SPATIAL VARIATION OF WITHIN-DAY AND BETWEEN-DAY CHLOROPHYLL A DYNAMICS DURING SUMMER IN GUANTING RESERVOIR, BEIJING, CHINA

Recognizing intra- and inter-daily dynamics of chlorophyll a (Chl-a) and its related environmental variables on consecutive days is essential to assessing and managing water quality and eutrophication. In this study, the Chl-a concentration, nutrients, water temperature and meteorological factors in Guanting Reservoir in Beijing were collected at six sampling times during summer. The Chl-a concentration generally decreased from May to September. At both test times, thermal stratification and mixing in the water column controlled temporal and vertical variations in the maximum Chl-a concentration layer. The position of the maximum Chl-a concentration layer between days generally followed that of the thermocline. Daily stratifications were temporary and downwelling variations in the maximum Chl-a concentration layer were wind driven; therefore, the vertical distribution of Chl-a was homogenized at night. Surface Chl-a concentrations increased at night and decreased during the day, except on rainy days. Principal component analysis and Pearson's correlations indicated that daily average and raw surface Chl-a concentrations generally changed as a negative function of air temperature, solar radiation, wind speed and water temperature. However, when a 5-hour time lag was considered, the relationship between surface Chl-a concentration, all meteorological factors and water temperature became significantly positive.