Pacific Coastal Biome
393
day. Nanoflagellates dominate during winter but form only 10% of the 2–4 m phytoplankton in summer. During the summer, a very shallow DCM develops at 10–20 m,
below a primary production peak at about 5 m. At other times primary production takes
highest values just under the surface.
Observations of nutrient dynamics, especially rates of nitrate depletion, made along
the Seward line across the wide shelf in the northern part of the province (Childers et al.,
2005) suggest that new production rates here are equivalent to 25–70 M NO 3 m
−1 d
−1
with highest values occurring either in midshelf or at the edge, depending on conditions
obtaining in individual years. Seasonally, productivity on the inner shelf leads productivity
on the outer shelf by about 30 days: March–April compared with April–May, respectively,
although satellite images suggest greater accumulation of surface chlorophyll within the
Alaska Current at the shelf edge, especially in the northern and Aleutian sections of
the shelf.
Recent studies of the comparative distribution of mesozooplankton species finally
unravel some of the uncertainties about their maintenance in appropriate locations in
such a dynamic environment: we are fortunate in having a recent study of the Seward
line in the northern part of the province (Coyle and Pinchuk, 2005) and a more general
regional analysis by Mackas and Coyle (2005). On the anomalously wide northern shelf,
the preferential distribution of neritic and oceanic species of subarctic zooplankton can
be determined perhaps more easily than elsewhere. A neritic community, dominated by
Pseudocalanus spp., Metridia pacifica, Calanus marshallae, and Limacina helicina is most
abundant in the Coastal Current and in the fjords in spring and summer at the period
of maximum freshwater discharge. The oceanic community, beyond the shelf break,
is dominated by Eucalanus bungii, Neocalanus cristatus, and Oithona similis. Midshelf,
between the two salinity fronts, a mixed assemblage occurs in spring, although with lower
biomass than within the Coastal Current assemblage. As the summer advances, and the
low-salinity conditions of the inner shelf expand outward, the distinction between neritic
and midshelf zooplankton assemblages is reduced. More generally, Mackas and Coyle
point out, the major ecotone in this series occurs between oceanic and shelf communities
at the salinity front near the shelf break. The distinctions between oceanic and neritic
assemblages are profound: shelf zooplankton is much smaller, has more generations per
year so that multiple cohorts are often present, may lay dormant eggs, obtains less of
its food from protistan plankton, rarely exhibits ontogenetic vertical migration, and less
consistently exhibits diel vertical migration.
The winter downwelling regime discussed earlier may induce shoreward advection of
oceanic species after these have risen to the surface in early spring, prior to the reduction
of wind stress associated with the breakdown of the Aleutian Low. On the shelf, beyond
the salinity front of the Coastal Current, zooplankton biomass in the mixed assemblage
is therefore dominated by large oceanic species, although numerically neritic species
dominate. Nevertheless, mesozooplankton biomass is highest overall within the Coastal
Current.
The basins and fjords of the continental shelf, like the Straits of Georgia, are able to
support permanent populations of oceanic species provided sufficient depth is available
for their overwintering stages. The winter mesozooplankton here is dominated by small
copepods (Pseudocalanus, Oithona, and the rest), but their development in spring is
swamped by a massive influx of larvae and early copepodite stages of the offshore Neocalanus plumchrus together with immigration, also from the ocean, of Calanus pacificus,
C. marshallae, and M. pacifica. Predators also join the summer community: Sagitta spp.
and amphipods. Neocalanus plumchrus dominates biomass in spring and C. marshallae
dominates somewhat later in summer. Most copepods remain in the upper tens of
meters of the water column, with smaller numbers at about 50 m, whereas Sagitta,
amphipods, coelenterates, and euphausiids form deeper layers fueled largely by sinking
393
day. Nanoflagellates dominate during winter but form only 10% of the 2–4 m phytoplankton in summer. During the summer, a very shallow DCM develops at 10–20 m,
below a primary production peak at about 5 m. At other times primary production takes
highest values just under the surface.
Observations of nutrient dynamics, especially rates of nitrate depletion, made along
the Seward line across the wide shelf in the northern part of the province (Childers et al.,
2005) suggest that new production rates here are equivalent to 25–70 M NO 3 m
−1 d
−1
with highest values occurring either in midshelf or at the edge, depending on conditions
obtaining in individual years. Seasonally, productivity on the inner shelf leads productivity
on the outer shelf by about 30 days: March–April compared with April–May, respectively,
although satellite images suggest greater accumulation of surface chlorophyll within the
Alaska Current at the shelf edge, especially in the northern and Aleutian sections of
the shelf.
Recent studies of the comparative distribution of mesozooplankton species finally
unravel some of the uncertainties about their maintenance in appropriate locations in
such a dynamic environment: we are fortunate in having a recent study of the Seward
line in the northern part of the province (Coyle and Pinchuk, 2005) and a more general
regional analysis by Mackas and Coyle (2005). On the anomalously wide northern shelf,
the preferential distribution of neritic and oceanic species of subarctic zooplankton can
be determined perhaps more easily than elsewhere. A neritic community, dominated by
Pseudocalanus spp., Metridia pacifica, Calanus marshallae, and Limacina helicina is most
abundant in the Coastal Current and in the fjords in spring and summer at the period
of maximum freshwater discharge. The oceanic community, beyond the shelf break,
is dominated by Eucalanus bungii, Neocalanus cristatus, and Oithona similis. Midshelf,
between the two salinity fronts, a mixed assemblage occurs in spring, although with lower
biomass than within the Coastal Current assemblage. As the summer advances, and the
low-salinity conditions of the inner shelf expand outward, the distinction between neritic
and midshelf zooplankton assemblages is reduced. More generally, Mackas and Coyle
point out, the major ecotone in this series occurs between oceanic and shelf communities
at the salinity front near the shelf break. The distinctions between oceanic and neritic
assemblages are profound: shelf zooplankton is much smaller, has more generations per
year so that multiple cohorts are often present, may lay dormant eggs, obtains less of
its food from protistan plankton, rarely exhibits ontogenetic vertical migration, and less
consistently exhibits diel vertical migration.
The winter downwelling regime discussed earlier may induce shoreward advection of
oceanic species after these have risen to the surface in early spring, prior to the reduction
of wind stress associated with the breakdown of the Aleutian Low. On the shelf, beyond
the salinity front of the Coastal Current, zooplankton biomass in the mixed assemblage
is therefore dominated by large oceanic species, although numerically neritic species
dominate. Nevertheless, mesozooplankton biomass is highest overall within the Coastal
Current.
The basins and fjords of the continental shelf, like the Straits of Georgia, are able to
support permanent populations of oceanic species provided sufficient depth is available
for their overwintering stages. The winter mesozooplankton here is dominated by small
copepods (Pseudocalanus, Oithona, and the rest), but their development in spring is
swamped by a massive influx of larvae and early copepodite stages of the offshore Neocalanus plumchrus together with immigration, also from the ocean, of Calanus pacificus,
C. marshallae, and M. pacifica. Predators also join the summer community: Sagitta spp.
and amphipods. Neocalanus plumchrus dominates biomass in spring and C. marshallae
dominates somewhat later in summer. Most copepods remain in the upper tens of
meters of the water column, with smaller numbers at about 50 m, whereas Sagitta,
amphipods, coelenterates, and euphausiids form deeper layers fueled largely by sinking