Geographical, historical and physical aspects of Lake George

Abstract

The equatorial location gives to Lake George (surface area 250 km 2 ) a climatic régime which makes for an exceptionally unvaried physico-chemical environment within the water mass throughout the year. This is enhanced by peculiarities of the local geomorphometry and the morphometry of the lake. Incident solar energy, according to 10-day running means, varies during the year only ± 13% of the daily mean of 1970 J cm -2 . The mean air temperature of 23 °C is always equable (mean daily temperature = 29.5 ± 1.5 °C, mean nightly, 16.5 ± 1.5 °C), and is in equilibrium with the minimum water temperature of the bottom layers of the lake, the surface temperatures of which are usually about 30 °C. The effect of two, approximately equal, dry seasons is offset by the presence of mountainous catchment areas which have a high runoff and thus permit a continuous flow through the lake, which amounts to a mean flushing rate of 2.8 times the mean lake volume per year, so sustaining the supply of primary nutrients. These features promote a continuous productivity, although the allochthonous supplies of nutrients are not thought to be important compared to the total flux of nutrients within the lake. The wind régime, which is strongly influenced by local convectional rather than continental forces, assists convection currents in the water in effecting a nocturnal turbulence which provides an efficient recirculation of nutrients within the shallow water column (2.4 m). This turbulence, alternating with diurnal stratification, against the background of the relative constancy of the climate, means that the lake is dominated by a 24 h physiological cycle rather than the seasonal succession of other latitudes. Some evidence indicates that the concentric distribution of plankton in the lake is established by wind initiated rotary currents. Wave movements at the sediment/water interface are thought to disturb the majority of the sediment surface area possibly down to between 5 and 14 cm in depth, with a mean frequency of an order of magnitude of once every 3 weeks. Progressive organic enrichment has existed throughout the lake’s history (3600 ± 90 years), but during the last approximately 700 years an equilibrium appears to have been reached as shown by a retarded rate of change of organic deposition.