淡水河系感潮段水質與生態系統模擬研究

Abstract

本研究乃根據淡水河感潮河系之水生生態現況建構一水質生態數值模式，並於現場施行兩場平潮(slackwater)與四場定點密集(intensive)觀測採樣以輔助模式之發展。 實測資料顯示淡水河下游段的鹽分分佈屬於部分混和(partially mixed)的情況，垂直方向的鹽分分層情況可由均勻混和至嚴重分層。營養鹽濃度普遍偏高，葉綠素濃度可達數個至數十個ug/l，低潮時溶氧濃度嚴重偏低，僅在高潮時接近出海口處可藉由與較乾淨海水的稀釋作用而稍有提升。實測營養鹽的時空分佈皆顯示污染物來自上游河段，水質狀況僅在下游接近出海口處受到海水稀釋效應而稍有改善。研究區域內的浮游動物以橈足類(copepod)為主，乃經由潮水而由周遭海域進入淡水河下游河段；浮游植物以矽藻(diatoms)與綠藻(green algae)為主，矽藻乃由潮水作用經河口進入下游河段，綠藻則源於淡水河感潮河系內。 本研究所發展之生態模式，共有21個水質生態模擬變數，包含兩個浮游動物及三個浮游植物模擬變數，以及氮、磷、矽、有機碳與溶氧等，除了浮游動物攝食效應乃採用Legović (1989)的公式以外，其餘各模擬變數之間的反應關係式大部分與美國Chesapeake Bay模式相似。本生態模式與二維水理水質模式HEM-2D(2-dimensional Hydrodynamic Eutrophication Model, Park and Kuo, 1993)的水理模式部分直接相連接，並取代了HED-2D原有之水質模式部分。 生態模式建立後，續以環保署長期水質監測資料、海洋科學研究中心之葉綠素與營養鹽濃度實測數據，以及本研究於現場採樣分析所得之浮游生物量數據，進行模式之檢定與驗證。模式中所需之參數與反應係數盡可能由目前可得之現場實測數據進行推估，其餘參數則參照相關文獻所載數值，並藉由模式檢定過程獲得。 模擬結果顯示點源污染量是造成本河系水質污染的主要原因。低流量時，河川的自淨能力偏低，大量廢污水嚴重惡化此間水質，水體經常發生低氧/缺氧的情形，水質狀況在下游近出海口處受到海水稀釋與潮汐沖洗效應而稍有改善。模式亦能具體反映水質在潮週內呈現的週期性變化。 模擬結果與實測數據皆顯示淡水河水質狀況並無顯著之時間變化趨勢。根據模擬結果，在生長季節期間，此間水質與浮游生物量主要受河川流量影響，高河川流量將區域內的污染物與浮游生物量快速沖洗至河口外，而低河川流量持續發生的時間往往不足以提供浮游生物有累積足夠生物量的機會，僅在長時間的低流量情況下，浮游生物量才有明顯之生長與累積。淡水河系感潮段內的浮游生物大部分仰賴來自河口外沿岸水體的補充。

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.