Effects of resuspended sediments and vertical mixing on phytoplankton spring bloom dynamics in a tidal estuarine embayment

As a preliminary multidisciplinary modeling approach encompassing physical and ecological systems, a 1-D biogeochemical model was coupled with a 3-D hydrodynamic-sediment transport model through an off-line mode. The resulting modeling system was used to investigate the main controlling factors for the onset of phytoplankton spring blooms in 2001 at the Youngsan River Estuarine Bay (YREB), where tidal currents are relatively weak and the freshwater discharge from the river is artificially regulated by sluice gates on a dyke. The model simulations suggested that the strong vertical mixing caused by sea-surface cooling was a major controlling factor in the development of phytoplankton blooms in the YREB during the period from January to April 2001. In relation to the reduction in vertical mixing rates, frequent freshwater inflows from the reservoir during the neap tides contributed to the onset of phytoplankton spring blooms in 2001. Furthermore, sensitivity tests with different conversion factors for the estimation of the ‘photosynthetically active radiation’ (PAR) revealed that further examination and parameterization of PAR is required for improvement of the ecosystem model. It also showed that an increase in the PAR attenuation coefficient, taking into account the tidally varying structure of suspended sediment concentrations, inhibited phytoplankton blooms. Finally, it can be emphasized that in order to predict variability in phytoplankton production in turbid estuarine bays, it is essential to incorporate an ecosystem model with a 3-D hydrodynamic model coupled to a sediment transport model capable of resolving the variable vertical dynamics of sediment resuspension and mixing.