Phytoplankton Size-Fractionated Biomass in the Ross Sea: Spatial and Temporal Variations During the Austral Spring

Abstract

The present work analyzes, in terms of pigment content, the spatial and temporal variations of the micro, nano- and picoplankton fractions of the Ross Sea phytoplankton biomass. Special attention is given to plankton dynamics in the area of the polynya, during austral spring 1994. In total, 50 stations were sampled in the period from November 14 to December 15. From December 18 to 27, a discrete surface sampling at 60 positions, distributed along a 1200-nautical mile track, was also performed. Three subsystems were identified in the sampling area: the polynya, the marginal-ice zone and the pack-covered waters. In the first subsystem, the microphytoplankton predominated, but its relative abundance was certainly biased by the presence of large quantities of the palmelloid form of Phaeocystis cfr. antarctica. In the marginal ice zone, the micro- and nanophyto-plankton fractions were at first dominated by diatoms, which were replaced after a short time by the picophytoplankton. In the pack-covered waters, the bulk of the biomass consisted of pico-phytoplankton. The ratio phaeo/Ch| a suggests that in these two last subsystems the dimensional structure of phytoplankton is the product of the selective removal of larger cells by grazing, probably by krill. The oscillation of ChI a concentrations between high and low levels throughout the investigated area suggests the presence of recurrent blooms. Biomass concentrations also confirm the key role of the marginal ice zone in setting the conditions for the seasonal evolution of the open water system, even when it does not induce a stabilizing effect on the water column.