Rapid wind-driven fluctuations of the pycnocline drive phytoplankton blooms in a long narrow bay

Abstract

In long, narrow bays, the response to wind forcing is not mediated by Coriolis, allowing the depth of the pycnocline to respond more rapidly to changes in wind stress than offshore. The rapid, wind-driven Fuctuations of isopycnals -which expose deep, nutrient-rich waters to light-should lead to similarly rapid biological responses. However, due a lack of high-resolution observations, the dynamics and implications of these responses have not been explored. Here we investigated the response of phytoplankton to synoptic wind pulses in two bays (Rias Baixas, NW-Iberia) and offshore waters. We measured hydrographic properties, inorganic nutrients and chlorophyll-a during 3 weeks of spatial surveys and time-series stations in summer of 2018. The cruise began after an upwelling event, and sampled relaxation-downwelling conditions for a few days, followed by another upwelling pulse. The subsurface chlorophyll maximum was located on the same nutrient-rich isopycnal range (26.4-27 kg m3) in all surveys. During upwelling events, these isopycnals rapidly shoaled in the bays, but less so offshore. The transient exposure to light drove rapid increases in chlorophyll concentration in the bays. As the winds weakened, the isopycnals deepened, and phytoplankton biomass slowly decayed. These results show that phytoplankton respond rapidly to the transient exposure of deep, nutrient-rich isopycnals to light during upwelling pulses in bays. Short-lived phytoplankton blooms are thus controlled by the adiabatic, reversible upwelling of deep isopycnals rather than the fertilization of surface waters by diapycnal mixing