Seasonal variability of surface phytoplankton biomass in the Humboldt Upwelling System

Eastern Boundary Upwelling Systems host extremely high productive ecosystems and support highly diverse communities. Among the four Eastern Boundary Upwelling Systems, only the Humboldt Upwelling System (HUS) features a “Seasonal Paradox”, with higher surface chlorophyll, a proxy for phytoplankton biomass, during the season with weak upwelling and lower surface chlorophyll when upwelling intensity is strong. It is still unclear what causes the “Seasonal Paradox” in the HUS. The aim of this study is to understand what sets the HUS apart from other EBUS and resolving the “Seasonal Paradox”. To identify possible drivers, we use a regional coupled physical-biogeochemical model (ROMS-BioEBUS) and analyze simulated plankton dynamics in a coastal band of 200 km width in the HUS. We assess the bottom-up and top-down controls along with physical processes (advection and mixing) which drive surface phytoplankton biomass, and determine the major drivers of phytoplankton biomass seasonal variation. We investigate in particular the role of dilution of plankton biomass over a deepening mixed layer, along with seasonal variation of offshore export, in reducing surface biomass concentrations during the upwelling season. Preliminary results suggest that these physical processes play a major role in the “Seasonal Paradox” and even may dominate over limitation of phytoplankton production.