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-inver- tebrates associated with hyacinth mats were inventoried. The phytoplankton commu- nity 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 abun- dances the L- zones were more similar to the P than to the L + zones. There was an in- creasing importance of chlorophytes ( Staurastrum sp. and Pandorina morum ) and di- atoms (Cyclotella meneghiniana and pennales) and a decreasing dominance of Mycro- cystis along the discriminant axis from L +, L- to P. The zooplankton community was most dense in the unvegetated zones. Daphnids and bosminids 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 ac-

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