Modeling Study of Water Quality and Ecological System in the Danshuei River Estuary
Date Issued
2005
Date
2005
Author(s)
Wang, Chi-Fang
DOI
en-US
Abstract
In this study, an ecosystem model is developed for the ecological simulation of the Danshuei River estuary. A series of field observations have been conducted to support the development of the model, including two slackwater surveys and four intensive surveys.
The observed salinity distributions indicate that the lower Danshuei River estuary is a partially mixed estuary. The vertical distribution varies greatly from homogeneous to highly stratified. The nutrient concentrations are high. The total inorganic nitrogen and biogenic silicon concentrations are both of the order of several mg/l. The total phosphorus concentration is on order of tenth of mg/l. Chlorophyll ‘a’ concentration shows seasonal variation and ranges from several ug/l to tens of ug/l. Dissolved oxygen concentration is severely depressed during low tide and increases during flood tide as a result of dilution by cleaner sea water. The spatial gradients and intra-tidal variations of nutrient concentrations all indicate that the nutrients and pollutants come from the upper estuary. The estuary is heavily polluted. The water quality conditions improve only near the river mouth as the result of sea water dilution. Copepod is the major group of zooplankton in this estuary, intruding from the surrounding coastal waters. Two distinct groups of phytoplankton are observed in the estuary, the diatoms intruding from the sea and the green algae growing autochthonous in the estuary.
An ecosystem model simulating a total of 21 state variables is developed in this study. Biological variables are incorporated, including two groups of zooplankton and three groups of phytoplankton. Organic carbon, elaborate nutrient cycling of nitrogen, phosphorus and silicon, and dissolved oxygen budgeting are also included. Most of the kinetic equations describing the relations among the state variables are similar to those used in the Chesapeake Bay Model, except for the formulation of the grazing process. The Legović’s (1989) formulation for a multiple prey–predator system is adopted. The ecosystem model is internally linked with the hydrodynamic sub-model of the HEM-2D (2-dimensional Hydrodynamic Eutrophication Model) developed by Park and Kuo (1993) and replaces the original water quality sub-model.
Model calibration and verification are conducted by making use of the EPA long-term monitoring data, the chlorophyll and nutrient data of the National Center for Ocean Research and the phytoplankton and zooplankton biomass from the field surveys conducted under this investigation. Values of model parameters and rate coefficients are derived from field data if possible; otherwise they are determined through model calibration process, with the aids of reported literature values.
Results of model simulations indicate that the point source loadings of pollutants are responsible for the bulk of pollution in the estuary. The loadings are mainly from the waste discharge of the large population in the city of Taipei. Influence of the non-point sources of pollutants from the fluvial sections of the river is relatively small, except for the brief periods of very high flow. During the low flow period, the cleansing capacity of the river flow is minimal. The waste discharge is large enough to severely degrade the water quality Hypoxia/anoxia is a common occurrence throughout the estuary. The water quality improves only toward the river mouth as a result of sea water dilution and tidal flushing. The cyclic intra-tidal variation of water quality is evident by higher nutrient concentrations, lower salinity and dissolved oxygen concentration at low tide, and lower nutrient concentrations, higher salinity and oxygen concentration at high tide. Simulation results show that the ecosystem model generally reproduces the spatial distribution and intra-tidal variation of the plankton and water quality conditions in the estuary.
Both the model simulation and field data indicate that there is no apparent trend in the temporal variation of water quality in the Danshuei River estuary. The results of model simulations demonstrate that the water quality and plankton population are mainly controlled by the river inflows during the growing months. The high river flows cleanse the estuary by flushing out the pollutants and plankton populations. It happens quite often that the low river flow period between successive high flow events is too short to allow for plankton population to build up to significant levels. The estuary depends heavily on the intrusion of planktonic organisms from the surrounding coastal waters.
Subjects
數值模式
河川感潮段
水質
浮游生物
廢污水
物理傳輸機制之影響
numerical model
estuary
water quality
plankton dynamics
waste discharges
impacts by transport processes
Type
thesis
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