WETLAND VEGETATION COLONIZATION AND EXPANSION IN SMALL IMPOUNDMENTS IN YAOUNDE, CAMEROON, WEST-AFRICA

Temporal changes in the pattern and distribution of tropical wetland v egetation in four small impoundments over a 28-year period were docume nted for a wetland ecosystem in the Olezoa drainage basin in Yaounde, Cameroon, West Africa. These impoundments were constructed along the O lezoa 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 vege tation. Evaluation of the impoundments and the surrounding catchment w as accomplished using low altitude aerial photographs for the years 19 64, 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 th an 50% reduction from original depths of 300-400 cm to less than 150-2 00 cm in 1992. We suggest that vegetation colonization and expansion i n the impoundments is a function of the reduction of water depth due t o siltation accompanying increasing urbanization. During the period of evaluation, urbanization in the surrounding catchment tripled. Siltat ion in the impoundments resulted mainly from the accumulation of soil eroded from the upland accompanying devegetation and construction acti vities. The shallowing of the impoundments by siltation provided a fav orable water-depth regime and substrate for colonization by aquatic an d emergent wetland vegetation. Continued shallowing resulted in develo pment of vegetation zones along a water-depth gradient, and the migrat ion of these zones toward the interior of the impoundments. The histor y of degradation of the catchment area and siltation of the impoundmen ts is thus recorded by temporal changes in vegetation.