[Spatial and Temporal Distribution of Chlorophyll a and Its Relationship to Algae and Environmental Factors in Aha Reservoir]

As one of the drinking water sources for Guiyang City in southwest China, the Aha Reservoir has an area of 190 km² and a volume of 5.42×10⁸ m³. The water depth is less than 30 m, with an average depth of 13 m. Regulated by subtropical humid monsoon climate, it has cool summers and warm winters, with an annual mean air temperature of about 15.3℃ and an annual normal rainfall of approximately 1,129 mm. Impacted heavily by human activity (e.g., untreated industrial and domestic sewage and agricultural non-point pollution sources), the eutrophication problem in the Aha Reservoir has become more serious each year. In order to explore the spatial and temporal distribution characteristics of chlorophyll a (Chl-a) and its relationship to algae and the driving factors in the Aha Reservoir, phytoplankton and water samples were collected in the dry period, normal period, and flood period. The results showed a significant seasonal variation in Chl-a, same as biomass, but not the same as the algal abundance. Highest Chl-a concentration (91 μg·L^-1) occurred in the mean season with the dinoflagellate bloom but during dry and wet seasons, they were only 8 μg·L^-1 and 16 μg·L^-1, respectively. During the dry and flood periods, the Chl-a concentrations in surface waters were slightly higher than the other layers caused by sufficient light and dissolved oxygen. But in the normal period, the Chl-a concentrations in surface waters were far higher than the other layers because of the dinoflagellate bloom assembling in surface waters. Located at the reservoir entrance of Jinzhong River, Dam sampling point owned higher Chl-a concentration than Kuzhong as a result of higher nutrients. Correlation analysis indicated that Dinoflagellate was positively correlated with Chl-a (R=0.798, P<0.01). Chl-a was positively associated with total phosphorus, dissolved oxygen, pH value, and total nitrogen (R=0.762, P<0.01; R=0.792, P<0.01; R=0.658, P<0.01; R=0.388, P<0.05) and it had a negative correlation with the N/P ratio and nitrate nitrogen (R=-0.37, P<0.05; R=-0.435, P<0.05). Stepwise regression analysis showed that TP, N/P ratio, and DO were the most important factors influencing the temporal and spatial distribution of Chl-a. Thermal stratification and water temperature were also the significant factors that could not be ignored.