PHYTOPLANKTON BIOMASS IN THE GLOBAL OCEAN: PATTERNS, DRIVERS, UNCERTAINTIES

The relative contribution of different phytoplankton taxa to global plankton biomass is likely crucial for ecosystem services related to biogeochemical cycling, ecosystem productivity and the dynamics of higher trophic levels. Yet, the drivers of phytoplankton biomass dynamics are currently not particularly well understood, as bottom-up, top-down and competition effects influence global phytoplankton biogeography. Here, we attempt to understand the drivers of phytoplankton community composition in the pelagic ocean on the monthly, seasonal and annual scale. We extrapolate phytoplankton biomass estimates from the MARine Ecosystem DATa (MAREDAT) project to the global scale using statistical relationships between taxon-specific biomass and environmental predictor variables such as nutrient concentrations, light levels, sea surface temperature, or other plankton taxa. We discuss the spatiotemporal variability of global open ocean phytoplankton biomass for several major phytoplankton taxa, with a particular focus on diatoms and coccolithophores. Competition between diatoms and coccolithophores is an important driver for coccolithophore biomass dynamics, as indicated by the high diagnostic power of silicate in the coccolithophore model, and an inverse relationship between diatom and coccolithophore biomass. We estimate diatoms to contribute roughly 10-15%, and coccolithophores 0.3-0.6% to global carbon biomass within the mixed layer. The comparison of abundance-based with pigment-derived biomass estimates can serve as an indicator for the current uncertainty associated with these new estimates of the relative contribution of different phytoplankton taxa to total phytoplankton biomass.