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The response of Cyperus papyrus (L.) and Miscanthidium violaceum (K. Schum.) Robyns to eutrophication in natural wetlands of Lake Victoria, Uganda

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Interrelationships between nutrient concentrations and aerial biomass, root numbers, length and rhizomes size in Cyperus papyrus (L.) (papyrus) and Miscanthidium violaceum (K. Schum.) Robyns (synonymous to Miscanthus violaceus (K. Schum) Pilg.) were established in five different wetlands around the northern shores of Lake Victoria in Uganda. The two macrophytes were also grown experimentally under controlled nutrient conditions. There were no significant differences between the rhizome sizes of Miscanthidium in the different wetlands, but those of papyrus varied significantly. Papyrus showed no significant differences in root lengths and numbers under different nutrient conditions in the different wetlands studied. Miscanthidium on the other hand tended to have more and shorter roots in nutrient-rich wetlands as opposed to fewer and longer roots under low nutrient concentrations. Below-ground biomass values for floating papyrus swamps had an average of 1 296g dry wt m-2 and rooted papyrus swamps, 776g dry wt m-2. The aerial biomass of papyrus showed a positive response to higher nutrient conditions. In wetlands receiving domestic wastewater, values ranged from 3 973–6 255g dry wt m-2: in wetlands without wastewater, the aerial biomass values were lower (1 983–2 862g dry wt m-2). Miscanthidium did not show any clear response to nutrients, having a more or less uniform aerial biomass of 2 065 98g dry wt m-2. Results from growth experiments showed that above- and below-ground biomass values were higher for papyrus than Miscanthidium. Whilst papyrus gave a clear positive growth response to increased nutrient conditions, Miscanthidium did not. It was concluded that papyrus vegetation had a better potential for the treatment of nutrient-enriched water than Miscanthidium. Keywords: tropical wetland vegetation; biomass; Cyperus papyrus; Miscanthidium violaceum; nutrients; eutrophication; wastewater treatment (Afr J Aqua Sci: 2002 27(1): 11-20)

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