Diurnal Mixing and the Vertical Distribution of Phytoplankton in a Shallow Equatorial Lake (Lake George, Uganda)

In an optically deep lake, such as Lake George, where light attenuation depends chiefly upon the phytoplankton density, the growth success of individual species is related not only to the photosynthetic capacity of the algae, the nutrient status of the cells, and respiration losses but also to the time that the algae are resident within the euphotic zone. The vertical distribution of phytoplankton, and therefore the time that cells remain at specific depths (characterized by the light and nutrient climate at those depths), is a function of the excess density of the organism (e.g. see Smayda 1970) and water movements. Smayda distinguished three excess density states: (a) where the density of the organism (p') is greater than the density of the bathing medium (p) (i.e. p' >p), (b) p' = p, (c) p' <p. In a completely static water body these three states will result in sinking, no movement or an upward movement of organisms. For most lakes, however, water movements distort these simple relationships. The interaction between excess density and water movement is manifested by the observed patterns of vertical phytoplankton distribution. A great number of papers illustrate the multifarious distribution patterns of phytoplankton that occur in lakes and seas. Most blue-green algae contain gas vacuoles which are thought to act as regulatory buoyancy mechanisms (Fogg 1969). Theoretically, those blue-green algae that have extended vacuoles should float to the surface, whereas those with collapsed vacuoles would sink. Examples of accumulations of blue-green algae at the surface are well known in eutrophic lakes of Europe and North America. Examples within Africa include the differing responses of algal species to diurnal changes in a Nile reservoir (Talling 1957), and other marked differences in relation to the annual cycle of thermal stratification in Lake Victoria (Talling 1966). For a very small but strongly stratified temperate water, Happey (1970) has illustrated the inter-relationship between diatom distribution, silica concentration and thermal stratification in a small temperate water body. Lund (1959) remarked upon the seasonal differences of the vertical distribution of two species of blue-green algae, Anabaena circinalis and Oscillatoria agardhii. Reynolds (1971, 1972) successfully illustrated the influence of gas vacuoles upon the vertical distribution of planktonic blue-green algae in the North Shropshire meres, England. Fogg & Walsby (1971) have given a general account of buoyancy in the bluegreen algae, but tended to neglect the influence of water turbulence upon their theoretical depth distribution patterns of blue-greens. There appears to be no documented account * Present address: Department of Botany, University of Adelaide, Adelaide, South Australia 5001.