8
blooms form when the pyknoc1ine is not deeper than 40-S0m.
In all these
cases Sverdrup's concept of critical depth may serve as an Occam's razor,
i.e. explaining a maximum of phenomena with the simplest possible theory.
The scheme treated above is simplistic and real only in the most
general of terms. The hydrography along the ice edge may be extremely
complicated with upwelling and downwelling depending on the weather
(Buckley et al. 1979, Johannesen et al. 1983), and the ice edge itself
may be far from well defined for the very same reasons (Vinje 1977, Wadhams 1981). Blooms may be entirely absent if strong winds erode the
stability as soon as it is formed.
The need for water column stability has also been broughb into the
discussion of blooms in the Antarctic Ocean. The idea was brought up
first by Gran (1931) in connection with studies of the Weddell Sea where
he assumed that stability was formed for a limited period by melting ice.
Hart (1934, 1942) and Has1e (1969) have brought strong cases for this
idea. There seems a priori no reason to believe that the Arctic and
Antarctic Oceans are d'ifferent with this respect save that in the Arctic
the large ,amount of freshwater from rivers may also cause some additional
stability. Both Hart and Has1e have explained the low phytoplankton
stock at the Polar Front as a result of deep-reaching turbulence, and
the delay of the spring bloom with increasing latitude may be explained
by delayed melting of ice. On the VULCAN cruises (Holm-Hansen and Foster
1981) it was found that for all blooms (10 stations with >2 ~g ch1. 1- 1 )
the pykno1ine was situated at 20-40m, and SOm may be the maximum. depth
allowing blooms, just as in the Arctic Ocean. The importance of pyknocline depth is difficult to ascertain, however, without knowing the rate
of vertical movement of the phytoplankton through the submarine light
gradient (Falkowski and Wirick 1981). Unfortunately there are no convenient methods to determine the rates of such vertical motion.
When a spring bloom ends in the Arctic,it is probably due to nutrient depletion and grazing. Nutrient depletion is hardly likely in the
Antarctic. Stability erosion combined with heavy grazing is a more
likely proposal.
It is an open question as to how long stability formed
by ice melting may last. Quite obviously it is broken down in offshore
waters well before a bloom approaches nutrient depletion. Depending
upon the 'growth rate (0.4 or 0.2 div.day-1) it will take a bloom 16 or
30 days to develop from 0.1 to 8 ~g ch1. 1- 1 . Assuming optimum conditions for rapid growth the shorter period may not be too far from reality.
Based on a numerical plankton model for the Barents Sea (Slagstad 1982),
it appears that sudden erosion of stability (due to a sudden storm, for instance) combined with heavy grazing and rapid sinking may break down
a bloom in a matter of few days, whereas a bloom will' persist for several
blooms form when the pyknoc1ine is not deeper than 40-S0m.
In all these
cases Sverdrup's concept of critical depth may serve as an Occam's razor,
i.e. explaining a maximum of phenomena with the simplest possible theory.
The scheme treated above is simplistic and real only in the most
general of terms. The hydrography along the ice edge may be extremely
complicated with upwelling and downwelling depending on the weather
(Buckley et al. 1979, Johannesen et al. 1983), and the ice edge itself
may be far from well defined for the very same reasons (Vinje 1977, Wadhams 1981). Blooms may be entirely absent if strong winds erode the
stability as soon as it is formed.
The need for water column stability has also been broughb into the
discussion of blooms in the Antarctic Ocean. The idea was brought up
first by Gran (1931) in connection with studies of the Weddell Sea where
he assumed that stability was formed for a limited period by melting ice.
Hart (1934, 1942) and Has1e (1969) have brought strong cases for this
idea. There seems a priori no reason to believe that the Arctic and
Antarctic Oceans are d'ifferent with this respect save that in the Arctic
the large ,amount of freshwater from rivers may also cause some additional
stability. Both Hart and Has1e have explained the low phytoplankton
stock at the Polar Front as a result of deep-reaching turbulence, and
the delay of the spring bloom with increasing latitude may be explained
by delayed melting of ice. On the VULCAN cruises (Holm-Hansen and Foster
1981) it was found that for all blooms (10 stations with >2 ~g ch1. 1- 1 )
the pykno1ine was situated at 20-40m, and SOm may be the maximum. depth
allowing blooms, just as in the Arctic Ocean. The importance of pyknocline depth is difficult to ascertain, however, without knowing the rate
of vertical movement of the phytoplankton through the submarine light
gradient (Falkowski and Wirick 1981). Unfortunately there are no convenient methods to determine the rates of such vertical motion.
When a spring bloom ends in the Arctic,it is probably due to nutrient depletion and grazing. Nutrient depletion is hardly likely in the
Antarctic. Stability erosion combined with heavy grazing is a more
likely proposal.
It is an open question as to how long stability formed
by ice melting may last. Quite obviously it is broken down in offshore
waters well before a bloom approaches nutrient depletion. Depending
upon the 'growth rate (0.4 or 0.2 div.day-1) it will take a bloom 16 or
30 days to develop from 0.1 to 8 ~g ch1. 1- 1 . Assuming optimum conditions for rapid growth the shorter period may not be too far from reality.
Based on a numerical plankton model for the Barents Sea (Slagstad 1982),
it appears that sudden erosion of stability (due to a sudden storm, for instance) combined with heavy grazing and rapid sinking may break down
a bloom in a matter of few days, whereas a bloom will' persist for several