A one-dimensional flow model which accounts for turbulence and stratification in the upper ocean has been applied in conjunction with a one-dimensional production-advection-diffusion model to predict the initial evolution of the phytoplankton biomass concentration under given biological and physical conditions. Here the stratification is caused by the salinity which is modeled by a one-dimensional diffusion equation; the density is related to the salinity by a linear relation (state equation). The resulting one-dimensional model has been applied to investigate how local physical conditions affect the initiation of phytoplankton blooms in nutrition rich waters characterized by wind driven currents and salinity stratification. This is of particular relevance to fjords and near-coastal waters exposed to freshwater run-off, where the vertical salinity gradient causes the strongest stratification. It appears that the onset of phytoplankton blooms depends on a nontrivial balance between the wind-induced free surface stress and the strength, vertical location and thickness of the initial pycnocline, as well as the mean sinking velocity of the phytoplankton. This interaction has been elucidated by means of numerical simulations for a set of biological and physical parameters representative for the inner and outer part of fjords and for near-coastal salinity stratified waters.