優養化湖泊環境監測－以金門太湖為例

Abstract

台灣因氣候及地形條件，導致民生用水相當依賴湖泊水庫。根據環保署102年環境監測年報，台灣離島28座水庫優養比例高達96％。金門縣為台灣第二大離島，島上共有11座水庫，均處於優養化狀態，且水體中存在釋放藻毒之藍綠菌（cyanobacteria），對於當地的湖庫生態與居民飲用水安全造成威脅。微囊藻毒素有強烈的肝毒性，容易誘發肝炎以及肝癌，對皮膚與黏膜有強烈刺激性，所造成的危害在世界各地層出不窮，當中不乏人類死亡的案例。 本研究以優養化湖泊金門太湖為對象，自2013年11月至2014年8月進行4次的採樣，於湖面上佈下11個採樣點，進行全湖域的水質、懸浮固體物與底泥的調查。結果顯示金門太湖地區之主要營養鹽來源為山外溪，其次為幹訓班水之排水溝，此外位於太湖湖面的光復亭小島因有大型鳥類群聚，所排泄的鳥糞亦可能為營養鹽來源。金門全年葉綠素a濃度介於8.9-118.5 µg/L，遠高於Carlson優養化單一指標7.2 µg/L之標準，藻毒濃度在四季的採樣中，2月之藻毒濃度最高，Microcystin-LR濃度為0.38 µg/L、Microcystin-RR濃度為0.13 µg/L，低於歐盟1.0 µg/L之標準，但藻毒問題仍為太湖水庫之隱憂。營養鹽部分，總磷濃度最高之月份為2月，平均濃度高159.1 µg/L，推測可能原因為太湖水位下降，使得總磷在湖水產生濃縮作用與11月至2月間湖水翻騰現象所造成。而氨氮濃度最高月份為8月，濃度平均達198.7 µg/L。營養鹽循環中，除了外部的營養鹽來源之外，底泥釋放也是營養鹽的來源之一，根據調查結果，太湖湖域沉積最嚴重之區域為小太湖，而沉積物中磷之濃度最高之區域為太湖南邊，就整體磷通量而言，小太湖區域為最高。以SEDEX分析金門太湖底泥之磷鍵結態，其中被視為較易釋放磷於水體的Ex-P與Fe-P佔總磷之比例高於50％，顯示底泥可能為太湖重要營養鹽來源之一。

Due to climatic and topographic conditions, domestic water source in Taiwan is highly dependent on reservoirs. In recent decades, due to economic development, there are more and more human activities in catchment area. They caused eutrophication of reservoir by discharging plenty of nutrients into water body. This phenomenon is worsened at Taiwan’s isolated islands. According to Taiwan EPA, the percentage of eutrophic lake in Kinmen county is up to 96％ in 2013. Thus, the water quality on this isolated island becomes a highly imperative problem. In Kinmen, Tai-Hu Reservoir （THR） is one of the major drinking water sources. THR has been seriously affected by eutrophication because it is teemed with cyanobacteria. Here we focused on the water quality monitoring results of 11 stations in THR in terms of Chlorophyll a （Chl-a）, total phosphorus, ammonia nitrogen, nitrite, nitrate, microcystin-LR （MC-LR）, microcystin-RR （MC-RR）, phosphorus in sediment core samples, and sediment deposition rates. This research is focused on THR and finished a full-year monitoring from November in 2013 to August in 2014. Eleven sampling station was installed in the lake for whole lake monitoring in terms of water quality, suspended solids and sediments. The results indicated that the main source of nutrients is Shan-Wie Creek, and the drainage of Corporal Training Camp is the second major source. In addition, the feces of migrating birds on the Kuan-Fuh Island in the lake might be a source of phosphorus, too. The whole-year concentration of Chl-a is between 8.9 and 118.5 µg/L, which is much higher than the single-parameter eutrophic state index based on Chl-a only, i.e., 7.2 µg/L. Algal toxin level is at its highest in February with microcystin-LR concentration at 0.38 µg/L and microcystin-RR concentration at 0.13 µg/L. Microcystin-LR concentration is below EU standard at 1.0 µg/L. However, algal toxin is still an impending problem. For the nutrients, total phosphorus is highest in February with an average concentration at 159.1 µg/L probably because the reversing of water stratification and concentrating effects brought about by lowered water level or decreased water volume in winter season. The highest ammonium-nitrogen concentration falls in August at as high as 198.7 µg/L. In the nutrient cycle, apart from the external source for phosphorus, the lake sediment may also be a significant source. According to the results, the Small Tai-Hu Lake （at the west side of THR） is the area that has the highest deposition of sediments but the highest phosphorus concentration is at the south side of THR. For the flux of phosphorus, the Small Tai-Hu Lake is still at the top. According to the results by a sequential extraction method （SEDEX） exhibits that bioavailable phosphorus portion is higher than 50% indicated that the lake sediment could be a major source of phosphorus in THR.