8. Seasonality of Primary productivity of phytoplankton of Lake Bosomtwe, Ghana -West Africa

Abstract

The 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 gC m-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).