Abstract

Established microcosms containing surface water from the mesotrophic Lake Mokolo were subjected to enrichment of different concentrations with phosphorus. The microcosms were sampled through a 20 day succession period to determine the net effects of increased concentrations of phosphorus in the water column on the phytoplankton community structure during the winter and summer seasons. A significant increase in chlorophyll a (chl-a) was observed when treatments of 30, 40 and 60 µg L^-1 P were compared to the controls. On day zero in both the winter and summer microcosm experiments, all four phosphorus treatments had similar species diversity of phytoplankton of the specific seasoned tested (winter: Margalef index = 223; summer: Margalef index = 347). However, 13 days after the addition of 40 and 60 µg L^-1 P, the phytoplankton community exhibited a strikingly different species richness (winter: Margalef index = 123; summer: Margalef index = 114). In the winter microcosm experiments, the green alga Scenedesmus armatus dominated the phytoplankton composition at enrichment levels of 40 and 60 µg L^-1 P up to day 20. The biovolume of the dinophyceae Ceratium hirundinella declined rapidly after the addition of 40 µg L^-1 P in the different summer microcosms. In the summer microcosms, Spondylosium secedens and Microcystis aeruginosa dominated the 40 and 60 µg L^-1 P microcosm enrichment experiments.

Key words: Scenedesmus armatus, southern Hemisphere lake, enrichment, cyanobacteria, threshold phosphorus concentrations, chlorophyll a (chl-a) concentration