风生紊流导致微囊藻群体破碎和形态变化

Wind induced turbulence caused colony disaggregation and morphological changes in the cyanobacterium Microcystis*

微囊藻群体大小和形态决定其垂向迁移能力,从而影响着水华的形成.为了探讨湖泊中风生紊流对微囊藻群体大小和形态的影响,本研究于2012年8月26日至9月7日在太湖梅梁湾的围隔内进行了12 d的昼夜不间断的高频采样(采样间隔每2小时一次).研究期间,水面微囊藻密度呈现4次周期性消涨,藻密度变化范围为4×10^(4)~2671×10^(4)cells/m L.而整个水柱中的藻密度变化范围仅为3×10^(4)~18×10^(4)cells/m L.皮尔逊相关性分析表明微囊藻的原位生长速率与表面藻密度呈负相关而与风速呈正相关.强风速使微囊藻在水柱中均匀分散,增强了透光性,促进了微囊藻的生长.微囊藻群体粒径随着风速的增大逐渐减小,反之亦然.其中值粒径(D50)变化范围为66.2~768.0μm.在此期间微囊藻群体形态主要以鱼害微囊藻、不规则的惠氏微囊藻、球状的惠氏微囊藻和铜绿微囊藻群体形态为主,其占比也呈现出波动状态.皮尔逊相关分析结果显示微囊藻群体大小与风速呈负相关,说明湖泊中风生紊流会影响微囊藻群体大小.当紊流强度为2.33×10^(-5)m^(2)/s3时,微囊藻群体会发生破碎现象,该紊流强度相当于5 m/s的风在30 m深的水库或湖泊中所产生的紊流强度.微囊藻群体被风生紊流破碎后最大粒径与该风速下紊流的最小涡旋尺度相近,表明紊流的最小涡旋尺度决定了微囊藻所能形成群体的最终大小.监测期间,整水柱中不同群体形态的微囊藻占比发生了明显变化,在监测初期以鱼害微囊藻群体形态为主,随后不规则的惠氏微囊藻和铜绿微囊藻群体形态的比例不断增加,最后鱼害微囊藻群体形态又占据主导地位.球状的惠氏微囊藻群体形态在整个监测期中的比例随时间的增加而逐渐降低.不同群体形态微囊藻之间比例的大幅变化无法用微囊藻生长演替来解释.而皮尔逊相关分析结果显示鱼害微囊藻与惠氏微囊藻(不规则的和球状的惠氏微囊藻之和)群体形态之间存在负相关,且惠氏微囊藻与铜绿微囊藻群体形态呈负相关.但在今后研究中需进一步关注在微囊藻群体形态的动态变化过程中细胞大小、胶被、产毒特性和基因序列等特征,从而验证不同种微囊藻群体是否存在形态转换这一猜想.总而言之,普通强度的风生紊流能够破碎微囊藻群体,而气候变化导致的内陆湖泊周边风速下降会促使微囊藻形成更大的群体,从而有利于水华的形成.

Colony size and morphology influence the vertical migration capacity of Microcystis and therewith the occurrence of surface accumulations or blooms.To explore the influence of wind-induced turbulence on the colony size and morphology of Microcystis in field conditions,a high-frequency field investigation was conducted in an enclosure in Meiliang Bay,Lake Taihu,China,from26 August to 7 September 2012.A Pearson’s correlation analysis indicated that the in situ growth rate of Microcystis was negatively related to surface cell density and positively related to wind speed.Strong wind speed stimulated Microcystis growth by enhancing light transmission due to the homodispersion of Microcystis in the water column.The Microcystis colony size was negatively correlated with wind speed,suggesting that wind-induced turbulence could break up colonies in shallow lakes.The results indicated that Microcystis colonies could be broken up by a turbulence intensity of 2.33×10^(-5) m^2/s3,which corresponds to an average wind speed of5.00 m/s in a reservoir with 30-m depth.Different Microcystis morphotypes were present and negative relationships were detected between the proportion of Microcystis ichthyoblabe and the proportion of Microcystis wesenbergii and between the proportion of Microcystis aeruginosa and the proportion of Microcystis wesenbergii out of all Microcystis throughout the water column,but more evidence is need to support the hypothesis that the morphology of Microcystis colonies changes over time.Altogether,the results suggest that declining wind speed,driven by climate change,will promote surface blooms of Microcystis due to the formation of larger colonies.