Seasonal vertical stratification and the potential impacts of its fading on algal blooms in winter: Cases of Lake Chenghai and Lake Yangzonghai, Yunnan Plateau

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基于2014年10月-2016年7月在云南程海和阳宗海开展了4个季度（秋季，10月；冬季，1月；春季，4月；夏季，7月）的调查，研究了两个湖泊的水体分层特征，探讨了热力分层及其变化与其他环境因子对浮游植物生物量的潜在影响，结果显示：程海和阳宗海水体分层的特征均为冬季混合、春季形成分层、夏秋季分层稳定，两湖均属暖单次混合型湖泊；程海分层期温跃层的平均深度（顶界）、厚度和强度分别为17.70±3.89 m、5.54±4.44 m和0.67±0.43℃/m，阳宗海的分别为12.53±3.35 m、8.25±4.85 m和0.53±0.43℃/m.在热力分层稳定期，两湖底层达到缺氧甚至厌氧状态，底层的电导率总体较表层高.调查期间，两个湖泊水柱表层浮游植物生物量（以叶绿素a浓度表征）均在冬季出现峰值，程海和阳宗海的分别为19.22±11.08和45.82±9.41 μg/L；进一步分析发现，热力分层的消退可能是导致水体表层无机营养盐升高（底层供给）的重要原因，加之适宜的光热条件可诱导两湖冬季水华的发生；在其他季节转化期间，浮游植物生物量变化的主要影响因子亦具有一定的共性及湖泊与季节异质性.;The thermal stratification and its potential impacts on the phytoplankton biomass were studied through the four seasonal field investigations (October 2014-July 2016) in Lake Chenghai and Lake Yangzonghai, two mesotrophic deep lakes on the Yunnan Plateau, southwest China. The results revealed that these two lakes are both warm monomictic lakes, where the vertical stratification was occurred in spring, stabilized in summer and autumn, and disappeared in winter. The mean values of thermocline depth (the upper boundary), thickness and strength during the stratification periods (spring, summer and autumn) were 17.70±3.89 m, 5.54±4.44 m and 0.67±0.43℃/m in Lake Chenghai and were 12.53±3.35 m, 8.25±4.85 m and 0.53±0.43℃/m in Lake Yangzonghai, respectively. During the stabilized thermal stratification periods, the dissolved oxygen in the bottom of the water column reached less than 3 mg/L even 1 mg/L, and the specific conductance in the bottom was generally larger than that in the surface water column. In these two lakes, the maximum values of the seasonal-average algal biomass (represented by chlorophyll-a concentration) were both in winter, which was 19.22±11.08 μg/L in Lake Chenghai and 45.82±9.41 μg/L in Lake Yangzonghai. Moreover, the fading of thermal stratification may be an important factor that increases the inorganic nutrients in the surface of water column (from the bottom of water column). Thereafter, higher nutrients combined with suitable light conditions and water temperature could promote the occurrence of algal blooms in winter. Besides a certain similarity, the major influencing factors of seasonal phytoplankton biomass also had inter-lake and seasonal heterogeneity during the other periods of season changes.