Cross-Scale Physical Interactions Control Coastal Phytoplankton Productivity at the Deep-Chlorophyll Maximum

The coastal ocean experiences a myriad of physical processes ranging from geostrophic currents to internal waves that contribute to advection and irreversible mixing. These processes combine nonlinearly to influence primary productivity in the coastal ocean. Over 18 days, we observed a phytoplankton bloom on the Tasmanian Eastern Continental Shelf (TECS) within the deep-chlorophyll maximum (DCM) at two locations on the shelf. Winds were sporadic in both their direction and intensity, with multiple periods of intense (>0.2 N m^-2) downwelling favorable winds and less intense (>0.1 N m^-2) upwelling favorable winds. Intense downwelling winds generated near-inertial motion, deepened the surface-mixed layer, and was associated with estimates of dissipation of temperature variance (χ) exceeding 10^-6 K² s^-1; however, following these winds we also observed colder, nutrient-rich waters downwelling, resulting in decreasing diffusive vertical nitrate fluxes following the intense winds. During upwelling favorable winds, we observed upwelling via advective vertical nitrate fluxes that were three orders of magnitude larger than diffusive fluxes. In summary, the effect of mesoscale and sub-mesoscale processes on vertical fluxes of nitrate is highly interconnected with small adjustments in the mesoscale flow-field potentially having much larger effects on primary productivity than intense wind-mixing events.