Sensitivity analysis of a mathematical model of chlorophyll distribution in the tidal Keelung River

Large temporal and spatial variations in chlorophyll concentration found in estuarine and coastal waters have been attributed to a diverse set of phys ical, chemical and biological factors. The Keelung River has the largest ti dal excursion and the most serious pollution problem among the three tribut aries of the Tanshui River system in northern Taiwan. It receives extremely high anthropogenic nutrient loadings from municipal and industrial dischar ges with little or no treatment. The observed data show very high nutrient concentrations in the river, with total nitrogen of order of several millig rams per liter and total phosphorus of 1 mg/l. However, the summer chloroph yll 'a' concentrations are generally less than 10 mu g/l. 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 syst em to simulate the phytoplankton dynamics, nutrient cycling and dissolved o xygen distribution. The numerical model was calibrated and verified with ob served chlorophyll 'a' and nutrient distributions in the Keelung River, and then used to investigate the various factors regulating chlorophyll 'a' di stributions in that river. The sensitivity analyses include water-column li ght extinction, nutrient reduction, freshwater discharge, boundary conditio n and effect of salinity distribution. The result reveals that light availa bility is the predominant limiting factor of phytoplankton growth, while nu trient concentrations an not limiting. The reduction of nutrient N and P Lo adings have no effect on chlorophyll 'a' concentration in the Keelung River .