Observations on the limnology and primary production of a small man‐made lake in the West African savanna

Abstract

Summary1. The results of limnological studies on a small, 35‐ha, triangular man‐made lake and a small adjacent fish pond in the coastal savanna of Ghana are described.2. The lake, which has a renewal time of approximately 0.96 years, is subject to large, predictable seasonal changes. Water volume and surface area rapidly reach maximum values in the wet season from April to June despite the low runoff values which average only 1.2% of the precipitation. Minimum values occur in the dry season when up to 76% of the lake area may be exposed. Most of the water is lost by evaporation which may amount to 193 cm annually.3. The annual cycle in water temperature follows that in solar radiation with peaks towards the end of the dry season in March to April and deep troughs in the wet season from April to June. During the dry season the water column is characterized by greater stability, a higher degree of stratification, and a much higher amplitude of diurnal temperature change (4‐5°C) than in the wet season (1°C). Isothermal conditions tend to be approached nocturnally and even during the day in the wet season.4. The ionic content of the water is low and dominated by bicarbonate, sodium, silica and iron. During the wet season flood water results in a decline in conductivity from 214 to 30 μmhos/cm2, a decrease in pH and transparency, and an increase in oxidizable organic material. Except for the conductivity these values return to normal in about 6 weeks as a result of rapid decomposition of allochthonous materials.5. The oxygen pulse is strong and photosynthetic activity by phytoplankton high during the dry season whereas in the wet season the converse is true. As a result of the low concentration of organic carbon (0.10–0.35%) in the sediments their oxygen demands are low. Consequently, dissolved oxygen values near the sediment are relatively high and oxygen stratification poorly‐developed. Most of the organic carbon appears to be recycled in the water column rather than in the sediments as is generally the case in tropical ecosystems.6. Rooted vegetation became permanently established in the fish pond but not i n the lake itself. Emergent and floating vegetation which grows profusely in the draw‐down area in the wet season is for the most part exported from the system by wind and biological agencies and, therefore, contributes little to the energy and mineral budgets of the lake. Primary production in the water column is mainly by Myxophyceae and Bacillariophyceae and is low during the dry season (1.35 gC/m2/day) and very low in the wet season. Possible reasons for the low level of primary production are discussed.