Physical controls on the distribution of phytoplankton round the Crozet Plateau, Southern Ocean

Abstract

The spatial and temporal distribution of phytoplankton around the Crozet Plateau,Southern Ocean, is studied through a variety of satellite datasets and in situ cruise andArgo float measurements. Parts of the study area regularly contain phytoplanktonconcentrations significantly enhanced over the other parts, and over most of the rest ofthe Southern Ocean, due to natural iron fertilization from the Crozet Islands. Study ismade of these contrasts in order to further understand the processes that limitphytoplankton over the mostly High Nutrient Low Chlorophyll (HNLC) Southern Ocean.The longer timescales, larger areas and regularity of naturally iron-fertilizedphytoplankton blooms make them preferable to study compared to studies of the effectsof iron released from a ship. Data from these experiments, and from other islands in theSouthern Ocean, is investigated in the context of the results found around the CrozetPlateau.The circulation patterns around the Crozet Plateau are studied in detail, with particularemphasis on understanding where surface water that has passed close to the CrozetIslands flows, as this is a known source of iron to the surface water. The surface mixedlayer depth and incoming irradiance are also studied so that the variation in lightavailability with time and latitude can be understood. The distribution of phytoplankton isstudied through satellite chlorophyll-a images, with values adjusted to match thoseobserved in situ during the research cruises.The initial development of the bloom is controlled by variations in light availability butthe advection and lateral mixing of iron is sufficient to explain the spatial extent of thebloom and the variations in peak chlorophyll-a concentrations. The variability of thestrength of the bloom is driven mostly by variations in the iron utilization byphytoplankton during the preceding winter and wind-driven variations in the circulation