藻华高风险期微囊藻在洱海中的垂直分布特征及其影响因素

大量微囊藻浮力调节及其垂向迁移是形成蓝藻水华的重要机制之一.为研究微囊藻的垂直分布特征及影响因素，以洱海北部湖心为监测点位，采用野外采样室内分析的方法，于2016年9—12月对微囊藻生物量、伪空胞体积、粒径等指标进行了测定.结果表明:9—11月微囊藻生物量逐月增长并于11月达到峰值（1.77 mg/L），在12月出现下降（月均值为0.34 mg/L）；9月、11月微囊藻伪空胞体积（18.0~22.6 μm3/cell）较高，10月、12月较低（10.2~17.3 μm3/cell）；9—11月微囊藻的垂向迁移速率（5.4~14.5 m/d）、漂浮百分率（57%~96%）及群体粒径（91~305 μm）逐渐增大，12月明显减小.微囊藻生物量、伪空胞体积及群体粒径的垂直分布规律均表现为表层最高，中层次之，底层最低.9—11月，微囊藻在洱海中出现垂直分层现象，伪空胞提供的浮力作用和藻群体粒径作用大于风力扰动作用，促使微囊藻在水柱中主动迁移并聚集在水面；而在12月，风力扰动对微囊藻垂直分布的影响远超过浮力及粒径的作用，微囊藻在水柱中趋于均匀分布.研究显示，藻华高风险期微囊藻在洱海呈弱分层分布，且浮力及群体粒径是影响其垂直分布的主要因子. 

Vertical Distribution of Microcystis and Its Influence Factors in Lake Erhai during High Risk Period for Algal Bloom

The buoyancy regulation and vertical migration of Microcystis in the water column are one of the most important mechanisms for the formation of cyanobacteria blooms. To reveal the vertical distribution characteristics of Microcystis and its influence factors, the central site in the north of Lake Erhai was selected as the study area. Microcystis biomass, gas vesicle volume and colony size were investigated through field sampling and laboratory analysis from September to December, 2016. The results showed that Microcystis biomass increased steadily from September to November, when it reached the peak value (1.77 mg/L) and then decreased significantly in December, in average of 0.34 mg/L. The gas vesicle volumes of Microcystis were higher in September and November (18.0-22.6 μm3/cell) than those in October and December (10.2-17.3 μm3/cell). The floating rates (5.4-14.5 m/d), percentage of floating colonies (57%-96%) and colony sizes (91-305 μm) increased consistently from September to November and then decreased in December. The biomass, gas vesicle volume and colony sizes all decreased with the increase of water depth. From September to November, Microcystis showed a weak stratified distribution in Lake Erhai and the effect of buoyancy provided by gas vesicle volumes and colony size defeated the wind turbulence. Thus, Microcystis could migrate actively in the water column and accumulate on the water surface. In December, the impact of wind turbulence on the vertical distribution of Microcystis greatly exceeded that of buoyancy and colony size. Microcystis tended to be distributed uniformly in the water column. According to the study, Microcystis showed a weak stratification distribution in Lake Erhai during the high risk period of algal blooms and the buoyancy and colony size were the main factors influencing its vertical distribution.