Quantum yields of phytoplankton photosynthesis in the Gulf of Aqaba (Elat), Northern Red Sea

Abstract

The Gulf of Aqaba (Elat) is characterized by oligotrophic waters and low productivity conditions in summer, when the euphotic zone of the stratified water column is severely nutrient-depleted. Winter is moderately productive, when mixing injects nutrients into the lit upper waters. The annual phytoplankton cycle and the bathymetric distribution of biomass and photosynthetic activity in the Gulf are analyzed in conjunction with the harvesting and utilization of light by cells. The harvesting of light energy at any depth is shown as an interaction between the optical properties of cells, a*, and the intensity and spectral distribution of the underwater light field. The efficiency by which phytoplankton cells utilize the absorbed light energy is the quantum yield of the photosynthetic process, Φ. The efficiency of light utilization by phytoplankton depends on the nutrient status of cells, their photoacclimation to ambient light, and the irradiance to which they are exposed. We show that the seasonal and spatial changes in the quantum yield of photosynthesis control the wax and wane of the different phytoplankton assemblages in the Gulf, thereby ultimately determining the flux of energy fueling all of the Gulf's planktonic and benthic biota.