Spatio-temporal Variability of Phytoplankton Community Species Composition, Biomass and Primary Productivity of Lake Bosomtwe (Ghana)

In this thesis, the community composition, biomass and primary productivity of the phytoplankton of Lake Bosomtwe on spatio-temporal scales are presented for the first time in relation to the physico-chemical environment. Horizontal and temporal/seasonal variabilities in the wet weight biomass, chlorophyll a, gross productivities, and community respiration as well as growth rates are also assessed. Phytoplankton community composition and biomass were assessed from 14 different stations in spatial surveys and 1 index central station for temporal studies. Samples were collected biweekly (temporal study) and in stratified and mixing periods (spatial study) with a 6-Litre van Dorn sampler and preserved with acid Lugol. Samples were counted using inverted microscopy and converted to wet weight biomasses by approximating cell volume through routine measurements of 30-50 cells of an individual species and the application of the geometric formula best fitted to the shape of the cell. Identifications were performed on preserved whole and net (10 µm mesh) samples. The in situ light and dark bottle oxygen method was used for studying photosynthesis. An adapted version of the phytoplankton production model was employed to quantify phytoplankton photosynthesis and community respiration. Both chlorophyll a and total phosphorus were estimated. Coefficient of variance, t-test, ANOVA and the Levine test of homogeneity of variance were used to assess annual, inter-annual and seasonal variability of the means of physico-chemical and biological variables. Community species composition consisted of a total of 56 in horizontal and 75 in temporal studies in 7 major groups dominated by the Chlorophyceae. But the biomass dominated by the Cyanobacteria. Mean biomass of phytoplankton was 1570.0 in the first year (2004-2005) and 2262.3 mg m-3 in the second year (205-2006). Annual mean gross primary productivity of 4.73 gC m-2d-1 is high but net productivity was very low i.e. 0.38 gC m-2d-1. This was due to the very high community respiration that represented 90 % of the gross primary productivity and may imply higher loss rates. Mean growth rate in the Zeu of 0.13 d-1 is low and is commensurate with the low biomasses observed in the lake. Very weak relation between biomass and chlorophyll a and the little change in community composition in an annual cycle show that the changes in the chlorophyll a concentration are due to adaptation of the same community to differing conditions within the water column. Variabilities in the community biomass that range from a Coefficient of Variance of 28-79 % and that of the gross primary productivity of 33.1 % are high and seem to be regulated by the similarly high variabilities in the physico-chemical parameters. Significant inter-annual differences in the community biomass were observed. Physical factors such as Zmix and Zeu seem to exert more control in the dynamics of the phytoplankton growth and productivity compared to chemical factors even though nitrogen limitation is suggestive because of the abundance of heterocyst-bearing filamentous Cyanophyceae. Selective grazing may also be contributing to some extent to the observed dynamics.

Source: KNUST