Temporal distribution of phytoplankton in Lake Nyamusingiri in the Albertine Rift Valley, Uganda

摘要

AbstractA study aimed at investigating the temporal variation of phytoplankton assemblages in Lake Nyamusingiri was carried out during the period of December 1997-May 1998. Uganda's freshwaters are ecologically diverse but a few are intensively studied. Research on phytoplankton has been restricted to large water bodies. There is little information on phytoplankton of the western Uganda crater lakes, which are important water and biodiversity resources. This study provided baseline data on phytoplankton, which will serve as a basis for monitoring the effects of human activities on the lake that might result in ecological transformations like loss of biodiversity because of overexploitation. A laboratory thermometer and Winker's method were used to determine temperature and dissolved oxygen concentration, respectively. Lake transparency was measured by using the Secchi disc. A Van Dorn sampler was used to collect water samples. Nutrient and chlorophyll a concentrations were determined by using facilities at the Fisheries Resources Research Institute (FIRRI), Jinja. The Sedgwick-Rafter counting chamber was used to analyse phytoplankton. Variation in temperature was small (25.4-26.2C). Stable thermal stratification was not evident. The Secchi disc transparency was less than unity. The chlorophyll a value was high. Biomass was found to be light-limited by nonalgal materials. Dissolved oxygen concentration was more than 100% in the surface waters but declined to <20% at the bottom, which reflected the eutrophic nature of the lake. Diversity indices were low. Eighteen species and five classes of phytoplankton were revealed by this study. The phytoplankton flora was dominated by chlorococcal green algae characteristic of the large eutrophic East African lakes. ResumeUne etude destinee a investiguer la variation temporelle des assemblages de phytoplancton dans le lac Nyamusingiri a ete realisee entre decembre 1997 et mai 1998. Les eaux douces ougandaises sont variees au point de vue ecologique, mais peu sont etudiees en profondeur. La recherche sur le phytoplancton s'est limitee aux grandes etendues d'eau. On dispose de peu d'informations sur le phytoplancton des lacs de cratere de l'ouest de l'Ouganda, qui sont d'importantes ressources en eau et en biodiversite. Cette etude fournit des donnees de base sur le phytoplancton, qui serviront de reference pour suivre les effets des activites humaines sur le lac, qui pourraient entrainer des transformations ecologiques telles qu'une perte de biodiversite due a la surexploitation. Nous avons utilise un thermometre de laboratoire et la methode de Winker pour determiner respectivement la temperature et la concentration d'oxygene dissous. La transparence du lac fut mesuree au moyen d'un disque de Secchi. Nous avons utilise une bouteille de Van Dorn pour prelever des echantillons d'eau. Nous avons determine les concentrations de nutriments et de chlorophylle a en utilisant les installations du Fisheries Resources Research Institute (FIRRI), a Jinja. Une cuve de comptage de Sedgwick-Rafter fut utilisee pour analyser le phytoplancton. Les variations de temperature etaient faibles (25.4-26.2C). La stratification thermique stable n'etait pas evidente. La transparence du disque de Secchi etait de moins de un. La valeur de la chlorophylle aetait elevee. On a decouvert que la biomasse etait limitee parce que la lumiere etait filtree par des materiaux autres que des algues. La concentration d'oxygene dissous etait superieure a 100% dans les eaux de surface, mais diminuait jusqu'a moins de 20% au fond, ce qui reflete la nature eutrophique du lac. Les indices de diversiteetaient bas. Cette etude a aussi revele la presence de dix-huit especes et cinq classes de phytoplancton. Celui-ci etait domine par des algues vertes chlorococcales caracteristiques des grands lacs eutrophiques de l'Est africain.