The role of papyrus plants (Cyperus papyrus) and internal waves in the nutrient balance of Lake Victoria, East Africa

Dissertação mest., Gestão da água e da costa, Universidade do Algarve, 2007

Amounts of nitrogen and phosphorus removed by the papyrus dominated wetlands of Lake Victoria basin were quantified. Lake levels and frequency of upwelling were assessed because they contribute to eutrophication. In the 10,235.17 km2 area of papyrus dominated wetlands, denitrification removes 53% annual nitrogen input, while aboveground biomass harvest can remove 26% and 19% annual nitrogen and phosphorus input per year respectively. To remove the existing load of nitrogen, it requires 1.3 years by denitrification alone while through above-ground biomass harvest; it requires 3.8 and 5.2 years for nitrogen and phosphorus respectively. Overdrawing of water through Jinja outlet caused a lake level decrease of 2.5 m from 2004 to 2006. Because they were out of water, papyrus wetlands lost 80% of their functionality as refuge and breeding grounds for tilapia. Because dried out papyrus die, burn, and are returned into the water, they can add 17 to 18,500 years in one year of current levels of nitrogen input into the lake, and emit 5.2 * 107 tonnes of CO2 to the atmosphere. Wedderburn number application shows upwell occurs when mean winds over the lake blows for 6 consecutive days by a speed of at least 5 m/s without the lake-land breeze. Surface water temperature recorded at hourly basis around Rubondo Island gives a lowest temperature value of 21.79oC, and shows patchy; island generated internal waves causing upwelling. Both lake level decrease and upwelling enhance eutrophication, emphasizing the need to conserve papyrus wetlands for the future health of Lake Victoria basin.