(Table 1) Productive characteristics of phytoplankton and some relevant factors in the North Atlantic in September-October 1991

During the study period values of primary production in the water column (C_pc) and average chlorophyll a concentration in the photosynthetic layer along the transect across the North Atlantic Current (NAC) were 140-348 mg C/m2/day and 0.15-0.47 µg/l, along the transect across the Gulf Stream these values were 201-407 mg C/m2/day and 0.13-0.24 µg/l; at stations of the sub-latitudinal (21°-28°N) transect across the Atlantic Ocean they were 95-360 mg C/m2/day and 0.09-0.20 µg/l; and in the region of the Canary upwelling values were 951-1740 mg C/m2/day and 0.56-5.35 µg/l. Along the transects across NAC and across the Atlantic Ocean the variability of C_pc signifi¬cantly depended on level of incident solar radiation. Transition from sub-arctic high-productive coastal waters to oceanic subtropical waters was accompanied by a downwelling of the main mass of chlo¬rophyll from the upper layers of the photosynthetic zone to its lower layers; this resulted in slower decrease of primary production with depth and in more effective utilization of solar radiation by phytoplankton. The photosynthetic layer at both oceanic and coastal stations was limited by irradiance level about 0.1% of its subsurface PAR values (E_0). In the subtropical waters, a deep chlorophyll maximum was formed at depths 100-120 m at low irradiance (0.4-1.2% E_0) and at nutrient concentrations (NO3 + NO2, PO4, Si) exceeding their very low concentrations in the upper 100 m layer by factor ~1.5.