台灣南部海岸溼地浮游藻之光限制與生物量

Light Limitation and Phytoplankton Biomass in the Coastal Wetlands of Southern Taiwan

The biomass of phytoplankton in an aquatic ecosystem is highly unpredictable due to complicated interactions between algal communities and the physicochemical environment. However, given enough observed data, relationships between phytoplankton biomass and certain limiting resources can be established. In this study, a resource-based model was adopted for the simulation of light-limited phytoplankton biomass in coastal wetlands of southern Taiwan. A total of 22 waterbodies, including three tidal wetlands and 19 closed impoundments were surveyed during a year-long study. Light-limiting data were identified and analyzed for the determination of model parameters, including minimum light requirement and critical light requirement. Results indicate that, light utilization efficiency of the phytoplankton communities in these saline coastal waterbodies were similar to those of freshwater lakes, with critical light requirement ranged from 0.077 to 0.165 mol-photon day-1 mg-Chl-a -1 m and minimum light requirement ranged from 2.51 to 2.72 mol-photon day-1 m -1 . Impoundments were more productive and more efficient in light utilization than tidal wetlands. Algal cells accounted for 66.9% of water column light attenuation under light-limiting conditions, as compared with 39.6% for non-light-limiting situations. Self-shading presented a major regulating mechanism on the algal biomass of highly eutrophic coastal waterbodies. Under a light-limited condition, algal biomass can be managed to prevent ecosystem deterioration caused by excessive eutrophication through the control of light availability. Measures such as surface shading using wetland plants, and water depth augmentation through hydrological manipulation, can be employed for wetland management purposes. An enhanced water circulation can also lower wetland productivity, as shown in the case of Spoonbill Reserve of this study. 光限制水體的藻類生物量受到藻類群聚光照需求，以及水體混合水深與光衰減特性等複雜因素影響，其預測相當困難。然透過大量調查數據的分析，可以判別光限制情況，並使用這些數據建立藻類生物量與光照之相關模式。本研究運用此一概念，探討台南地區海岸溼地浮游藻的光限制，並建立光限制下的浮游藻生物量模式。所選定的水體共 22 個，包括 19 個封閉的池塘與 3 個潮汐濕地，在一年期間每水體進行 2 到 4 次調查，篩選出光限制數據，並據以決定藻類生物量模式參數，包括最小光照（minimum light requirement）與臨界光需求（critical light requirement）。結果顯示，濱海水體藻類群聚的光利用效率與淡水湖泊相當，臨界光需求在 0.077 與 0.165 mol-photon day-1 mg-Chl-a -1 m 之間，最小光照在 2.51 與 2.72 mol-photon day-1 m -1 之間。封閉池塘的初級生產力以及藻類光利用效率皆高於開放的潮汐濕地。藻類自蔭作用（self-shading）在這些水體的光限制扮演重要角色，藻細胞構成 66.9%的光衰減，遠高於非光線限制水體的 39.6%。初級生產過高可導致水域生態劣化，濱海濕地營養鹽濃度高，其初級生產一般無法透過營養鹽進行控制。瞭解浮游藻的光線限制，可以經由光照調節來維持合適的初級生產，方法包括以濕地植物進行遮光，或透過水文操作提高濕地水位。促進水流循環亦可降低濕地初級生產，如本研究所調查的黑面琵鷺保護區濕地。

光限制水體的藻類生物量受到藻類群聚光照需求，以及水體混合水深與光衰減特性等複雜因素影響，其預測相當困難。然透過大量調查數據的分析，可以判別光限制情況，並使用這些數據建立藻類生物量與光照之相關模式。本研究運用此一概念，探討台南地區海岸溼地浮游藻的光限制，並建立光限制下的浮游藻生物量模式。所選定的水體共 22 個，包括 19 個封閉的池塘與 3 個潮汐濕地，在一年期間每水體進行 2 到 4 次調查，篩選出光限制數據，並據以決定藻類生物量模式參數，包括最小光照（minimum light requirement）與臨界光需求（critical light requirement）。結果顯示，濱海水體藻類群聚的光利用效率與淡水湖泊相當，臨界光需求在 0.077 與 0.165 mol-photon day-1 mg-Chl-a -1 m 之間，最小光照在 2.51 與 2.72 mol-photon day-1 m -1 之間。封閉池塘的初級生產力以及藻類光利用效率皆高於開放的潮汐濕地。藻類自蔭作用（self-shading）在這些水體的光限制扮演重要角色，藻細胞構成 66.9%的光衰減，遠高於非光線限制水體的 39.6%。初級生產過高可導致水域生態劣化，濱海濕地營養鹽濃度高，其初級生產一般無法透過營養鹽進行控制。瞭解浮游藻的光線限制，可以經由光照調節來維持合適的初級生產，方法包括以濕地植物進行遮光，或透過水文操作提高濕地水位。促進水流循環亦可降低濕地初級生產，如本研究所調查的黑面琵鷺保護區濕地。