lawrence as soon as ice
cover clears in spring, and algal growth is no longer light-limited; it has particularly high
values (<200 mg C m
−2 hr
−1 ) in the region of upwelling and also further down the Gulf
where it is reinforced by strong tidal mixing over the Magdalen shallows, and also in the
region of the tidal front between Anticosti Island and the Gaspé peninsula. Knowledge
of the pelagic ecology of this region was advanced by the Canadian JGOFS process study
performed there in 1992–94 (Roy et al., 2000a). This revealed yet another example of the
now-familiar pattern of seasonality in the large and small autotrophic cell fractions; the
small fraction forms a “background” of autotrophy that has a relatively small seasonal
cycle of relative abundance, because the growth of protistan herbivores is so closely
matched to autotrophic cell-division rate. Against this background, episodic blooms of
larger (>5 m) cells are responsible for the much more variable total chlorophyll biomass.
At least in part, the strength of the chlorophyll enhancement here is a consequence of the
climatological strength of wind mixing. Strong mixing induces strong nutrient flux from
below, but limits the access of autotrophic cells to surface light, whereas weak mixing
restricts the cells to a shallow, well-lit mixed layer with weak nutrient flux: moderate
mixing, says Doyon, et al (2000), is responsible for strong phytoplankton growth here.
The pelagic ecosystem of the Gulf of St. Lawrence is dominated during winter and
spring by large phytoplankton cells that are heavily grazed by mesozooplankton, whereas
the heterotrophic loop dominates in summer when nanophytoplankton and heterotrophic
dinoflagellates and ciliates replace the diatoms and copepods of winter. Despite this, the
flux of sinking organic material is approximately similar summer and winter (Savenkoff
et al., 2000). Of the total POC flux, ∼50% comprised mesozooplankton fecal pellets—with
some seasonality—while <10% comprised intact phytoplankton cells. The composition
of this flux responds to the taxonomic composition of the superjacent zooplankton
population, so that where the smaller copepods (e.g., Temora) were more relatively
numerous, the fecal pellet fraction was relatively larger (Roy et al., 2000b).
In the Gulf of Maine (see Townsend et al., 1992) upwelling occurs at many small
estuarine fronts and is thought to be a major factor in overall productivity, because
the water in the deep basins is constantly renewed by episodic pumping from deep
slope-water sources. However, the effects of tidal mixing dominate the distribution and
strength of algal blooms, and the principles discussed already for the northeast Atlantic
continental shelf (see Northeast Atlantic Shelves Province) apply here. Blooms are initially
light limited in tidally mixed areas, then persist longer because of the constant renewal of
nutrients due to benthic regeneration. In stratified areas, an oligotrophic profile rapidly
forms when the initial nutrient charge is utilized. In winter, very dense aggregations of C.
finmarchicus and the euphausiid M. norvegica occur in the deep troughs and basins, both