Seasonal shifts in phytoplankton community structure in the Southern Ocean observed from space

Seasonal shifts in phytoplankton community structure in the Southern Ocean observed from space

Marine diatoms are key players in the ocean carbon cycle, responsible for ~ 20% of global marine productivity. In the High-Nitrate Low-Chlorophyll (HNLC) regions of the Southern Ocean (SO), phytoplankton communities are dominated by a diverse assemblage of small eukaryotic cells under low light (winter) and low nutrient (summer) conditions (Bouman et al. 2012).  Yet in the spring period, the increase in availability of light and nutrients, in particular dissolved silica (Si) and Fe, allows large cells (diatoms) to bloom, potentially increasing the efficiency of the SO’s biological pump (Coale et al. 2004).  In this grossly undersampled region of the global ocean, to study the succession in phytoplankton taxonomic composition in response to changes in growth conditions, and its interannual variability, requires sustained large-scale observations which only earth-orbiting satellites can provide.

The student will use data collected as part of the NERC Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth (CUSTARD) project in the Southeast Pacific, where Antarctic Intermediate Waters and SubAntarctic Mode Waters form, along with historical data from other regions of the SO.  The relationships between surface chlorophyll concentrations and the light absorption characteristics of phytoplankton will be compared with global trends.  Implications of these findings for the retrieval of chlorophyll biomass using conventional remote sensing reflectance algorithms will be explored. The assembled bio-optical dataset will be used to develop a satellite algorithm specific for HNLC seas to improve the retrieval of surface chlorophyll, primary production and information on phytoplankton functional types (Nair et al. 2008) from space.

Aims of the Project

  1. Assess the distribution of phytoplankton groups using in situ measurements pigment markers, marine bio-optics and microscopic cell counts.
  2. Determine the key environmental variables that govern changes in community structure in space and time.
  3. Identify light absorption signatures that may be used to detect the presence or absence of phytoplankton functional types using ocean-colour radiometry.
  4. Produce a resolved time series of community structure during key transition periods in a highly undersampled HNLC region of the SO using satellite data.

Methods to be used

Chemotaxonomic analysis of High Performance Liquid Chromatography pigment datasets.
Measurements of phytoplankton light absorption by visible spectrophotometry.
Examination of physical (CTD, light profiles), chemical (macronutrients and trace-metals) and biological (cell counts, particulate biomass) datasets from research cruises
Working with remote sensing images of ocean colour.

Specialised skills required

The successful applicant, whose first degree might be in marine, environmental or earth sciences, will have an aptitude for multidisciplinary research, good quantitative and computing skills, and an interest in laboratory and field-based experimental work.

if interested please contact Heather Bouman mailto:heather.bouman@earth.ox.ac.uk

References

Bouman HA, Lepère C, Scanlan DJ & Ulloa O (2012) Phytoplankton community structure in a high-nutrient, low-chlorophyll region of the eastern Pacific Subantarctic region during winter-mixed and summer-stratified conditions. Deep-Sea Research I 69: 1-11
Coale KH et al. (2004) Southern Ocean iron enrichment: carbon cycling in high- and low-Si waters. Science 304, 408–414.

Nair A, Sathyendranath S, Platt T, Morales J, Stuart V, Forget M-H, Devred E & Bouman H (2008) Remote sensing of phytoplankton functional types. Remote Sensing of Environment 112:3366-3375

For more information on the CUSTARD project: https://roses.ac.uk/custard/