Assessing the role of dust deposition on phytoplankton ecophysiology and succession in a low-nutrient low-chlorophyll ecosystem: a mesocosm experiment in the Mediterranean Sea

Abstract. In this study, we investigate the response of the phytoplankton community, with emphasis on ecophysiology and succession, after two experimental additions of Saharan dust in the surface water layer of a low-nutrient low-chlorophyll ecosystem in the Mediterranean Sea. Three mesocosms were amended with evapocondensed dust to simulate realistic Saharan dust events, while three additional mesocosms were kept unamended and served as controls. The experiment consisted in two consecutive dust additions and samples were daily collected at different depths (−0.1, −5 and −10 m) during one week, starting before each addition occurred. Data concerning HPLC pigment analysis on two size classes (< 3 and > 3 μm), electron transport rate (ETR) vs. irradiance curves, non-photochemical fluorescence quenching (NPQ) and phytoplankton cell abundance (measured by flow cytometry), are presented and discussed in this paper. Results show that picophytoplankton mainly respond to the first dust addition, while the second addition leads to an increase of both pico- and nano-/microphytoplankton. Ecophysiological changes in the phytoplankton community occur, with NPQ and pigment concentration per cell increasing after dust additions. While biomass increases after pulses of new nutrients, ETR does not greatly vary between dust-amended and control conditions, in relation with ecophysiological changes within the phytoplankton community, such as the increase in NPQ and pigment cellular concentration. A quantitative assessment and parameterisation of the onset of a phytoplankton bloom in a nutrient-limited ecosystem is attempted on the basis of the increase in phytoplankton biomass observed during the experiment. The results of this study are discussed focusing on the adaptation of picophytoplankton to nutrient limitation in the surface water layer, as well as on size-dependent competition ability in phytoplankton.