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

• Main Author: Venables, Hugh James
• Format: Thesis
• Language: English
• Published: 2007
• Subjects: Southern Ocean; Crozet Islands
• Online Access: https://eprints.soton.ac.uk/66356/; https://eprints.soton.ac.uk/66356/1/Venables_2007_PhD.pdf

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 and Argo float measurements. Parts of the study area regularly contain phytoplankton concentrations significantly enhanced over the other parts, and over most of the rest of the Southern Ocean, due to natural iron fertilization from the Crozet Islands. Study is made of these contrasts in order to further understand the processes that limit phytoplankton over the mostly High Nutrient Low Chlorophyll (HNLC) Southern Ocean. The longer timescales, larger areas and regularity of naturally iron-fertilized phytoplankton blooms make them preferable to study compared to studies of the effects of iron released from a ship. Data from these experiments, and from other islands in the Southern Ocean, is investigated in the context of the results found around the Crozet Plateau. The circulation patterns around the Crozet Plateau are studied in detail, with particular emphasis on understanding where surface water that has passed close to the Crozet Islands flows, as this is a known source of iron to the surface water. The surface mixed layer depth and incoming irradiance are also studied so that the variation in light availability with time and latitude can be understood. The distribution of phytoplankton is studied through satellite chlorophyll-a images, with values adjusted to match those observed in situ during the research cruises. The initial development of the bloom is controlled by variations in light availability but the advection and lateral mixing of iron is sufficient to explain the spatial extent of the bloom and the variations in peak chlorophyll-a concentrations. The variability of the strength of the bloom is driven mostly by variations in the iron utilization by phytoplankton during the preceding winter and wind-driven variations in the circulation.