Composition et distribution spatio-temporelle des protozoaires ciliés dans un petit lac hypereutrophe du Cameroun (Afrique centrale) Composition and spatio-temporal distribution of protozoan ciliates in a shallow and hypereutrophic lake in Central Africa (Cameroon)

Résumé

La composition et la distribution spatio-temporelle des ciliés du Lac Municipal de Yaoundé (Cameroun) ont été analysées d’avril à décembre 1997, en relation avec des paramètres physico-chimiques du milieu. Trois stations pélagiques et l’herbier littoral ont été échantillonnés. 58 espèces de protistes ciliés ont pu être identifiées, parmi lesquelles environ 60 % d’espèces littorales et périphytiques et 37 % d’espèces pélagiques. Les espèces les plus courantes sont les espèces pélagiques Uronema nigricans et Coleps hirtus. L’abondance saisonnière des ciliés a atteint une valeur maximale de 21 800 ind•L^-1, les valeurs les plus élevées étant enregistrées dans la couche trophogène du lac, particulièrement au cours de la grande saison des pluies. Les variations de la biomasse totale des ciliés sont plus faibles, avec des maxima correspondant également aux fortes pluies. Les espèces dominantes en matière de biomasse sont Prorodon africanum, Coleps hirtus et Uronema nigricans. Enfin, les fluctuations saisonnières de l’abondance et de la biomasse des ciliés sont discutées en relation avec les paramètres environnementaux, parmi lesquels le régime pluvial tient apparemment un rôle central.

Mots clés:

• lacs tropicaux,
• protozoaires ciliés,
• dynamique des populations,
• hypereutrophe

Summary

The Municipal Lake of Yaoundé (3° 51’ 37’’ N and 11° 30’ 40’’ E) is a shallow water body (Z_m = 4.3 m), situated in downtown Yaoundé, the political capital of Cameroon (Central Africa). The composition and the spatio-temporal distribution of ciliates in combination with physical and chemical water variables were measured in this lake from April to December 1997. Sampling was carried out weekly, at 5 depths (surface, 0.5 m, 1 m, 1.5 m and 2.5 m) and at 3 pelagic stations, A, B and C, representing the upper, middle and the lower parts of the lake, using a Van Dorn sampling bottle. In the littoral station, the periphytic fauna is collected by agitation of the vegetation in a bucket and filtration of water. The water temperature was measured with a thermometer and the pH was measured in the field with a portable pH-meter. The oxygen concentration was determined according to the Winkler method and the transparency of the water column was determined with a Secchi disk. Light availability was measured with a luxmeter. Water colour, suspended solids and ammonium-nitrogen (NH₄-N) were analyzed spectrophotometrically using APHA methods. Ciliate species were identified using appropriate and specific keys and were counted under a stereomicroscope at magnifications of 250X and 500X.

Results showed that the lake was hypereutrophic. Throughout the study, the temperature and pH values of the water remained around 25°C and 7, respectively. The dissolved oxygen concentration decreased with depth to almost zero at the lake bottom, while the Secchi disk transparency did not exceed 1 m. Spatio-temporal variations of these variables indicated that the middle zone (0.5 to 1.5 m) was the stratum with a critical role in the functioning of the lake. Indeed, it represented the transitional zone between the trophogenic upper layer where photosynthetic activity occurred and the bottom water layer, formed by the microaerophilic to anaerobic tropholitic layers where reducing processes were important.

Fifty-eight ciliate species, belonging to 10 orders and 28 families, were identified among which five (Holophrya sp., Lagynophrya rostrata, Lagynophrya simplex, Pseudoprorodon sp. and Histriculus sp.) were new records for Cameroon fauna. This population contained 62% of littoral species and 37% of pelagic species respectively. Within these taxa, dominant species were Uronema nigricans (present at all pelagic stations) and Coleps hirtus (present at more than 97% of the pelagic stations). Their abundance was greater than 2,000 ind•L^-1 during the study period showing variation with many peaks. Prorodon africanum occurred sporadically between October and November when rainfall decreased and their densities reached 5,700 ind•L^-1 in November at 0.5 m depth at station A. After November, this species was replaced by Prorodon sp. Most of the species collected were cosmopolite or characteristic of shallow tropical water bodies.

The total abundance of the ciliate community was high reaching 21,800 ind•L^-1 at station A, at 0.5 m depth. According to several authors, the abundance of ciliates in such tropical water bodies varies from 3.6 to 9.75 x 10⁴ ind•L^-1. The highest abundances were observed at depths of 0.5 m to 1.5 m at all 3 stations, especially during the rainy season. This corroborated the importance and the role of this stratum in the functioning of Lake Yaoundé. The lowest abundance of ciliates was observed at station A, at the bottom (2.5 m depth). The total abundance of ciliates showed one or several peaks during the period from July to November. With respect to the total biomass, the variation was more regular with generally one peak during the rainy season. At certain depths at stations B and C, several peaks were observed during the period of April to June and from September to November. The total biomass values obtained were higher than those reported by other authors. This can be explained by the fact that reported values in these latter studies were expressed as dry weight while values reported in this study were expressed as wet weight. With respect to biomass, the dominant species was Prorodon africanum (5,300 µg•L^-1), followed by Coleps hirtus (3,800 µg•L^-1) and Uronema nigricans (3,100 µg•L^-1). The highest biomass was 130 x 10⁵ µg•L^-1, and was observed at station A, at 0.5 m depth.

Generally large size ciliates were more abundant during the dry season whereas small size species were abundant during rainy season. Chilodonella uncinata and Prorodon africanum grew in surface layers (between the surface and 1 m depth) while Paradileptus conicus and Uronema nigricans preferred deep water (1 to 2.5 m depth). There was a great development of ciliates during the transitional period between the rainy and dry seasons. Moreover there was a close relationship between environmental parameters and the ciliate community. In fact, 2 to 3 physico-chemical predictors (dissolved oxygen, temperature and conductivity or pH) explained spatio-temporal distributions of different species. The variation of the total biomass in station A was explained by the dissolved oxygen concentration (r² = 0.366; p<0.001) and by the pH (r² = 0.274; p<0.001); at stations B and C, the variation was explained by ammonium-nitrogen (r² = 0.178 and r² = 0.294 for p<0.01). The most important abiotic factor that influenced the density and biomass variation of ciliate communities of Lake Yaoundé was the precipitation rate. Throughout the study, station B was the most characteristic of the structure of the ciliate community of this lake.

Key words:

• ciliates,
• spatio-temporal abundance,
• tropical lake,
• hypereutrophic