東沙環礁潟湖浮游生物群聚呼吸率時空變異之研究

Abstract

為瞭解東沙環礁潟湖之浮游生物群聚呼吸率(Plankton community respiration；CR)及有機碳循環的變化，本研究於民國99年的5、7、9月和100年2月在東沙潟湖進行相關調查研究，並分析其物理、化學水文及生物參數，以期瞭解影響有機碳循環之群聚呼吸率的時、空變異，並探討影響其變化之主因。結果顯示東沙潟湖內群聚呼吸率在春季(五月)、夏季(七月)、秋季(九月)和冬季(二月)的變化範圍分別為41.1~279.2、9.0~116.0、41.9~113.9和0.0~69.8 mg C m-3 d-1，其中以冬季最低。而浮游生物群聚生物量及生產力之多寡可能是群聚呼吸率變化的主控因素，此結果可由群聚呼吸率、葉綠素甲濃度、異營性細菌生物量或生產力呈顯著相關(p<0.05)得到應證。此外，由不同季節分析結果顯示，春季的群聚呼吸率可能主要受控於浮游植物生物量的多寡，其中又以矽藻的貢獻度最大，其可由群聚呼吸率與葉綠素甲或矽酸鹽的顯著相關得到驗證；夏季的群聚呼吸率未與任何參數有顯著相關，若以細菌生長效率推估異營性細菌呼吸率，其約佔24.2%，但無法解釋群聚呼吸率的主要來源；秋季群聚呼吸率與異營性細菌生物量有顯著相關(p<0.05)，顯示其應為影響群聚呼吸率變化之主因；冬季的群聚呼吸率極低，可能肇因於浮游生物量太低，亦可能受到潮汐作用或海流影響，致使潟湖內之無機營養鹽和浮游生物遭到稀釋或攜出，進而降低其群聚呼吸率。研究期間，初級生產力和群聚呼吸率之平均比值(P/R ratio)接近或大於1(春季：1.35±1.07；夏季：1.01±0.65；秋季：0.90±0.63；冬季：3.03±3.82)，顯示東沙環礁潟湖為平衡性或自營性生態系，此結果建議本潟湖應有額外的有機碳，供其它生物利用或輸出至鄰近海洋生態系。This study was designed to explore spatio-temporal variations of plankton community respiration (CR) and organic carbon cycle in Dongsha Atoll. Four curises were performed in different seasons. Results showed that the ragne of CR in spring (May), summer (July), autumn (September), and winter (February) were 41.1~279.2, 9.0~116.0, 41.9~113.9 and 0.0~69.8 mg C m-3 d-1, respectively. The lowest rate was observed in winter. Further analyses suggest that the CR was mostly related to biomass and production of plankton communities, especially phytoplankton and bacterioplankton. This assumption can be verified by significant relationships between mean values (per m3) over water depth of CR vs. chlorophyll a (Chl a), heterotrophic bacterial biomass (BB) or production (BP; all p<0.05). Seasonally, the rate of CR might mostly attribute to phytoplankton, particularly diatom, in spring. This assumption could be validated by significant regressions between CR and Chl a or silicate. In summer, the CR was not related to any observed variables. Even though the estimated bacterial respiration accounting for 24.2% of CR, it can not explain the variation of CR rate. In autumn, CR was significantly related to BB (p<0.05), and it suggests that the CR was dominated by bacterioplankton. As state above, the lowest CR was observed in winter, and it might be due to low plankton biomass. Moreover, the mean ratio of primary production to community respiration was close to or higher than 1, and it indicates that the ecosystem of Dongsha Atoll was either carbon balance or autotrophic. It also suggests that there was residual organic carbon deposited to the bottom of Dongsha Atoll or exported to nearby ecosystems.