Size-structure of phytoplankton biomass and driving factors in east Lake Dongting

不同粒径大小浮游藻类的养分吸收速率、沉降特性和能流方向等都不相同，浮游藻类生物量的粒级组成变化对湖泊生态系统的结构与功能具有重要影响.为了解通江湖泊浮游藻类粒级组成演替规律及其驱动机制，于2018年9月—2019年9月对东洞庭湖进行了年度采样调查，研究了不同粒级浮游藻类的时空分布特征及其与环境因子的关系.结果表明：东洞庭湖浮游藻类叶绿素a总浓度呈现显著的时空分布差异；季节上表现为夏季（22.43 μg/L） > 秋季（16.95 μg/L） > 春季（11.69 μg/L） > 冬季（3.28 μg/L），空间上表现为北部湖区（26.12 μg/L）>南部湖区（15.81 μg/L） > 东部行洪道（5.88 μg/L）.纳微型藻（3~20 μm）是东洞庭湖浮游藻类生物量的主要贡献者，其在冬季优势度最高，为68.0%；春季开始，超微型藻（0~3 μm）的贡献量逐渐增加，到夏季达到最高值，为42.1%；粒径最大的微型藻（>20 μm）占比最低，全年平均占比16.2%.RDA限制性排序结果表明，不同粒级浮游藻类对环境因子的响应趋势相同，但适应能力不同；温度、水位、营养盐和pH等是影响东洞庭湖浮游藻类粒级结构的重要因素.;Cell size is an important element determining phytoplankton physiological and ecological processes, including nutrient uptake, sinking and grazing; thus, phytoplankton size-structure plays an important role in the structure and function of lake ecosystem. To understand the succession patterns and driving factors of total chlorophyll-a and size structure of phytoplankton, field in-situ investigation was conducted from September 2018 to September 2019 in the east Lake Dongting, a Yangtze River- connected lake. The total phytoplankton chlorophyll-a biomass showed significant seasonality and spatiality. The highest concentration of total chlorophyll-a was observed in summer (22.43 μg/L), followed by autumn (16.95 μg/L), spring (11.69 μg/L) and winter (3.28 μg/L). Spatially, the total phytoplankton chlorophyll-a was highest in the north (26.12 μg/L) and lowest in the east (5.88 μg/L) of the lake. Phytoplankton was overwhelmingly dominated by nanophytoplankton (3-20 μm). The proportion of nanophytoplankton to total phytoplankton chlorophyll-a biomass was highest in winter (68.0%), whereas the proportion of picophytoplankton (0-3 μm) increased gradually in spring and peaked in summer (42.1%). The contribution of microplankton (>20 μm) to total phytoplankton chlorophyll-a biomass was lowest with a mean of 16.2% all year round. The results of redundancy analysis indicated that phytoplankton with different cell sizes showed similar responsive trends to environmental changes, while their adaptabilities were different. Temperature, water levels, nutrients and pH were the key factors explaining size-structure of phytoplankton in east Lake Dongting.