棲蘭山鴛鴦湖水體分層和混合作用對湖泊代謝之影響

Influence of Stratification and Mixing Regime on Lake Metabolism in the Yuan-Yang Lake, Chi-Lan Shan, Taiwan

鴛鴦湖為棲蘭山上的一處高山淺水湖泊，具一處主要湖水出流口，終年湖水不結冰。湖泊代謝研究主要對進出湖泊生態系統的碳通量進行研究，分析湖泊碳平衡的生物程序，並由季節性的湖泊代謝變化分析氣象因素所帶來的影響。計算湖泊代謝的簡要概念為將湖泊視為一個系統，經由量測此系統的總初級生產率（Gross primary production, GPP）和呼吸率（Ecosystem respiration, ER），可得湖泊系統的淨生產率（Net ecosystem production, NEP）。淨生產率為正值時，湖泊系統處於自營性狀態；若計算結果為負值，則湖泊系統處於異營性狀態。淨生產率計算結果可用以判斷湖泊系統利用碳源之情形，當湖泊系統屬自營性狀態時，為初級生產率大於呼吸率的固碳狀態，可將空氣中的二氧化碳固定於湖泊中；當湖泊呈異營性狀態時，呼吸率大於初級生產率，湖泊成為大氣二氧化碳的排放源。春季氣溫及水溫開始由寒冬之後開始回升，藻類開始生長，總初級生產力增加，此時呼吸量仍在低值，是藻類主要生長季節。夏季溫度最高，水體分層作用明顯，細菌及浮游動物生長頻繁，呼吸量快速增加，總初級生產力較春季微高。秋季氣溫開始下降，藻類與細菌及浮游動物活動力趨緩，總初級生產力及呼吸率略有降低。夏、秋季因颱風、豪雨等劇烈天氣變化，大量降雨、徑流進入湖泊，陸域碳源隨降雨沖刷進入湖泊，混合作用使湖底營養鹽揚起至水體表層，造成呼吸率大幅增加。冬季因水溫驟降造成生物活動力減弱，總初級生產力及呼吸率隨之降低，其值為四季最低。鴛鴦湖四季淨生產量皆為負值，屬異營性湖泊。 Yuan-Yang Lake (YYL), located in Chi-Lan Mountain, is an alpine shallow lake and ice-free all year round, and the lake has one primary outlet. Lake metabolism which may be influenced by climatic changes is the carbon budget in the whole lake system. Net ecosystem production (NEP) summarizes biological activities of the lake in the ecosystem. Gross primary production (GPP) represents the amount of inorganic carbon fixation and ecosystem respiration (ER) represents the amount of organic carbon consumption. NEP equals to the resultant of GPP minus R and reveals the integrated trophic state of lake system. When NEP is positive, the lake system is the source of producing and exporting organic carbon and belongs to the autotrophic state. When lake metabolism tends to heterotrophic, suggesting negative NEP, CO2 in water body reaches to supersaturation that will be released to the atmosphere. After the cold weather, water temperature in spring increases and GPP recovers to a level implying the significant growth of phytoplankton, but the ER in the lake system being still kept low. Stratification in YYL is apparent in summer because of high water and air temperature occurring in this season and activities of bacteria and plankton are active so that GPP and ER in summer are higher than in spring. When water temperature decreases in fall, NEP and EP decrease owing to lowering biological activities. On the other hand, heavy rains and high winds in typhoon events promote the water body toward full mixing in summer and autumn. Terrestrial nutrients which are brought to the lake with runoff and re-suspending from sediment during the typhoon period by the mixing regime, resulting in the sharp increase of ecosystem respiration When the water temperature decreases in winter, biological activities are weak that GPP and ER descend to the lowest level in the whole year. NEP is negative all year round in YYL which is regarded as the heterotrophic lake.

Yuan-Yang Lake (YYL), located in Chi-Lan Mountain, is an alpine shallow lake and ice-free all year round, and the lake has one primary outlet. Lake metabolism which may be influenced by climatic changes is the carbon budget in the whole lake system. Net ecosystem production (NEP) summarizes biological activities of the lake in the ecosystem. Gross primary production (GPP) represents the amount of inorganic carbon fixation and ecosystem respiration (ER) represents the amount of organic carbon consumption. NEP equals to the resultant of GPP minus R and reveals the integrated trophic state of lake system. When NEP is positive, the lake system is the source of producing and exporting organic carbon and belongs to the autotrophic state. When lake metabolism tends to heterotrophic, suggesting negative NEP, CO2 in water body reaches to supersaturation that will be released to the atmosphere. After the cold weather, water temperature in spring increases and GPP recovers to a level implying the significant growth of phytoplankton, but the ER in the lake system being still kept low. Stratification in YYL is apparent in summer because of high water and air temperature occurring in this season and activities of bacteria and plankton are active so that GPP and ER in summer are higher than in spring. When water temperature decreases in fall, NEP and EP decrease owing to lowering biological activities. On the other hand, heavy rains and high winds in typhoon events promote the water body toward full mixing in summer and autumn. Terrestrial nutrients which are brought to the lake with runoff and re-suspending from sediment during the typhoon period by the mixing regime, resulting in the sharp increase of ecosystem respiration When the water temperature decreases in winter, biological activities are weak that GPP and ER descend to the lowest level in the whole year. NEP is negative all year round in YYL which is regarded as the heterotrophic lake.