Implications of seasonal mixing for phytoplankton production and bloom development

Based on a 1D model considering phytoplankton and nutrients in a vertical water column, we investigate the consequences of temporal and spatial variations in turbulent mixing for phytoplankton production and biomass. We show that in seasonally mixed systems, the processes controlling phytoplankton production and the sensitivity of phytoplankton abundance to ambient light, trophic state and mixed-layer depth differ substantially from those at steady state in systems with time-constant diffusivities. In seasonally mixed systems, the annually replenished nutrient pool in the euphotic zone is an important factor for phytoplankton production supporting bloom development, whereas without winter mixing, production mainly depends on the diffusive nutrient flux during stratified conditions. Seasonal changes in water column production are predominantly determined by seasonal changes in phytoplankton abundance, but also by seasonal changes in specific production resulting from the transport of nutrients, the exploitation of the nutrient pool and the increase in light shading associated with phytoplankton growth. The interplay between seasonal mixing and the vertical distribution of mixing intensities is a key factor determining the relative importance of the processes controlling phytoplankton production and the sensitivity of the size and timing of the annual maximum phytoplankton abundance to the abiotic conditions.