The impact of water hyacinth (Eichhornia crassipes) in a eutrophic subtropical impoundment (Lake Chivero, Zimbabwe).: II.: Species diversity

We compared abundance and diversity of phytoplankton, zooplankton, and fishes among limnetic (P: always without macrophytes) and littoral habitats with (L+) and without (L-) hyacinths in Lake Chivero, a man-made hypertrophic reservoir near Harare (Zimbabwe). In addition, the littoral macrophyte community, and macro-invertebrates associated with hyacinth mats were inventoried. The phytoplankton community was dominated by blue-green algae (mainly Microcystis aeruginosa), typical for a hyper-eutrophic lake. Total absolute densities were about 10 to 30 times higher at the L+ sites than at the unvegetated L- and P sites. On the basis of relative species abundances the L- zones were more similar to the P than to the L+ zones. There was an increasing importance of chlorophytes (Staurastrum sp. and Pandorina morum) and diatoms (Cyclotella meneghiniana and pennales) and a decreasing dominance of Mycrocystis along the discriminant axis from L+, L- to P. The zooplankton community was most dense in the unvegetated zones. Daphnids and bostminids were more abundant in the pelagic than in both littoral zones. Calanoids and Diaphanosoma were dominantly represented in the unvegetated zones. The two littoral zones were characterised by higher densities of chydorids, while they could be discriminated by the dominance of cyclopoids in the vegetated site. Seventeen different fish species were captured by at least one of the different fishing methods. Apparent habitat preferences differed according to fishing method. Generally, Oreochromis niloticus and Pharyngochromis acuticeps preferred the vegetated sites, while mature specimens of Clarias gariepinus were caught in deeper water at pelagic sites. Barbus paludinosus and Labeo cylindricus preferred the rocky shores of the lake. The smaller size classes of O. niloticus, Tilapia sparrmanii and P. acuticeps preferred the littoral zone rather than the open waters. Water hyacinth mats generally seem to have a positive effect on taxon diversity only in fishes and were the preferred sites for only a limited number of groups, mainly zooplankton (cyclopoids and Daphnia laevis) and fishes (O. niloticus and R acuticeps). The most important function of this weed could be to offer shelter and feeding grounds for small fishes.