A zero-dimensional model for phytoplanktonic production in turbid, macro-tidal, well-mixed estuaries is proposed. It is based on the description of light-dependent algal growth, phytoplankton respiration and mortality. The model is forced by simple time-functions for solar irradiance, water depth and light penetration. The extinction coefficient is directly related to the dynamics of suspended particulate matter. Model results show that the description of phytoplankton growth must operate at a time resolution sufficiently high to describe the interference between solarly and tidally driven physical forcing functions. They also demonstrate that in shallow to moderately deep systems, simulations using averaged, instead of time-varying, forcing functions lead to significant errors in the estimation of phytoplankton productivity. The highest errors are observed when the temporal pattern of light penetration, linked to the tidal cycle of solids settling and resuspension, is neglected. The model has also been applied using realistic forcing functions typical of two locations in the Scheldt estuary. Model results are consistent with the typical phytoplankton decay observed along the longitudinal, seaward axis in the tidal river and oligohaline part of this estuary.