Atlantic Polar Biome
139
equal density, but of different T and S to produce water of greater density). Upwelling
at ice fronts by this mechanism occurs very commonly in BPLR, even alongside large
free-floating icebergs.
More Extensive Open Water Situations Where extensive open water develops during
summer, as in Baffin Bay and the Chukchi Sea, a “spring” bloom occurs in the open
water, although it is not long sustained because nutrients are rapidly stripped from the
surface brackish layer. Strong blooms, sustained by topographic upwelling, occur in many
places in the Canadian archipelago, notably in Lancaster Sound. Here, the chlorophyll
maximum occurs consistently during summer between the 1% and 10% light levels,
usually near the base of the pycnocline, at which depth a nitracline also occurs. The
timing of the bloom is dependent, naturally, on the schedule of the breakup of fast ice
and hence of local “spring”; in Barrow Straits, for example, this occurs only in July.
It is largely by virtue of its relatively early opening that the North Water “polynya” is
exceptional in the Canadian region. On this timing, of course, depends the instantaneous
state of development of the entire planktonic ecosystem; the recruitment of the first
cohort of copepodites of Calanus and Pseudocalanus occurs 1.5–3.0 months earlier in the
North Water than in the Barrow Strait (74
N) (Ringuette et al., 2002).
A “typical arctic structure” for phytoplankton has been described from the open
North Water region in summer (Harrison et al., 1987): chlorophyll maxima consistently
occur between the 10% light level and the bottom of the photic zone, itself usually
deeper than the mixed layer. These maxima are usually six times the surface values of
around 125 mg chl m
3 . Below the photic zone, enhanced microbial activity associated
with aggregated sinking cells reaches maxima usually below 500 m. NO 3 values are often
below the limit of detection in the mixed layer and a nutricline is coincident with
the pycnocline. Reduced nitrogen is relatively available throughout the summer and
is importantly utilized by autotrophs, a fact apparently missed by earlier investigators
(Harrison et al., 1982). The photosynthetic index (PI), as gC (g chl)
−1 d
−1 , of arctic
open-water phytoplankton takes “normal” values (giving a mean of PI = 67 in a large
data set), but very low values obtain below the Arctic Ocean ice cover, where values of
PI < 05 are more typical.
Diatoms and coccolithophores were long assumed to dominate polar phytoplankton
under these open-water conditions, at concentrations reaching 750 and 925 × 10
3 cells
liter
−1 , respectively, but recent work has revealed that a substantial proportion of chlorophyll, cell numbers, and RuBPC activity are actually contributed by the pico fraction, just
as in other seas. These small cells perform 10–25% of all carbon fixation in polar seas
compared with 20–30% in mid-latitudes and >50% in the tropical ocean (Trotte, 1985).
Generally, 60% of polar algal biomass is contributed by >35-mm cells, whereas >50% of
respiration is contributed by <1-mm cells (bacteria and other microheterotrophs). Limited by low temperatures are the prokaryotic cyanobacteria and prochlorophytes, which
occur only in relatively low abundance and serve as a biological marker for the transport
of southern water in summer. No novel physiological mechanisms need be invoked to
explain the success of arctic phytoplankton in extremely cold water and it would, in fact,
have been possible to predict their performance by extrapolation from what is known of
the physiology of temperate-zone organisms (Li and Dickie, 1984).
The general ecology of arctic zooplankton (especially pteropods, copepods, and
euphausiids) has been well understood for many years. Growth rates are very slow, individuals are large compared with congeners in warm seas, seasonal ontogenetic migration
dominates vertical distribution, and copepods dominate the total biomass. Copepods
form >85% numerically of all mesoplankton and (as carbon biomass) polar zooplankton
have the following composition: 70% copepods, 11% pteropods, 10% amphipods, and
lesser amounts of ostracods, coelenterates, and appendicularians among the major taxa.
139
equal density, but of different T and S to produce water of greater density). Upwelling
at ice fronts by this mechanism occurs very commonly in BPLR, even alongside large
free-floating icebergs.
More Extensive Open Water Situations Where extensive open water develops during
summer, as in Baffin Bay and the Chukchi Sea, a “spring” bloom occurs in the open
water, although it is not long sustained because nutrients are rapidly stripped from the
surface brackish layer. Strong blooms, sustained by topographic upwelling, occur in many
places in the Canadian archipelago, notably in Lancaster Sound. Here, the chlorophyll
maximum occurs consistently during summer between the 1% and 10% light levels,
usually near the base of the pycnocline, at which depth a nitracline also occurs. The
timing of the bloom is dependent, naturally, on the schedule of the breakup of fast ice
and hence of local “spring”; in Barrow Straits, for example, this occurs only in July.
It is largely by virtue of its relatively early opening that the North Water “polynya” is
exceptional in the Canadian region. On this timing, of course, depends the instantaneous
state of development of the entire planktonic ecosystem; the recruitment of the first
cohort of copepodites of Calanus and Pseudocalanus occurs 1.5–3.0 months earlier in the
North Water than in the Barrow Strait (74
N) (Ringuette et al., 2002).
A “typical arctic structure” for phytoplankton has been described from the open
North Water region in summer (Harrison et al., 1987): chlorophyll maxima consistently
occur between the 10% light level and the bottom of the photic zone, itself usually
deeper than the mixed layer. These maxima are usually six times the surface values of
around 125 mg chl m
3 . Below the photic zone, enhanced microbial activity associated
with aggregated sinking cells reaches maxima usually below 500 m. NO 3 values are often
below the limit of detection in the mixed layer and a nutricline is coincident with
the pycnocline. Reduced nitrogen is relatively available throughout the summer and
is importantly utilized by autotrophs, a fact apparently missed by earlier investigators
(Harrison et al., 1982). The photosynthetic index (PI), as gC (g chl)
−1 d
−1 , of arctic
open-water phytoplankton takes “normal” values (giving a mean of PI = 67 in a large
data set), but very low values obtain below the Arctic Ocean ice cover, where values of
PI < 05 are more typical.
Diatoms and coccolithophores were long assumed to dominate polar phytoplankton
under these open-water conditions, at concentrations reaching 750 and 925 × 10
3 cells
liter
−1 , respectively, but recent work has revealed that a substantial proportion of chlorophyll, cell numbers, and RuBPC activity are actually contributed by the pico fraction, just
as in other seas. These small cells perform 10–25% of all carbon fixation in polar seas
compared with 20–30% in mid-latitudes and >50% in the tropical ocean (Trotte, 1985).
Generally, 60% of polar algal biomass is contributed by >35-mm cells, whereas >50% of
respiration is contributed by <1-mm cells (bacteria and other microheterotrophs). Limited by low temperatures are the prokaryotic cyanobacteria and prochlorophytes, which
occur only in relatively low abundance and serve as a biological marker for the transport
of southern water in summer. No novel physiological mechanisms need be invoked to
explain the success of arctic phytoplankton in extremely cold water and it would, in fact,
have been possible to predict their performance by extrapolation from what is known of
the physiology of temperate-zone organisms (Li and Dickie, 1984).
The general ecology of arctic zooplankton (especially pteropods, copepods, and
euphausiids) has been well understood for many years. Growth rates are very slow, individuals are large compared with congeners in warm seas, seasonal ontogenetic migration
dominates vertical distribution, and copepods dominate the total biomass. Copepods
form >85% numerically of all mesoplankton and (as carbon biomass) polar zooplankton
have the following composition: 70% copepods, 11% pteropods, 10% amphipods, and
lesser amounts of ostracods, coelenterates, and appendicularians among the major taxa.