Floodplains mostly consist of primary and secondary river beds, connecting channels and lakes that are either isolated from rivers and channels for most of the year, or lakes that are connected to rivers and channels for most (or all) of the year. We hypothesize that the differences in invertebrate communities, as calculated by beta-diversities, will be higher in isolated than in connected lakes. We use ostracod communities in the pleuston of the floating macrophyte Eichhornia crassipes in the Upper Paraná River Floodplain to test this hypothesis.
We have observed significant differences in species composition between the two types of lakes. However, although beta diversity values are indeed slightly higher in isolated than in connected lakes, these differences are not significant. This lack of clear effect of isolation on beta diversity could be owing to the fact that the period of isolation since the last homogenizing flood pulse had not been long enough, or because differences in degree of isolation become altogether insignificant in periods of low water flow.