SHORT-TERM VERTICAL-DISTRIBUTION OF PHYTOPLANKTON POPULATIONS IN A SHALLOW TROPICAL LAKE (LAKE MUNICIPAL, YAOUNDE, CAMEROON)

The circadian (i.e. 24-hours) vertical distribution of phytoplankton c ommunity was examined for the first time in a small, shallow and nutri ent-rich tropical lake (Lake Municipal, Yaounde, Cameroon), in relatio n to environmental factors. Sampling was done during the late dry seas on and the water column was very stable. Phytoplankton community was d iverse and was dominated by cyanobacterial species which averaged (+/- SD) 97 +/- 1% and 62 +/- 14 % of total phytoplankton abundance and bi ovolume, respectively. The abundances of these species consistently pe aked in the surface layer. Their dominance was related to their positi ve buoyancy in stable environments, and the resulting shading of surfa ce waters that possibly outcompeted eukaryotic algae. Although most of the extracted chlorophyll was apparently not from the dominant cyanob acterial species, chlorophyll-a concentrations (25.3 to 234 mu g/l), a s well as phytoplankton total abundance (2.42 to 11.73 x 10(8) cells/l ) and biovolume (18.24 to 135.78 x 10(9) mu m(3)/l), were substantial and characteristic of productive waters. The vertical distribution of the dominant phytoplankton species exhibited three different patterns, according to the depths where their maximum abundances were recorded: (1) surface water populations represented by the cyanobacteria Oscill atoria limosa and Synechocystis pevalekii, (2) mid-layer species, the euglenoid flagellate Lepocinclis fusiformis, and (3) populations that actively avoided the surface water during the nighttime, the euglenoid Trachelomonas hispida and the cryptomonad Cryptomonas ovata. We concl ude that these patterns were related to several factors, including lig ht conditions, temperature, cell buoyancy, the potential of motile fla gellates to avoid adverse conditions such as grazing pressure, and acc orded to the autoecologies of algal species involved.