The variation of microcrustaceans diversity downstream of small reservoirs is influenced by litoranean taxa

ABSTRACT The hypotheses tested were that, as well as in reservoirs of large surface areas, volumes and flow, in small reservoirs there is also a gradual and continuous dilution of the richness, diversity and abundance of microcrustaceans (Cladocera and Copepoda) downstream reservoirs, without the recomposition of richness downstream due to the lack of tributary rivers, marginal lagoons and even the small size of the stream, with no conditions for the development of its own potamoplankton. Two small reservoirs were studied and in their downstream stretches, with daily collections every six hours at eight sampling points, two lentic and six lotic downstream from each. Nineteen species were found (11 from Cladocera and eight from Copepoda), with emphasis on the greater number of coastal species than pelagic ones, and the greater abundance of coastal cladocerans at the points downstream of the reservoirs. There was no difference between the times, indicating that the spatial scale was more important than the temporal scale. There were higher values of ecological attributes in lentic environments, but there were significant increases in richness and diversity in the last points of the stretches of the stream, while the abundance decreased gradually downstream. No significant correlations were found for the abundance of taxa with any physical and chemical variable (temperature, pH, dissolved oxygen, turbidity, total dissolved solids and transparency), indicating little or no effect of these variables. The cluster analysis with Euclidean distance separated two large groups, the first formed by the stretch downstream of the first reservoir and the second with the other points. In addition to the presence of litoranean taxa coming from reservoirs, there was an increase in litoranean taxa resident in streams. It can be concluded that there was a dilution of the abundance of species downstream of the streams, but the richness and diversity remained stable due to the contribution of litoranean taxa.

摘要 本研究验证的假说为：与大表面积、大体积及大流量的水库类似，小型水库下游的微型甲壳动物（microcrustaceans）的物种丰富度、多样性及个体丰度同样会呈现逐渐且持续的稀释效应；且由于缺乏支流、沿岸泻湖乃至河道自身尺寸过小，不具备形成自身河流浮游生物（potamoplankton）的条件，下游无法通过类群补充实现物种丰富度的恢复。本研究选取两座小型水库及其下游河段作为研究对象，在每座水库下游设置8个采样点（2个静水生境点、6个流动水生境点），并以每6小时一次的频率开展每日采样。本研究共记录19个物种（11种枝角类（Cladocera）、8种桡足类（Copepoda）），其中沿岸类群的数量显著多于远洋类群，且水库下游采样点的枝角类个体丰度更高。不同采样时段间无显著差异，表明空间尺度对群落结构的影响强于时间尺度。静水生境的生态参数整体取值更高，但在河道下游末端的采样点中，物种丰富度与多样性出现显著提升，而个体丰度则沿下游方向逐渐降低。各分类群的个体丰度与所有理化变量（水温、pH、溶解氧、浊度、总溶解固体及透明度）均未呈现显著相关性，表明上述变量对群落丰度的影响微弱甚至无影响。基于欧氏距离（Euclidean distance）的聚类分析将所有采样点划分为两大类群：第一类为第一座水库下游河段，其余采样点构成第二类。除了来自水库的沿岸类群外，河道原生的沿岸类群数量亦有所增加。综上可见，河道下游的物种个体丰度呈现稀释效应，但由于沿岸类群的补充贡献，物种丰富度与多样性得以保持稳定。