Short-Term Drivers of Phytoplankton Community Structure and Primary Production in The Canary Current Region

Abstract

This thesis presents novel results on phytoplankton spatiotemporal distribution and variability at submesoscale range, and on the potential effects of climate change over primary productivity in the oligotrophic waters of the Canary region. Phytoplankton growth mainly depends on nutrient and light availability. Highly dynamic oceanic environments are dominated by physical processes that generally alter phytoplankton dynamics by controlling the access to these resources. Mesoscale motions have been considered the most important factor modulating the distribution of biogeochemical properties at the upper levels of the ocean. Nevertheless, recent studies have highlighted the role played by smaller processes that operate below the local Rossby radius of deformation (~40 in the Canary region), referred to here as submesoscale. Due to the inherent complexity of sampling at such high-resolution levels, our knowledge about submesoscale-influenced phytoplankton distribution and variability is mostly constrained to theoretical and modeling studies. In addition, little is known about the fate of meso-submesoscale impacts on phytoplankton communities under climate change stressors. Ocean warming leads to enhanced stratification in the oligotrophic ocean but also to the intensification of cross-shore wind gradients and thus of the eddy kinetic energy across eastern boundary regions of the subtropical gyres. Phytoplankton thriving in a warmer, acidified ocean could then be fertilized by enhanced meso-submesoscale activity. Consequently, meso-submesoscale contribution to global net primary production could be increased. In order to contribute to understand how physical and biogeochemical factors, resolved at a resolution close to submesoscale could affect the spatiotemporal distribution and variability of pico- and nanoplankton, the main components of planktonic communities in the Canary Islands waters, we conducted two interdisciplinary surveys across a highly variable mesoscale field and a submesoscale front south of Gran Canaria island (Canary Island). We found that autotrophic and heterotrophic pico- and nanoplanktonic organisms presented a heterogeneous distribution in response to nutrient inputs caused by meso- and submesoscale processes. On the other hand, temporal variability, which is rarely studied, was found to be a significant source of error in phytoplankton variability. Finally, we have tested the response of three size classes (0.2-2, 2-20 and >20 μm) of subtropical phytoplankton communities in terms of primary production, chlorophyll and cell biomass, to increasing CO2 concentrations and nutrient fertilization during an in situ mesocosm experiment in oligotrophic waters off Gran Canaria. Our results suggest that in a future acidified subtropical ocean, mesoscale and submesoscale features would drive nutrient pumping to the surface ocean favouring the development of diatoms and increasing new production in the global ocean.