Optical characteristics of Daning River in the Three Gorges Reservoir and its impact on algae biomass

Using monitoring data from the Daning River in different seasons from April 2012 to January 2013, the factors influencing optical properties (euphotic depth and light attenuation) were analyzed, and its impacts on algae biomass in typical tributaries of river-style reservoirs were revealed. The operational period of the Three Gorges Reservoir was classified into four stages according to the water level: stage I (pre-November-April), stage II (May-July), stage III (July-September) and stage IV (September-November). The results indicate that the Daning River has a comparatively high spatial heterogeneity in the euphotic depths (Deu)(ANOVA, P<0.01). The mean values of euphotic depth in stage I were significantly greater than those in stage III and IV, This indicates that the concentration of suspended substance (SS) was the most important factor impacting optical properties, followed by transparency and chlorophyll-a (chla). The results of regression analysis showed that the SS influence on the Deu was significant, and there was no significant correlation between the Deu and the transparency and chla. Locally weighted regression analysis was performed to illuminate the relationship between index of feasible energy for phytoplankton (Et) and algae biomass. The resule indicated that in a certain range (100<Et≤300), an increasing of algae biomass was associated with increasing Et, while an decreasing of algae biomass was associated with increasing Et when exceeding a threshold value (Et>300).