Spatial and temporal variation of the phytoplankton structure in cascading oligotrophic reservoirs of southeast Brazil

Abstract: Aim Structural changes in phytoplankton was evaluated in one ultraoligotrophic and two oligotrophic cascading reservoirs during two climatic periods (austral winter and summer) of 2014. Changes in phytoplankton biomass, species composition, and codons (functional groups) were evaluated along a vertical and a longitudinal profile. Methods Four sampling sites were selected for the first reservoir in the cascade (Cachoeira do França) and three for each of the other two reservoirs (Cachoeira da Fumaça and Serraria). Samples were collected along a vertical and a longitudinal profile. Species biomass and codons ‘sensu’ Reynolds were identified and expressed in terms of biomass. Results 18 codons were identified and associated to the reservoirs’ limnological conditions. Composition and biomass of descriptor species changed among seasons. Codon B consisting mainly of Discostella stelligera was the most frequent in all reservoirs in both climatic seasons. Conclusions Our findings showed that seasonality was the key factor for the species composition and phytoplankton codons changes. However, the cascade effect on phytoplankton structure was shown by the continuous contribution of Ceratium furcoides (codon LM) during the winter, and of Discostella stelligera (codon B) in both sampling periods in all three reservoirs.

Abstract：研究目的：于2014年的两个气候期（南半球冬季与夏季），针对1座超贫营养（ultraoligotrophic）及2座贫营养（oligotrophic）梯级水库（cascading reservoirs）中的浮游植物（phytoplankton）群落结构变化展开评估。研究沿垂直与纵向断面，分析浮游植物生物量、物种组成及功能群（codons，即Reynolds分类体系下的划分单元）的动态变化。

研究方法：选取梯级首座水库——弗朗萨瀑布水库（Cachoeira do França）的4个采样点，另外两座水库——弗马萨瀑布水库（Cachoeira da Fumaça）与塞拉里亚水库（Serraria）各设置3个采样点。于垂直与纵向断面采集样本，鉴定物种生物量及Reynolds所定义的功能群，并以生物量为单位表征各项结果。

研究结果：共鉴定出18个功能群，其分布与各水库的湖沼学条件密切相关。指示物种的组成与生物量随季节发生显著改变。以星冠盘藻（Discostella stelligera）为主要类群的功能群B，在两个气候期的所有水库中均为最优势的功能群。

研究结论：本研究结果表明，季节动态是驱动浮游植物物种组成与功能群结构变化的核心因素。但梯级效应同样对浮游植物群落结构产生影响：冬季时段，叉状角藻（Ceratium furcoides，功能群LM）持续发挥优势贡献；在两个采样期内，星冠盘藻（Discostella stelligera，功能群B）在三座水库中均保持稳定的优势地位。