Phytoplankton production characteristics in the Eastern Atlantic and Atlantic sector of the Southern Ocean in October–November 2004

Production parameters of surface phytoplankton were measured along three transects: La Manche-Cape Town (I); Cape Town-54°S (II); 0°-49°W (along 54°S) (III). The Canary upwelling waters were most productive along transect I, where the surface chlorophyll a ( Chl 0 ) and the surface primary production ( PP 0 ) were as high as 4.3 mg/m 3 and 173 mg C/m 3 per day, respectively. Mosaic patterns in the distribution of these parameters were recorded in the northeastern regions of the South Subtropical Anticyclonic Gyre ( Chl 0 = 0.03–0.35 mg/m 3 ; PP 0 = 1.6–12.6 mg C/m 3 per day). Along transect II, the average twofold southward increase in Chl 0 (from 0.2 to 0.4 mg/m 3 ) and the concurrent decline of the phytoplankton assimilation activity ( AN 0 ) resulted in deviations from typical latitudinal changes in PP 0 . At most sites, PP 0 values varied between 6 and 15 mg C/m 3 per day. Negligible changes in Chl 0 (0.36–0.85 mg/m 3 ), PP 0 (8–19 mg C/m 3 per day), and AN 0 (0.7–1.6 mg C/mg chl a per hour) were registered for the oceanic waters along transect III. Along all the transects, PP 0 depended on Chl 0 to a greater extent than AN 0 . The values of the latter parameter were largely determined by the water temperature and showed a slight correlation with the insolation. Along transect II, the integrated primary production ( PP int ) and the layer-integrated chlorophyll a in the upper 200 m ( Chl 0–200 ) generally varied from 180 to 360 mg C/m 2 per day and from 30 to 70 mg/m 2 , respectively. In the Polar Front region, an increase in Chl 0–200 , PP int , Chl 0 , and PP 0 up to respective values of 190 mg/m 2 , 520 mg C/m 2 per day, 1.2 mg/m 3 , and 32 mg C/m 3 per day was observed. A comparison of the water column (0–100 m) stability with the vertical distribution of the primary production and chlorophyll content along transect II implies that the thick (>100 m) upper mixed layer (UML) formed in response to the strong water cooling and wind forcing was largely responsible for the limited primary production in the Subantarctic and Antarctic regions. The large UML thickness resulted in an intense removal of plant cells from the photosynthetic layer and light starvation of a significant (up to 60%) part of the phytoplankton community.