Eutrophication development and its key regulating factors in a water-supply reservoir in North China.

Yanghe Reservoir is an important source of drinking water for Qinhuangdao City, North China; however, in recent decades this water source has been eutrophic with recurrent summer cyanobacterial blooms. The trophic grade of the system in summer was mesotrophic-eutrophic in 1990 and became hypertrophic in 2011. The nutrient availability is extremely high during the entire year, and the water temperature should be the primary driver of the summer blooms. In May-October of 2010 and 2011, abrupt variations were observed in the Secchi depth (SD) and chlorophyll a (Chl-a), and both the correlated analysis of Chl-a-SD and trophic status indices (TSI) deviation (TSI(Chl-a)-TSI(SD)) showed that algal cell density dominated light attenuation. During the algal bloom outbreak, the microcystin concentration was found to vary between 0.35-2.12 microg/L in 2010 and 0.11-1.86 microg/L in 2011. The maximum microcystin content was more than two times the safety limit required for drinking water. Inflow discharges were most concentrated in the summer, with periods of lower residence time and the largest water level fluctuation over the entire year. When a high availability of nutrients promoted a high Chl-a concentration in the whole system, it appeared that the instability caused by the decrease in residence time could not produce effective changes in the cyanobacterial abundance. The results indicated that nutrient enrichment in the aquatic systems of Yanghe Reservoir is the most serious problem and that the status would not been modified effectively by increasing hydrological fluctuations (e.g., decreasing the residence time). Therefore, decreasing the nutrient concentrations is the only route to improve the water quality of this reservoir.