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Abstract
Large temporal and spatial variations in chlorophyll concentration found in estuarine and coastal waters have been attributed to a diverse set of physical, chemical and biological factors. The Keelung River has the largest tidal excursion and the most serious pollution problem among the three tributaries of the Tanshui River system in northern Taiwan. It receives extremely high anthropogenic nutrient loadings from municipal and industrial discharges with little or no treatment. The observed data show very high nutrient concentrations in the river, with total nitrogen of order of several milligrams per liter and total phosphorus of 1 mg/1. However, the summer chlorophyll ‘a’ concentrations are generally less than 10 μg/1. A time‐dependent, laterally averaged, two‐dimensional hydrodynamic and water quality model (HEM‐2DT) was applied to the tidal reach of the Tanshui River estuarine system to simulate the phytoplankton dynamics, nutrient cycling and dissolved oxygen distribution. The numerical model was calibrated and verified with observed chlorophyll ‘a’ and nutrient distributions in the Keelung River, and then used to investigate the various factors regulating chlorophyll ‘a’ distributions in that river. The sensitivity analyses include water‐column light extinction, nutrient reduction, freshwater discharge, boundary condition and effect of salinity distribution. The result reveals that light availability is the predominant limiting factor of phytoplankton growth, while nutrient concentrations are not limiting. The reduction of nutrient N and P loadings have no effect on chlorophyll ‘a’ concentration in the Keelung River.