混合剧烈、透明度浅和光质(紫外光与可见光比例)随水深变化大是河口海域最显著的物理特征；生活在其中的浮游植物随混合上下移动，势必经受着剧烈的光环境变化(光强与光质)。当前普遍认为光环境变化是河口浮游植物生物量和生产力变化的最关键因素之一，对于其中间过程—浮游植物如何响应和适应光环境变化而存活下来、成为优势种甚至爆发性生长的内在机理并不清楚，而该中间过程的关键在于它们如何维持光系统II的活性。珠江是我国年入海水量最大的河，其河口最大浑浊带区混合速率极高；因此项目拟在该区域研究原位光环境变化与浮游植物群体光合固碳变化关系的同时，分离、培养优势浮游植物单种，在室内和室外可控条件下研究光环境变化与其光合能力变化的关系，并深入研究光环境变化与光系统II的损伤、修复动力学过程及修复酶变化等的关系，阐明其如何响应和适应该光环境变化，从光生理学角度揭示光环境变化引起河口浮游植物生物量和生产力变化的内在原因。

Estuarine environments often drasticly change in the spatical and temporal scales, no exception for underwater light conditions. The land-derived inputs and water-exchanges between innner and outer esturies lead to a dramatic verital mixing within water column; simutaniously, the abundant suspensions and dissolved matters in the water column also lead to a steep light decay. Furthermore, the light with different wave-bands such as photosynthetically active radiation (PAR, 400-700 nm) and ultraviolet radiation(UVR, 280-400 nm) have differential decay rates in water, which results in the great changes in light quality with increasing water depth. Therefore, the light quantity (intensity) and quality (wave-band) vary greatly within different depths of the esturies. Phytoplankton living in estuaries would be periodically transferred from surface to bottom of the water column and vice versa due to the vertical mixing, and suffer from the drastic changes of underwater light environments i.e. being exposed to different light intensities and qulities (i.e., PAR and UV) at different depths. At present, it is usually believed that light environment change is a key reason for estuarine phytoplankton biomass and productivity, while it is unclear so far that how they response and acclimate to such light changes although it is vital for them to survive, to be dominant species and even blooms, and this process mainly lie on how they maintain the activities of Photosystem II. Pearl River has the largest annual land runoffs among Chinese rivers, with a vertial mixing volecity reached as high as 10 cm s-1 in its maximum turbidity zone. Therefore, this project is designed to carry out studies in this area as follows: (1) Using onboard experiments in a seasonal scale, to study the relationships of the mixing-caused changes in field sunlight quantity and quality and photosynthetic carbon fixation by natural phytoplankton assemblages from the maxiamum turbidity zone of the Pearl River estury, in order to reveal the effects of field light environment changes upon estuarine carbon fixation; (2) Laboratorial experiments will be performed using the mono-species of phytoplankton that are the dominating species (mainly diatoms and dinoflagellates) and isolated from the maximum turbidity zone of Pearl River estuary, to study the relationships of light environment changes and photosynthetic rate [including 14C carban fixation rate and maximal quatam yield (Fv/Fm)], and also to further study the effects of such light changes upon the PSII damage and repair processes and the repair-dependent FtsH protein enzyme contents and activities. This study can clearify how phytoplankton response and acclimate to the estuarine light environment changes and potentially elucidate the physilogical reasons for the alteration of field light environment changes on estuarine phytoplankton biomass and production in the photophysiological view of point.