Phytoplankton blooms in the Ross Sea, Antarctica: Interannual variability in magnitude, temporal patterns, and composition

The continental shelf of the Ross Sea, Antarctica, is a unique region within the Southern Ocean. Phytoplankton growth is believed to be seasonally limited, first in austral spring by irradiance, and then in summer by biologically available iron. It also is historically known to have taxonomically distinct regimes: the south‐central portion is dominated by Phaeocystis antarctica and to the west diatoms are abundant. We measured photochemical yield to interpret the health of the phytoplankton assemblage from 2001–2004 and interfaced these measurements with satellite remote sensing of pigments. The bloom of 2001–2002 was similar in both temporal and spatial distributions to the climatological mean of the Ross Sea, with a peak in biomass being observed in mid‐December within the Ross Sea polynyas; Fv/Fm values averaged 0.43. We found high (0.50–0.65) Fv/Fm for most of the seasonal phytoplankton bloom for 2002–2003, suggesting that it was not seasonally iron limited. An unusual, large bloom occurred during 2003–2004, with an initial bloom of P. antarctica during austral spring followed by an extensive diatom bloom in summer that may have been enhanced by an intrusion of modified circumpolar deep water. On the basis of an analysis of the historical SeaWiFS records, accumulation of phytoplankton biomass in February may occur approximately every 2–4 years, potentially being a significant source of carbon on the continental shelf.