Ecological stability in a shallow equatorial lake (Lake George, Uganda)

Abstract

The ecological stability of Lake George is illustrated by the relatively small biomass fluctuations that occur in the fauna and flora of the lake. For the phytoplankton it is shown that the gradient of these oscillations is within the range depicted by a specific rate of population increase of +0.05 to -0.05 day -1 . The relative stability of the phytoplankton community is discussed in the terms of a mathematical model (Tailing 1957, 1971) which interrelates the variables, photosynthesis, respiration, biomass and the underwater light climate. Examples of the interrelationship, and the extreme susceptibility of either a positive or a negative increase rate to diurnal environmental changes, are drawn from both long and short term measurements in Lake George. The optical characteristics of the water column are shown to be very dependent upon the mixing regime sustained over the previous 24 h. The occurrence of negative or positive increase rates is dependent not only upon the incident radiation but also upon the wind induced turbulence. It is shown that, given measurements of the rates of photosynthesis and respiration and the other variables included in Tailing’s model, the biomass at which the Lake George phytoplankton will stabilize is predictable. Although light is likely to limit photosynthesis under all circumstances in Lake George, it is demonstrated that a more favourable photosynthetic response to the light climate may be induced by nutrient enrichment of the nutrient-poor phytoplankton. Because nutrient enrichment can induce increased photosynthetic capacity values, the rates of nutrient supply are thought to be major factors in controlling the ecological stability of the phytoplankton. The mixing regime of the lake is of paramount importance in the decomposition of the sediments and the rate of nutrient release from them. The importance of the nutrient recycling via zooplankton excretion is compared with the total input of nitrogen and phosphorus to the lake. That the ecological stability of the lake is a delicately balanced equilibrium between climate and biota is illustrated by calculations on the oxygen budget of the water column, and by the occurrence of fish-kills. The general characteristics of the lake are discussed in the context of the problems associated with eutrophication.