ANNUAL AND INTERANNUAL VARIABILITY IN PHYTOPLANKTON AT A PERMANENT STATION OFF KERGUELEN ISLANDS, SOUTHERN-OCEAN

From November 1992 to February 1995 a quantitative and qualitative phy toplankton study was conducted at a permanent station (Kerfix) southwe st off the Kerguelen Islands, in the vicinity of the Polar Front (50 d egrees 40'S-68 degrees 25'E). Phytoplankton populations are low in thi s area both during summers and winters. They consist, in order of decr easing cell abundance, of pico- and nanoflagellates (1.5-20 mu m), coc colithophorids (<10 mu m), diatoms (5-80 mu m) and dinoflagellates (6- 60 mu m). Flagellates form the dominant group throughout the year and attain the highest summer average of 3.0 x 10(5) cells l(-1). Next in abundance year-round are coccolithophorids with the dominant Emiliania huxleyi (highest summer 1992 average 1.9 x 10(5) cells l(-1)), diatom s (summer 1992 average 1.0 x 10(5) cells l(-1)) and dinoflagellates (a verage 3.8 x 10(4) cells l(-1)). Winter mean numbers of flagellates an d picoplankton do not exceed 8.4 x 10(4) cells l(-1); those of the thr ee remaining algal groups together attain 2 x 10(4) cells l(-1). Summe r peaks of diatoms and dinoflagellates are mainly due to the larger si ze species (> 20 mu m). The latter group contributes most to the total cell carbon biomass throughout the year. Dominant diatoms during summ er seasons include: Fragilariopsis kerguelensis, Thalassionema nitzsch ioides, Chaetoceros dichaeta, C. atlanticus, Pseudonitzschia heimii, a nd P. barkleyi/lineola. This diatom dominance structure changes from s ummer to summer with only F. kerguelensis and T. nitzschioides retaini ng their first and second positions. Any one of the co-dominant specie s might be absent during some summer period. The variable diatom commu nity structure may be due to southward meandering of the Polar Front b ringing ''warmer'' species from the north, and to the mixing of the wa ter masses in this area. The entire community structure characterized both during summer and winters by the dominance of flagellates can be related to deep mixing (ca. 40-200 m) of the water column as the proba ble controlling factor.