Short-term vertical distribution of phytoplankton populations in a shallow tropical lake (Lake Municipal, Yaounde, Cameroon)

Abstract

The circadian (i.e. 24-hours) vertical distribution of phytoplankton community was examined for the first time in a small, shallow and nutrient-rich tropical lake (Lake Municipal, Yaounde, Cameroon), in relation to environmental factors. Sampling was done during the late dry season and the water column was very stable. Phytoplankton community was diverse and was dominated by cyanobacterial species which averaged (± SD) 97 ± 1% and 62 ± 14% of total phytoplankton abundance and biovolume, respectively. The abundances of these species consistently peaked in the surface layer. Their dominance was related to their positive buoyancy in stable environments, and the resulting shading of surface waters that possibly outcompeted eukaryotic algae. Although most of the extracted chlorophyll was apparently not from the dominant cyanobacterial species, chlorophyll-a concentrations (25.3 to 234 μg/l), as well as phytoplankton total abundance (1.42 to 11.73 x 108 cells/l) and biovolume (18.24 to 135.78 x 109 μm3/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 Oscillatoria 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 conclude that these patterns were related to several factors, including light conditions, temperature, cell buoyancy, the potential of motile flagellates to avoid adverse conditions such as grazing pressure, and accorded to the autoecologies of algal species involved.