Water circulation and fish larvae recruitment in papyrus wetlands,Rubondo Island,Lake Victoria

A field study was undertaken of the hydrodynamics, water quality and adult and larval fish abundance in papyrus wetlands and surrounding coastal waters at Rubondo Island, Lake Victoria. Because they were exposed to the prevailing wind, the two bays facing east had small (0.01 km²) wetlands and were well flushed, with minimal accumulation of organic detritus and little oxygen depletion. Because it faced west, the third bay, Mlaga Bay, was sheltered and poorly flushed; organic matter accumulated in a large (1.41 km²) papyrus wetland. The rises and falls of the lake level at decadal time scales, may, by drowning or drying out wetlands, contribute significantly to storing organic matter as detritus. Vegetation decay in the wetlands, as well as primary production, resulted in large diurnal fluctuations of pH and dissolved oxygen concentration. Shading in daytime and cooling at night, cooled the wetlands water sufficiently to generate a baroclinic circulation whereby cold, wetland water sank and moved offshore in the lake while warm, lake water intruded in the wetlands near the surface. This flushing prevented the occurrence of anoxic conditions in the wetlands which were used by adult and larval fish, mainly tilapia, and freshwater shrimps (Caridina nilotica). No fish larvae were found at more than 150 m from the shore in the three bays. Both larval fish and Caridina nilotica appeared to move in and out of the wetlands at night, possibly as a result of low dissolved oxygen as well as sheltering from predation. Their abundance varied with the lunar phase, with maximum near new moon and first quarter. Being the only protected wetlands in Lake Victoria, Rubondo Island may increasingly become an important source of replenishment for fisheries in the lake which elsewhere appears overfished.