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Awortwi et al.: Mixing and stratification relationship on phytoplankton of Lake Bosomtwe (Ghana) 43 West African Journal of Applied Ecology, vol. 23(2), 2015: 43–62. The Relationship Between Mixing and Stratification Regime on the Phytoplankton of Lake BoThe seasonal changes in the phytoplankton community in terms of biomass composition and associated physicochemical parameters of the Lake Bosomtwe (Ghana) located in West Africa were studied between 2004 and 2006 to assess the mixing and stratification regime of the lake on the phytoplankton dynamics. From water samples obtained from a central index station, biomass composition was assessed by converting phytoplankton counts to wet weights-based approximation into cell volume values; whiles mixed layer and euphotic depths were analyzed using temperature and light profiles of the lake respectively. Total phosphorus was estimated using the Ascorbic Acid Method. Results from the dataset showed that the phytoplankton biomass was dominated by the Cyanophyceae throughout the study period despite the seasonal changes associated with the mixing and stratification regimes. There were significant inter-annual differences in the mean values of the euphotic depth and the wet weight biomass (P < 0.05). However differences in the mean values of the mixed layer depth, the ratio of the mixed layer depth:euphotic depth, and total phosphorus concentration (P > 0.05) were insignificant. High variations in the mixed layer depth (CV > 34 %) and the euphotic depth (CV > 32) drive similarly high variations in the wet weight biomass (CV > 28) as is the case for many stratifying tropical lakes. However, both were poor predictors of the phytoplankton wet weight biomass behaviour (mixed layer depth, r2 = 0.1034; euphotic depth, r2
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Seasonality of Primary productivity of phytoplankton of Lake Bosomtwe, Ghana -West AfricaThe primary productivity and seasonality of phytoplankton and its seasonality in Lake Bosomtwe (Ghana), West Africa were studied from September 2005 to August 2006 using the dissolved oxygen method. The wet weight biomass (0.41±0.37 gCm-2d-1, n=25), Chlorophyll a (52.11±19.51 mg m-2, n=17), mixed layer depth (9.28±3.47 m, n=25), euphotic depth (4.43±1.76 m, n=25), ratio of mixed layer to euphotic depth (2.17±0.74, n=25), secchi disc depth (1.52±0.34 m, n=25), irradiance (1180±341.28 µE m-2s-1, n=25), extinction coefficient (0.97±0.46, n=25), and total phosphorus concentrations (1.84±0.49 µmol L-1, n=20) were measured to assess their influence on the productivity of the phytoplankton. Our assessment revealed high areal gross productivity of the phytoplankton (4.72 ± 1.56 gC m-2 d-1, n = 25) which is within the range for tropical African lakes, but concurrent high areal community respiration rates (4.34 ± 2.78 gC m-2 d-1, n = 25) contributed to a low net productivity (0.37 ± 2.32 gC m-2 d-1, n = 25) and growth rates (0.14 ± 0.33 d-1, n = 21). Variabilities in the gross productivity of the phytoplankton were high (CV =33.10 %) and driven by similarly high variabilities in the physicochemical and biological parameters. Productivity also exhibited clear seasonality associated with the mixing and stratification of the lake. The physicochemical and biological parameters (mixed layer depth, euphotic depth, secchi disc depth, irradiance, chlorophyll a, and the community respiration) all had a positive relationship with the primary production) whiles ratio of mixed layer to euphotic depth, total phosphorus, wet weigh biomass had a negative relationship with the gross productivity. Of all the measured parameters, only areal community respiration (r2 = 30.6 %) had a significant predictive value (r2 < 0.05).