THE MACROINVERTEBRATE FAUNA OF WATER HYACINTH FRINGES IN THE SUDD SWAMPS (RIVER NILE, SOUTHERN SUDAN)

The water hyacinth now forms an almost ubiquitous fringe to open-water habitats in the permanent wetland of the Sudd. In a typical river-lake the fringe varied between 9 and 16 m in breadth with highest plant biomasses in the centre. Largest quantities of macroinvertebrates in hyacinth root-mats were obtained in drop-trap samples within 6 m of the open water. They were dominated by coleopterans, odonatans, gastropods and Caridina nilotica. Outer edges of the fringe offer easy access, good dissolved oxygen concentrations, and a variety of potential food resources for aquatic invertebrates. By contrast the less hospitable landward zone contained a reduced fauna in which coleopterans, hydracarines and gastropods were prominent. A marked reduction in the numbers of invertebrates in lift-net samples of detached hyacinth rafts was attributed to browsing by fish. Hyacinth root-mats appear to have replaced the niches formerly provided by those of the Nile cabbage for aquatic invertebrates in the Sudd.