Name: Tslil Bar, Ph.D student

Institute: Department of Marine Biology, Faculty of Natural Sciences, University of Haifa.

Title of PhD project: Reconciling nutrient supply, phytoplankton C:N:P ratios and bacterial community diversity in the eastern Mediterranean Sea

Alfred Redfield suggested that the elemental composition of phytoplankton reflects the dissolved nutrient ratios of the deep ocean (C:N:P 106:16:1). He also hypothesized that phytoplankton elemental stoichiometry controls ocean chemistry through the remineralization of exported material. Yet, many studies since have demonstrated that phytoplankton cellular C:N:P is flexible and changes both between taxa and as a response to nutrient supply and  availability (physiological plasticity). My aim is to evaluate Redfield’s hypothesis regarding the predominant phytoplankton groups (Prochlorococcus, Synechococcus, and picoeukaryotes) and assess their contribution to the carbon flux in the eastern Mediterranean Sea (EMS) with its high N:P supply ratio and low nutrient availability.

My main goals in this research are to determine seasonal changes in cell stoichiometry (C:N:P) of the predominant phytoplankton groups in the coastal and open sea in the EMS, and the impact of increased N:P supply ratios on these groups and the nitrogen sources that they are utilizing (using δ15N isotopic signatures).

The elemental composition of marine plankton and particulate organic matter has biogeochemical implications for C cycling and export, for, understanding nutrient limitation, and for ecosystem and food web models.

This study will provide the first taxa specific C:N:P and N isotopic signatures of phytoplankton in the EMS and will help to understand more thoroughly the interaction between phytoplankton ecophysiology and sinking materials.