THE PRODUCTION O F MARINE I’LANKTON
151
appears to coincide with the period of stability. Holmes (1956) finds for
the Labrador Sea that from November to April there is very little
phytoplankton with the marked instability of the water column. Over
the May/June period, increasing stability rtnd increased radiation
apparently are mainly responsible for the phjtoplankton peak. In the
Arctic waters off Bear Island, Marshall (19,58) showed that during
March/April the mixed layer shallowed to less than the critical depth
and effective production of phytoplankton commenced, whereas in the
warmer Atlantic water nearby, the mixed layer only became shallower
than the critical depth by about May and June, and phytoplankton
production was correspondingly delayed. At these high latitudes the
stability of the water in late spring may be due in part to the melting of
ice; the phytoplankton outburst in polar regions frequently seems to
follow the melting ice edge (cf. Braarud, 1935; Hart, 1934). Zenkevitch
(1963) also mentions that in Arctic seas the algal bloom follows the ice
melt and believes that this is due t o the importance of critical depth
exceeding the mixed turbulent layer.
The vernal blooming of phytoplankton, while mainly due to increasing light intensity and length of daylight, is tlius dependent to a considerable extent on stabilization. Some of the cliscrepancies in the time
of commencement of the spring increase, observed by many workers
(e.g. Kreps and Verjbinskaya, 1932; Corlett, 1953; Conover, 1956; Fish,
1925; Bigelow et al., 1940) may be explained as due to differences in stabilization of the water column. Strong winds will reduce stability; thus
shelteredareasmay bloom earlier than exposedregionsat similarlatitudes.
The continued rise in temperature during late spring and early
summer in temperate regions causes increased stratification of the
water, so that typically a marked seasonal thermocline exists over the
summer. In normal seas this thermocline restricts the continued supply
of essential nutrients to the euphotic zone where they are being extensively utilized. Reduced surface salinity with a stable water column
may also be detrimental to nutrient replenishment later in the season
when nitrogen and phosphorus have been extensively utilized, and thus
may lower productivity. Steemann Nielsen (1 958) has shown that in
coastal areas off Greenland, zones of very low piioduction are associated
with lowered salinity surface water which prevents the vertical transport
of nutrient rich water from below. Where there is considerable mixing of
the water layers, a high production follows. Anderson (1964) refers to
the outflow of the Columbia River off the M’ashington and Oregon
coasts, producing a permanent halocline. This may effectively reduce
the thickness of the mixed layer in winter, but in summer, with the
addition of a seasonal thermocline, may act ;LS a barrier to vertical
movement of nutrients.
151
appears to coincide with the period of stability. Holmes (1956) finds for
the Labrador Sea that from November to April there is very little
phytoplankton with the marked instability of the water column. Over
the May/June period, increasing stability rtnd increased radiation
apparently are mainly responsible for the phjtoplankton peak. In the
Arctic waters off Bear Island, Marshall (19,58) showed that during
March/April the mixed layer shallowed to less than the critical depth
and effective production of phytoplankton commenced, whereas in the
warmer Atlantic water nearby, the mixed layer only became shallower
than the critical depth by about May and June, and phytoplankton
production was correspondingly delayed. At these high latitudes the
stability of the water in late spring may be due in part to the melting of
ice; the phytoplankton outburst in polar regions frequently seems to
follow the melting ice edge (cf. Braarud, 1935; Hart, 1934). Zenkevitch
(1963) also mentions that in Arctic seas the algal bloom follows the ice
melt and believes that this is due t o the importance of critical depth
exceeding the mixed turbulent layer.
The vernal blooming of phytoplankton, while mainly due to increasing light intensity and length of daylight, is tlius dependent to a considerable extent on stabilization. Some of the cliscrepancies in the time
of commencement of the spring increase, observed by many workers
(e.g. Kreps and Verjbinskaya, 1932; Corlett, 1953; Conover, 1956; Fish,
1925; Bigelow et al., 1940) may be explained as due to differences in stabilization of the water column. Strong winds will reduce stability; thus
shelteredareasmay bloom earlier than exposedregionsat similarlatitudes.
The continued rise in temperature during late spring and early
summer in temperate regions causes increased stratification of the
water, so that typically a marked seasonal thermocline exists over the
summer. In normal seas this thermocline restricts the continued supply
of essential nutrients to the euphotic zone where they are being extensively utilized. Reduced surface salinity with a stable water column
may also be detrimental to nutrient replenishment later in the season
when nitrogen and phosphorus have been extensively utilized, and thus
may lower productivity. Steemann Nielsen (1 958) has shown that in
coastal areas off Greenland, zones of very low piioduction are associated
with lowered salinity surface water which prevents the vertical transport
of nutrient rich water from below. Where there is considerable mixing of
the water layers, a high production follows. Anderson (1964) refers to
the outflow of the Columbia River off the M’ashington and Oregon
coasts, producing a permanent halocline. This may effectively reduce
the thickness of the mixed layer in winter, but in summer, with the
addition of a seasonal thermocline, may act ;LS a barrier to vertical
movement of nutrients.