WETLANDS, Vol. 15, No. 4, December t995, pp. 354~364 ,~ 1995. The Society of Wetland Scientists WETLAND VEGETATION COLONIZATION AND EXPANSION IN SMALL IMPOUNDMENTS IN YAOUNDE, CAMEROON, WEST AFRICA Eliot A. Atekwana, I Philip L. Agendia,: Estella A. Atekwana, I and Theophilc H. Fonkou-" Dept. of Geology Western Michigan University Kalamazoo, MI "~ Dept. ~" Plant Biology FacutO' of Science University of Yaounde C~tmeroon, West Africa Abstract: Temporal changes in the pattern and distribution of tropical wetland vegetation in four small impoundments over a 28-year period were documentcd for a wetland ecosystem in the Olezoa drainage basin in Yaounde, Cameroon, West Africa. These impoundments were constructed along the Olezoa stream for the purpose of aquaculture and ranged in size from 1.4 to 3.0 ha, had depths between 200 to 400 cm, and were devoid of vegetation. Evaluation of the impoundments and the surrounding catchment was accomplished using low altitude aerial photographs for the yeurs 1964, 1974, and 1986 and field surveys for the periods 1985 through 1992. The results reveal a progressive decline in open water surface area of the impoundments ranging from 70 to 100% due to plant colonization and expansion. Measured impoundment depths along profiles show more than 50% reduction from original depths of 300-400 cm to less than 150~200 cm in 1992. We suggest that vegetation colonization and expansion in the impoundments is a function of the reduction of water depth due to siltation accompanying increasing urbanization. During the period of eval- uation, urbanization in the surrounding catchment tripled. Siltation in the impoundments resulted mainly from the accumulation of soil eroded from the upland accompanying devegetation and construction activities. The shallowing of the impoundments by siltation provided a favorable water-depth regime and substrale for colonization by aquatic and emergent wetland vegetation. Continued shallowing resulted in development of vegetation zones along a water-depth gradient, and the migration of these zones toward the interior of the impoundments. The history of degradation of the catchment area and siltation of the impoundments is thus recorded by temporal change~ in vegetation. Key Words: Atemengue, aquaculture, aquatic, Cameroon, effluent discharge, impoundment, Melen, Obili, Olezoa, open water surface area, Retenue, tropical, sewage, siltation, vegetation colonization, vegetation expansion, wastewater, West Africa, wetland, Yaounde INTRODUCTION In tropical West Africa, the monitoring of riparian aquatic and other wetland-type vegetation has been limited to large man-made lakes and their associated riverine systems. Visser (t973) surveyed preimpound- ment wetland-type vegetation in the Kainji area, Ni- geria to determine possible impacts on the newly formed Lake Kainji. Aquatic and semiaquatic vegeta- tion associations for the Lake Kainji reservoir have been documented by Cook (1968), while the effects of large annum drawd~wn in the reservoir on vegetation expansion has been studied by Morton and Obot (1984I. Factors controlling the development of aquatic weeds have been studied in the Volta basin, Ghana (Hall et at. 1969, Hall and Okali 1974). Other studies of aquatic vegetation changes in these large man-made lakes were carried out because of their association with infectious parasites that caused a public health nui- sance (Imevbore 1969, Little 1969, Obeng 1969), con- cerns about fish production (Andesanya 1969, Little 1969, El-Zarka 1973, Obeng 1973, ) or traffic obstruc- tions (Little 196% EI-Zarka 1973). Smaller impound- ments in this region have received little or no attention. In Yaounde, Cameroon, several small impound- ments between 1.4 and 3.0 ha were constructed along the Olezoa ~trcam for the purpose of aquaculture by the Cameroon Ministry of Agriculture, Industries and Fisheries (MAIF). Over time, these impoundments were colonized by vegetation, leading to difficulties in maintenance and resulting in their abandonment. Be- 354