Global warming can intensify the El Niño phenomenon that recurs every 2-7 years, which will lead to a great interannual variability of climate and may induce the deterioration of the water quality of reservoirs. To study the influence of the extreme El Niño events on the water quality of stratified reservoirs during summer, field surveys were conducted in Zhoucun Reservoir and its inflow rivers from May to August in a normal year (2012) and a strong El Niño year (2015). Temporal variations of physical and chemical index were investigated during monitoring. The results showed that the Zhoucun Reservoir was stratified during the study period. The precipitation in the summer of the normal year was significantly higher than that in the El Niño year at the same period. In the summer of the normal year, the water level increased from 124.26 m to 127.14 m and the hypolimnion thickness increased by 3.1 m. However, in 2015, the rapid decrease of the water level from May to August (from 121.65 m to 119.46 m) led to the decrease of the hypolimnion thickness (by 3.2 m). The inflow rivers belonged to surface current and its nutrients concentrations were obviously higher than those in the epilimnion. The inflow nutrients loads increased significantly in the summer of the normal year, as a result, total nitrogen increased from 1.00 mg·L-1 to 2.06 mg·L-1, nitrate increased from 0.19 mg·L-1 to 1.28 mg·L-1, and total phosphorus increased from 0.023 mg·L-1 to 0.088 mg·L-1 in the lacustrine zone of the reservoir. In contrast, the nutrients concentrations changed little in the summer of the El Niño year due to the decrease in runoff. Nonetheless, the reducing pollutants concentrations of the hypolimnion in the El Niño year were significantly higher than those in the normal year, which may be due to the temporal variations of hypolimnion thicknesses. The maximum concentrations of iron, manganese, ammonium and sulfide in the summer of the El Niño year were 0.38, 1.36, 2.36 and 1.67 mg·L-1, respectively. All these index exceeded the standards for surface water Class Ⅲ. We conclude that the extreme El Niño event has an apparent influence on the nutrients concentrations in the epilimnion and the pollutants concentrations in the hypolimnion in Zhoucun Reservoir.
Keywords: El Niñ; hypolimnion; nutrients; o; stratification; thermocline.