Wind-driven vertical mixing and light availability control the evolution of phytoplankton blooms on a tidally energetic shelf

Phytoplankton blooms play an important role in the global carbon cycle and regulate marine ecosystems. The balance of physical and biological processes governing the onset of seasonal blooms is widely debated in the literature. Most studies focus on the open ocean, yet rates of carbon fixation are often greater over shallow shelves, which are dynamically and optically different environments. Here, we present ocean glider observations of a spring phytoplankton bloom during April 2015 in the Celtic Sea, Northwest European Shelf. We provide new insights into the drivers of phytoplankton growth in a tidally active temperate shelf sea. Uniquely, we have measurements of turbulent kinetic energy dissipation, which together with incident irradiance were used to drive a 1-D phytoplankton growth model. The greatest changes in phytoplankton biomass took place within the wind-driven active mixing layer, which was decoupled from the deeper, seasonal pycnocline. Maximum carbon production and biomass accumulation rates occurred following periods of reduced wind stress and shoaling, which trapped phytoplankton cells at shallower depths in the euphotic zone. Day-to-day variability in cloud cover subsequently determined the exact timing and magnitude of biomass peaks. We also observed tidally driven turbulent entrainment of phytoplankton from the base of the biomass maximum. This is a physical erosion process unique to shallow shelf systems leading to the loss of new biomass to deep waters. The importance that changes in the depth of the surface mixing layer play during the spring bloom emphasises the need for accurate turbulent mixing parameterisations in shelf sea models.