Dominance of microflagellates over diatoms in the Antarctic areas of deep vertical mixing and krill concentrations

Phytoplankton data obtained during six summer Polish expeditionsto the Antarctic Peninsula area, are compared with concurrentlyrecorded data on water column stabilities and krill abundance.The results show that flagellates (1.5–20 µm) arenumerically dominant over diatoms in the areas of deep verticalmixing and/or extensive krill concentrations. Of 102 stationsdominated by flagellates, 85 (83.3%) are located in a well mixedwater column (>100 m) and correspond to a mean krill densityof 15–346 t Nm^–2. In the same areas, estimated flagellatecarbon biomass exceeds diatom carbon. On the other hand, ofthe 40 stations dominated by diatoms, 36 (90%) are located inareas of increased water column stability (upper mixed layerof 10–50 m) and correspond to a low mean krill biomassof 0.34–4.6 t Nm^–2. Positive correlations of flagellateto diatom (F:D) cell number ratios with the depth of the uppermixed layer suggest light limitation of diatom growth and anincreased sinking rate of diatoms relative to flagellates inthe areas of deep vertical mixing. The relationship of the F:Dratio with krill abundance suggests that krill prefer feedingon diatoms and are less efficient in grazing particles of thesize of microflagellates (<20 µm). Flagellates exceeddiatoms in an unstable water column when the phytoplankton populationsare low; both algal groups increase in numbers with growingstability. The results provide field evidence that deep verticalmixing and krill grazing create conditions for the dominanceof flagellates over diatoms. Both factors acting together arelikely to suppress diatom blooms in the Antarctic.