Abstract:

 The Danjiangkou reservoir, located in the junction area of Hubei Province, Henan Province, and Shanxi Province, is the only water source of the Central Line Project of SouthtoNorth Water Diversion of China, which serves as an important drinking water source for most cities facing severe water shortage in the North China Plain (e.g. Beijin, Tianjin, Shijiazhuang). Understanding the ecological condition of the Danjiangkou reservoir is a valuable knowledge for the regional water resource protection and management. Euphotic depth is one of the most important factors to assess the condition of the aquatic ecosystem because it represents a zone where almost all of the primary production occurs. Investigating the variation of euphotic depth in Danjiangkou reservoir and assessing the influencing factor can improve our understanding on the ecosystem dynamics of the Danjiangkou reservoir and its relationships with light. Relevant results could provide useful information for the sustainable water resource management of SouthtoNorth Water Diversion.
To investigate the spatiotemporal dynamics of euphotic depth and its influencing factors, a total of 25 field sampling sites were investigated during the high flow period, median flow period, and low water period in 2011-2012 in the Danjiangkou Reservoir. First, we investigated the spatiotemporal variations of the underwater photosynthetic active radiation and the concentration of three main substances (inorganic suspended particulate, algal chlorophyll a, dissolved organic carbon). Then, we used the step-wise multiple regression analysis to determine the most important factors affect the variation of euphotic depth in the Danjiangkou Reservoir. The results showed that, the euphotic depth of the Danjiangkou Reservoir range from 104 m to 1535 m, with average values of 608 m, 535 m, and 765 m in the high flow period, median flow period, and low flow period, respectively. In the temporal scale, the highest fluctuation of the euphotic depth was observed in the high flow period (CV=070), and the lowest fluctuation of the euphotic depth was observed in the low water period (CV=019). In the spatial scale, an increasing trend of euphotic depth was observed from the head to the tail in both Hanjiang and Danjiang reservoir basins. The result suggested that, the spatiotemporal variation of euphotic depth was mainly based on the hydrological conditions and impoundment regulation in the river-reservoir hybrid system. The stepwise regression analysis revealed that, the euphotic depth was determined by different factors in different hydrological periods. In the high flow period and median flow period, the euphotic depth was mainly determined by the suspended inorganic particulate and phytoplankton; however, in the low flow period, the he euphotic depth was determined by the chlorophyll a and dissolved organic carbon.