Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

We describe the coupling between upper ocean layer variability and size-fraction- ated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarc- tica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton ( 20 µm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (ZUML), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying ZUML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.