The impact of water hyacinth, Eichhornia crassipes (Mart) Solms on the abundance and diversity of aquatic macroinvertebrates along the shores of northern Lake Victoria, Uganda

Abstract

This study examined the impacts of the alien waterweed, water hyacinth, on the abundance and diversity of aquatic macroinvertebrates in the littoral areas of northern Lake Victoria in Uganda. The weed had undergone explosive growth on the lake causing serious disruption to people, the economy and the ecosystem. This study was confined to impact of the weed in the littoral zone, not to the large floating mats of vegetation which float across the lake and clog large areas of shoreline. The littoral area studied comprised of fringing mats of Eichhornia crassipes (Mart) Solms (water hyacinth) to the lakeward of Cyperus papyrus; water hyacinth mats undergoing colonisation by Vossia cuspidata (Roxb.) Griff.; and a typical Cyperus papyrus L shore with no outer floating mat of water hyacinth. Numerical abundance (Nos. m−2) and diversity (No. of taxa) of macroinvertebrates recovered from pure Eichhornia crassipes and the Eichhornia-Vossia succession increased from the fringe of the Cyperus papyrus towards the open water. In the typical Cyperus papyrus fringe, in the absence of water hyacinth, abundance was highest at the papyrus/open water interface and dropped off sharply towards open water. The Shannon–Weaver diversity index (H′) of macroinvertebrates decreased progressively from pure Eichhornia crassipes stands, to Vossia/Eichhornia beds and Cyperus papyrus stands (H′=0.56, 0.54 and 0.34, respectively) but were not significantly different. Dissolved oxygen decreased from open water into vegetation where it approached anoxia. Water hyacinth appeared to enhance the abundance and diversity of aquatic macroinvertebrates at the interface with the open water. The impoverished abundance and diversity of the macroinvertebrates deeper into the vegetation mats suggested negative environmental impacts of the water hyacinth when the fringe is too wide. Further research is recommended to establish the optimum width of the fringe of stationery water hyacinth that promotes maximum abundance and diversity of aquatic macroinvertebrates and, possibly, of other aquatic life. Since this study in 1997, there has been a dramatic decrease in Eichhornia infestations and by June 2000 it appeared largely to exist only as fringing vegetation in bays and inlets.