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 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.

More information

Identifiers

Catalogue record

Export record