Wetland vegetation colonization and expansion in small ...

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 o f 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 evaluation, 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

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 nuisance (Imevbore 1969, Little 1969, Obeng 1969), concerns about fish production (Andesanya 1969, Little 1969, El-Zarka 1973, Obeng 1973, ) or traffic obstructions (Little 196% EI-Zarka 1973). Smaller impoundments in this region have received little or no attention. In Yaounde, Cameroon, several small impoundments 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-

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 preimpoundment wetland-type vegetation in the Kainji area, Nigeria to determine possible impacts on the newly formed Lake Kainji. Aquatic and semiaquatic vegetation associations for the Lake Kainji reservoir have been documented by Cook (1968), while the effects o f 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 354

Atekwana et aL, V E G E T A T I O N IN S M A L L I M P O U N D M E N T S cause of the urban setting and small sizes of these impoundments, the dynamics of vegetation establishment cannot be directly inferred from studies of large man-made lakes and thcir associated riverine systems. This study, therefore, investigates the causes of the establishment of wetland vegetation in small impoundments in a tropical urban setting by examining the temporal changes in the distribution o f wetland vegetation for a period of 28 years, STUDY AREA AND B A C K G R O U N D The study is situated near the headwaters of the Olezoa stream in the Olezoa drainage basin, located in Yaounde, the capital of Cameroon, West Africa (Figures l a and b). The climate of Yaounde is dominated by a rainy season from March to June, a short dry season in July and August, a short rainy season in September and November, and a dry season from December through February (Institute de Recherche Scientific du Cameroun 1956, Lacaze 1977). The average annual precipitation is 1500 m m and temperature is 24°C for this area (Institute de Recherche Scientific du Cameroun 1956), The present Olezoa wetland complex is a modified riparian system. In 1948 and 1957, small basins were excavated and earthen dams constructed within the Olezoa floodplain to lorm a series of impoundments with connecting stream segments (.see Table 1 for dates of construction, clam elevations, impoundment stage, depth ranges, and original open water surface areas). Topography in the study area is moderate to steeply sloping with elevations ranging from about 711 m above mean sea level at the lowest dam to approximately 800 m above mean sea level along the northern portion of the drainage divide. Agriculture in the study area is limited to crop farming, which is limited to small manually cultivated plots that cover less than 5% of the study area. Between 1960 and 1975, the establishment of the University of Yaounde, the National Higher School of Polytechnic (NHSP), and the University Center for H e a l t h S c i e n c e s and U n i v e r s i t y Hospital C e n t e r ( U C H S / U H C ) stimulated rapid urbanization of the study area. Because the City o f Yaounde lacked a centralized sewage treatment lacility, a small conventional primary and secondary sewage treatment plant was constructed for the University of Yaounde in 1967. A similar sewage treatment plant was constructed for the U C H S / U H C in 1975. The design of the treatment plants incorporated mechanical screening, aeration, and biological digestion chambers, with the effluent discharged through sand filtration beds into the subsurface. Operational problems at these facilities led to the discharge of untreated sewage and wastewater into

355

the Olezoa stream and the impoundments. Surface water quality data within the study area (Tchuinkam 1987, Agendia et al. 1992 ) indicate high nutrient and inorganic contaminant levels that have been attributed to sewage discharge. Since 1975, aquaculture in these impoundments has been plagued with problems of excess weed growth and disease. Fish harvests have declined through time, leading to the abandonment of fish farming in all the impoundments by 1988 (Souley Roua pers. comm. 1992). Four of the impoundments; the Retenue, Atemcnguc, Mclcn, and Obili (Figure lc) were selected for study. METHODS The use of remote sensing (aerial photography, Landsat Multispectral Scanner) as a tool for cultural and environmental resource management has been well-documented (e.g., Richason 1983, Colwell 1983, Holz 1985). The collection of remotely sensed data at different times and scales allows for the detection, description and analysis of temporal and spatial landscape changes. Photointerpretation techniques, when combined with other types of data, provide the most accurate classification and assessment of temporal landscape changes (Dunn et al. 1991). This method is used routinely by the United States Environmental Protection Agency's (USEPA) Environmental Photographic Interpretation Center for Wetland Studies (Norton and Prince 1987). Aerial photographs have been used to document temporal changes of wetlands in an urbanizing agricultural landscape near Syracuse, New York (Thibault and Zipperer 1994). In this study, aerial photograph interpretation is combined with field investigations, reviews o f historical records, interviews with long-term residents, personal observations, and discussions with the M A I F personnel in order to characterize temporal changes in open water surface, vegetation within the impoundments, and urbanization in the upland areas of the study site. Aerial photographs from 1964, 1974, and 1986 were used in this study. All photographs are at a scale of I: 10,000, are orthogonal, and were acquired during the month of December (dry season). Although the Obili impoundment was constructed in 1948 and the others in 1957, 1964 aerial photographs were used as the starting point for monitoring temporal changes because they are the earliest available aerial photos of the study area. From these photographs, a c o m m o n map base was generated using A U T O C A D ~---~by digitizing the foltt~wing emitie~; impoundment boundaries; arc~t of the open water surface within the impoundments; area covered by vegetation within the impoundments and along the connecting stream segments, and the extent of urbanization in the uplands.

