[Long-term Changes and Drivers of Ecological Security in Shahe Reservoir, China]

Monthly datasets of ecological indicators from 2010 to 2020 in Shahe Reservoir, Tianmuhu, China, were examined to reveal the long-term variations in water ecological security and its driving factors. The results of Secchi disk depth(SD) measurements revealed significantly spatial variation(P<0.05) within the reservoir. The highest SD was recorded in the downstream-linked reservoir, and the lowest SD was recorded in the upstream tributaries. In contrast, the values of other water ecological indicators were higher in the upstream tributaries than in the transition region and the downstream-linked reservoir area. In summer and autumn, the SD was low, while the concentrations of total phosphorous(TP), chlorophyll a(Chl-a), the permanganate index, and cyanobacterial biomass(BM_c) were high. During the thermal stratification period from May to September, the concentrations of 2-methylisoborneol(MIB) and Chl-a were highest at a depth of 4 m, while diatom biomass(BM_b) and BM_c reached their maximum at depths of 2 m and 0.5 m, respectively. Therefore, spatial and temporal variations should be fully considered when evaluating aquatic ecological security. Focusing on spring and summer, when the risk of water ecological security was high, Chl-a combined with SD and MIB along with their correlation with other water quality indexes, was used to evaluate and optimize the ecological security of Shahe Reservoir. The evaluation results showed that the aquatic ecological security of the reservoir was excellent over the last 10 years; however, annual fluctuations have been large and the evaluation scores were spatially variable. While seasonal sampling strategies focusing on three layers depths are economical and reliable for lake regions with thermal stratification, our results indicate that tailored monitoring may be required to determine the aquatic ecological security of lakes and reservoirs. In Shahe Reservoir, the decrease in the SD and the increase in MIB caused by high TP and algal blooms were the most important drivers of ecological service function in the reservoir. Furthermore, hydrometeorological factors may also play important roles in the aquatic ecological security of reservoirs.