Algal Bloom and Mechanism of Hypoxia in the Metalimnion of the Lijiahe Reservoir During Thermal Stratification

: The present study was completed at the Lijiahe Reservoir in Xi'an, northwest China. Based on persistent monitoring of the physical and chemical indexes, in addition to phytoplankton activity, the correlation between phytoplankton distribution and dissolved oxygen depletion in metalimnion were analyzed during the thermal stratification periods. The results showed that:①During the summer stratification (from June to September), an obviously steady structure of dissolved oxygen, pH, as well as conductivity, was formed in a water column with RWCS/H indexes between 2 and 10 m-1. ②The algal bloom was induced by the intense stratification with maximum biomass concentration of 2.95×108 cells·L-1 and chl.a of 46.42 mg·m-3. With a sharp density gradient due to thermal stratification, the decayed algae cells accumulated above the metalimnion and largely consumed the dissolved oxygen, which affected the pH, conductance, as well as the permanganate index. ③Decomposition of high-density algae was, therefore, the main cause of metalimnion hypoxia during the summer stratification periods.