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Atekwana et al., V E G E T A T I O N IN S M A L L I M P O U N D M E N T S Table 1. Date of construction, dam elevation, impoundment stage, impoundment depth range, and open water surface area for impoundments at the study site upon construction. Date of ConstrucImpot,ndment tion

Dam Elevation (m)*

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719.3 717.5 715.6 711.9

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Open ImpoundWater ment Depth Surface Stage* Range Area tin) (m) (ha) 719.6 716.8 714.3 711.4

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* M e a s u r e m e n t s a r e a b o v e m e a n s e a level. D a t a o b t a i n e d f r o m the Ministry of Agriculture, Induslries and Fisheries (MAIF), Came r(-)orl.

Open Water Surface Area of the Impoundments Calculations of the open water surface area were made for each impoundment. From the 1964 photographs, the areas enclosed by polygons defining the open water surface within these impoundments were determined using the A U T O C A D ~ area calculation function. Open water surface areas for 1974 and 1986 were determined similarly. As no post-1986 aerial photographs were available for the study site, open water surface areas for the impoundments were determined in 1992 from field measurements using a 1 × 1 m grid. The percent relative change in the surface area for each impoundment since 1964 was calculated using the following formula: C~=((A-r~ - AT,)/A-m) × 100%, where C,4 is the percentage change in the 1964 impoundment open water surface area. ATr, is the open water surface area of the impoundments in 1964. A.rl is the open water surface area of the impoundments in 1974, 1986 or 1992. Vegetation and Water Depth Even though textural elements identified in the I: 10,000 scale photographs permitted differentiation of water and vegetation, the types of plants and their spatial distribution within the impoundments could not be

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resolved. Therefore, the identification and classification of vegetation was accomplished by field analysis. Two nearly perpendicular transects were laid out across each impoundment (Figure lc), along which plant species were identified and their spatial distribution noted. The transects consisted of a 3-m-wide strip along which vegetation was continuously sampled by placing a 1 × I m frame divided into 16 quadrats of 25 × 25 cm over the vegetation. The plant species present along the transects were identified and the percent areal cover estimated. The vegetation along the transect was subsequently placed into three categories based on the dominant plant species: an aquatic zone, dominated by aquatic floating-leaved and surface floating vegetation, a mixed zone consisting of both surface floating and rooted emergent vegetation, and an emergent zone dominated by rooted emergent vegetation only. A species was considered dominant if it accounted for more than 10 percent cover along a transect. Random sampling of vegetation at other locations within the impoundments was conducted to check for significant deviations from the measurements along transect lines. Additionally, the depth from the water surface to the top of the sediment layer within the impoundments was measured at l-m intervals along profiles coincident with the vegetation transects. The depths were measured using a 3-m rod fitted with a 25 cm 2 plate at the zero mark to minimize penetration into the bottom sediment layer. The rod was lowered to the bottom and the height Lo the water surface recorded.

Urbanization Changes Areas where natural vegetation had been altered by construction, farming, and other activities were considered "urbanized" in this study. The extent of urbanization in the upland portion of the study site was restricted to areas in the drainage basin that contribute direct runoff from precipitation to the impoundments. The urbanized portions of the upland were outlined on the 1964, 1974, and 1986 aerial photographs. Urban extent in 1992 was estimated by mapping urban changes onto a 1990 topographic map at a scale of 1: 10,000. The outlined area was transferred to the 1964 base map. The urbanized areas outlined by polygons

Figure I. (a) The location of Yaounde, (b) location of tile study area within the Olezoa drainage basin, and (c) study site. Shown are points of sediment sample collection and the location of vegetation transects/depth profile surveys. The direction of vegetation and depth surveys are indicated by arrows. NHSE National Higher School of Polytechnic and UCHS/UHC, University Center for Health Sciences and University Hospital Center.

