What causes the ocean to bloom? Revisiting Sverdrup’s critical depth hypothesis

This project targets a question that has puzzled biological oceanographers for decades: What controls the timing of the phytoplankton spring bloom? The phytoplankton population undergoes seasonal changes with the biggest increase occurring in spring, the 'bloom'. The timing of the bloom is important as a food source, and may also affect the efficiency of carbon transfer to the deep ocean, altering the balance of atmospheric and oceanic carbon dioxide. The timing of the bloom changes every year, in response to physical conditions during winter and spring. The first hypothesis on controls of bloom initiation was proposed in 1953 by Harald Sverdrup and is now known as the 'critical depth model'. However, some observations collected over the intervening 58 years are not consistent with Sverdrup's model. Recently, several alternative hypotheses have been proposed. In particular, one theory links the start of the bloom to a reduction in turbulent mixing. This theory remains untested. In collaboration with the National Oceanography Centre (NOC) at Southampton, the control of bloom initiation by physical processes will be investigated using ocean gliders and a mooring array between September 2012 and September 2013. The student can participate in the deployment cruise for the gliders and will process and calibrate the biogeochemical data (fluorescence, optical backscatter, oxygen) they collect. The student will also collate data from previous deployments of gliders across blooms. This will be the first time that a dataset with the necessary combination of physical and biological data at high vertical and temporal resolution and over the key winter/spring period is used to tackle this fundamental topic. The student will analyse time-series of chlorophyll fluorescence, oxygen, stratification and turbulent mixing to test the hypothesis that reduced mixing controls the onset of the bloom. The student will join a multidisciplinary biogeochemical and physical research group. Full training will be provided in the range of techniques required, including use of gliders and calibration and analysis of their data. The student will be co-supervised by Henson, Martin and Allen and spend between 3-18 months at NOC. The student will also hopefully have the opportunity to visit the group of Prof. Raf Ferrari at MIT, USA.

Applications are processed as soon as they are received, so early application is encouraged. NB applicants who wish to start their studies in October 2012 should submit their applications by 31 July 2012. Applications received after this date will be considered for the January 2013 entry point if the project is still available.