285
and on the continental shelves, where, in addition, "tidal streams ( ... ) equivalent to
hurricane-jorce winds in the atmosphere blowing regularly twice per day" (Simpson, 1981)
occur in some places. Because phytoplankton are mixed down below the critical depth,
irradiance limitation prevails throughout the winter so that no bloom can develop. Stabilization
of the water column in the spring favours a diatom bloom, which takes place sooner or later
in different locations, depending on the time when mixing no longer exceeds the compensation
depth (e.g. Pingree et al., 1976; Morin et al., 1985). More moderate vertical mixing favours
a coccolithophore bloom, often as the second stage after the diatom bloom in the annual cycle.
This bloom is apparently most intense over the European continental slope (Holligan et aI.,
1983), but it is also a major characteristic of the production cycle over large areas of the open
ocean (e.g. Holligan et al., 1988a) and occurs on the continental shelves as well (e.g.
Holligan et al., 1988b). In summer, when full stratification has developed, zooplankton
biomass has increased and the nutrients are exhausted in the surface mixed layer, primary
production is mainly effected by dinoflagellates and smaller flagellates (e.g. Holligan and
Harbour, 1977). In shelf areas where the depth is shallow enough and/or the tidal streams
strong enough for the water column to remain well-mixed year round, the tendency is for a
single phytoplankton maximum, occurring in early summer (e.g. Grall, 1972a, 1972b; Boalch
et al., 1978; Sournia et al. 1987). Diatoms usually dominate this summer bloom, with a
significant contribution by small flagellates and other nano/picoplankton (Atkins, 1945; Grall,
1972b; Klein and Sournia, 1987). In places where a seasonal thermocline develops, a second
bloom may occur in autumn, involving diatoms whose growth responds to the increase in
vertical mixing. One specific case where enhanced phytoplankton growth takes place in late
summer and autumn is the continental slope, where, as exemplified by the European
shelf-break, nutrient supply is brought about by the interaction between wind-induced mixing
and internal tides (e.g. Pingree and Mardell, 1981; Maze et al.; 1986; Le Fevre and Frontier,
1988; Serpette and Maze, 1989). The same apparently exists on the North American side of
the ocean, where similar physical forcing is found (e.g. Sandstrom and Elliott, 1984).
Secondary production also follows a distinctive annual cycle. The situation in the open ocean
has been reviewed by Parsons and Lalli (1988), mainly from data collected at Ocean Weather
Station I, in the North Atlantic Current. The herbivore biomass there is dominated by the
copepod Calanus finmarchicus, which overwinters below the euphotic zone at the last
and on the continental shelves, where, in addition, "tidal streams ( ... ) equivalent to
hurricane-jorce winds in the atmosphere blowing regularly twice per day" (Simpson, 1981)
occur in some places. Because phytoplankton are mixed down below the critical depth,
irradiance limitation prevails throughout the winter so that no bloom can develop. Stabilization
of the water column in the spring favours a diatom bloom, which takes place sooner or later
in different locations, depending on the time when mixing no longer exceeds the compensation
depth (e.g. Pingree et al., 1976; Morin et al., 1985). More moderate vertical mixing favours
a coccolithophore bloom, often as the second stage after the diatom bloom in the annual cycle.
This bloom is apparently most intense over the European continental slope (Holligan et aI.,
1983), but it is also a major characteristic of the production cycle over large areas of the open
ocean (e.g. Holligan et al., 1988a) and occurs on the continental shelves as well (e.g.
Holligan et al., 1988b). In summer, when full stratification has developed, zooplankton
biomass has increased and the nutrients are exhausted in the surface mixed layer, primary
production is mainly effected by dinoflagellates and smaller flagellates (e.g. Holligan and
Harbour, 1977). In shelf areas where the depth is shallow enough and/or the tidal streams
strong enough for the water column to remain well-mixed year round, the tendency is for a
single phytoplankton maximum, occurring in early summer (e.g. Grall, 1972a, 1972b; Boalch
et al., 1978; Sournia et al. 1987). Diatoms usually dominate this summer bloom, with a
significant contribution by small flagellates and other nano/picoplankton (Atkins, 1945; Grall,
1972b; Klein and Sournia, 1987). In places where a seasonal thermocline develops, a second
bloom may occur in autumn, involving diatoms whose growth responds to the increase in
vertical mixing. One specific case where enhanced phytoplankton growth takes place in late
summer and autumn is the continental slope, where, as exemplified by the European
shelf-break, nutrient supply is brought about by the interaction between wind-induced mixing
and internal tides (e.g. Pingree and Mardell, 1981; Maze et al.; 1986; Le Fevre and Frontier,
1988; Serpette and Maze, 1989). The same apparently exists on the North American side of
the ocean, where similar physical forcing is found (e.g. Sandstrom and Elliott, 1984).
Secondary production also follows a distinctive annual cycle. The situation in the open ocean
has been reviewed by Parsons and Lalli (1988), mainly from data collected at Ocean Weather
Station I, in the North Atlantic Current. The herbivore biomass there is dominated by the
copepod Calanus finmarchicus, which overwinters below the euphotic zone at the last