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(solid lines) and urbanized area (dashed line) for 1964, 1976, 1986, and 1992. were calculated in a similar manner to the open water surfaces. Nature of Sediments in the Impoundments In order to determine the nature and composition of sediments in the impoundments, three grab samples (500 to 1000 g) were collected from each impoundment in 1994 (see sample locations in Figure lc). The samples were oven dried at 110°C for 72 hours. Sediments were examined microscopically to determine the presence of plant litter. To determine the amount of organic and mineral matter present, 10 g of sediments from each sample location within the impoundments were placed in a crucible and heated in a high temperature furnace at 260°C for 5 hours. The samples were weighed and the process repeated until a constant weight was obtained. Any loss in weight was attributed to oxidized organic matter. The percentage of organic and mineral matter was calculated. RESUI~TS Free Water Surface and Changes in Urbanization Measured opcn water surface areas for the impoundments and urbanized area in the uplands is summarized in Figure 2. Open water surfaces declined from 1964 to 1992. No open water surface was observcd for the Retenue and Obili impoundments in 1992. Compared to the 1964 open water surface areas, the decline in open water surface area was relatively slow between 1964 and 1974, moOerate between 1974 and 198fJ, and rapid between 1986 and 1992, The calculated percent change (decline is indicated with a ( - ) ) in the open water surfaces of the impoundments from i964 to 1974, 1974 to 1986, and 1986 to 1992 are as follows:

Retenue: - 1 8 . 2 , - 3 6 . 4 , - 1 0 0 ; Atemengue: I 1.5, - 4 5 . 3 , - 9 6 . 9 ; Melcn: - 15.4, - 3 0 . 8 , - 6 9 . 2 ; and Obili: - 2 2 . 7 , - 1 3 . 6 , - 1 0 0 . The minimum calculated open water surface decline from 1964 to 1992 is about 70% for the Melen impoundment over the 28-year period. Maximum calculated open water surface decline rates per year are as follows: 2.2% measured for the Obili impoundment for the period 1964-1974, 4.0% measured for the Atemengue impoundment for the period 1974-1986, and 16% measured for the Retenue impoundment for the period 1986-1992. Measured urbanized area (Figure 2) and cunmlative change in urbanization (Figure 3) show the nature of urban changes in the upland portions of the study area from 1964 to 1992. Estimated urbanized areas for 1964, 1974, 1986, and 1992 are 21.4, 59. 3, 81, and 85.9 ha respectively. The most rapid urbanization occurred between 1964 and 1986_ There seem to have been minimal changes in the extent of urbanization (4.9 ha) from I986 to 1992. Most of the urbanization occurred around the Obili impoundment in the southern portion of the study area. Water Depth Measured depths of the impoundments along profiles are plotted in Figure 4. Compared to the original depths, there is significant depth reduction in all im poundments, in most cases exceeding 50%. The Rctenue impoundment, the deepest constructed, had measured depths in 1992 of approximately 100-200 cm compared to original depths of 300--400 cm. The other impoundments had maximum depths of approximately 150 cm in 1992, compared to their original depths of 200-400 cm. Water depth is defined as the depth to the water stage, which is equivalent to the water depth below the elevation of the spillway at each dam. Profilcs constructed ti'om depth measurements for the impoundments show steeply sloping sides to near maximum depth in a roughly symmetrical pattern (except for profile 2 in the Retenue impoundment) along both the long and short profiles. The bottom topography for the longer profiles, particularly in the Retenue impoundment, are flat to undulatory. Vegelation The wetland vegetation apparent in the 1964 photographs occurred mainly in the northern portion of the impoundments and in the connecting stream segments. In the 1974 photograplas, the denned impoundment boundaries remained consistent with those observed in 1964. However, expansion of vegetation is noticeable around the northern fringes of the impoundments. There are also isolated pockets of vegetation in

A t e k w a n a et at.. V E G E T A T I O N IN S M A L L I M P O U N D M E N T S

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Figure 4. Vegetation zoilation and measured water depths along vegetation transects/water dcpth survcy line~ within impoundments. the interior of the impoundments. Water is visible in the vegetated areas in the photographs. In the 1986 aerial photographs, the outlines of the 1964 impoundment boundaries still persist. Within the impoundments, the interior was dominated by open water but interrupted by pockets of vegetation. Between the open water surface and the edges of the impoundments, vegetation zonation could be observed. There were moderately vegetated areas where water was visible in the photographs, which graded to densely-vegetated areas where water was not observed around the fringes of the impoundments. The broad vegetation zonation observed in the aerial pliotograph,s was also apparent in the |]dd. :~tudics oil w e t l a n d - t y p e vegetation in C a m e r o o n ( k e t o u z e y t968), tropical Africa (Schnell 1976, Raynal-Roques 1980) and West Africa (Hutchinson and Dalziel 1958, Nielsen 1965) formed the basis for identification of

wethtnd pl:lnts in the study area. Vegetalion associations from the Kahlji reservoir (Cook 1968), studies on submerged and floating-leaved aquatic macrophyles (Denny 1985a) arid wetland vcgctaticm and associated plant lifc-furni (Denny 1985b) were used as guides in resolving vegetation zonation within the impoundments. The plant species identified are emergents and euhydrophytes, They were grouped into emergent. mixed, and aquatic 7ones along vegelalion transects. These zones are typically dominated by wetland floating-leaved, surface floating, and rooted emergent plants. The phtrits observed and their eslimated percentage cover along transects within each impoundment are shown m Table 2.

Emergent zone, The zone near and along the margin of the impoundments is characterized by emergenl vegetation dominated by Echino