Modeling the effect of freshwater inflows on the development of spring blooms in an estuarine embayment

Low vertical mixing rates are a key physical condition associated with the development of phytoplankton spring blooms in coastal and ocean waters. Vertical mixing rates in shallow coastal regimes are decreased not only by thermal stratification but also by haline stratification due to river runoff. In this paper, the main physical processes involved in the onset of phytoplankton spring blooms are examined in a tidedominated estuarine embayment, characterized by weak current velocities, using a 1-D ecosystem model (the European Regional Seas Ecosystem Model) coupled with a 3-D physical model (the Princeton Ocean Model coupled with a sediment transport model) via the off-line method. Simulation results show that a reduction in vertical mixing, caused by the episodic input of buoyant, freshwater inflows from a reservoir during the period of neap tides, is the main physical controlling process on the occurrence of spring algal blooms. Furthermore, sensitivity tests using: (1) layered and (2) depth-averaged monthly vertical eddy diffusivity values reveal that the timing of phytoplankton spring blooms in the model is strongly affected by the parameterization of vertical diffusivity.