Phytoplankton biomass and primary production dynamics in Lake Kariba

Abstract

The present study examined spatial, seasonal and depth variations in phytoplankton biomass and primary production (PP), compared with those reported for other tropical African lakes, to determine whether or not measured phytoplankton changes might be linked to climate warming. The biomass of three major phytoplankton classes (Cyanophyceae; Chlorophyceae; Bacillariophyceae) and net PP were measured during the midwinter and midsummer at six different depths at 35 sampling sites distributed across the lake’s five basins. A more rigorous sampling regime was used in the fifth basin, with phytoplankton biomass and PP rates measured every second month over a 24 month period at six different depths at ten sampling sites located in riverine and lacustrine waters. Cyanophyceae, which displayed a gradient of decreasing biomass from Basins 2 to 5, contributed 69% of the total phytoplankton biomass in the lake’s five basins during summer. This percentage was approximately four times greater than that contributed by the Bacillariophyceae and about ten times greater than that contributed by the Chlorophyceae. During winter, Bacillariophyceae biomass was equivalent to that of the Cyanophyceae, but displayed an opposing gradient of increasing biomass from Basins 1 to 3, with a subsequent biomass decline from Basins 3 to 5. Chlorophyceae exhibited no distinct biomass gradient across the five lake basins, being undetectable during winter. The biomass of all three phytoplankton classes and the net PP varied in magnitude and direction monthly between the lacustrine and riverine waters, with increasing water depth and with no distinct seasonal patterns being evident. The monthly variations in biomass were related to the thermal stratification cycle, hydrological gradients and the extent of water mixing, being similar to those reported for other tropical African lakes. It is noteworthy that total phytoplankton biomass and PP in Lake Kariba have declined by about 95% and 50%, respectively, since the 1980s. These declines correspond to an upward shift in the depth of the thermocline, associated with an average temperature increase of 1.9 °C and a 50% reduction in the depth of the euphotic zone, since the 1960s.

Temporal patterns in phytoplankton, zooplankton and fish composition, abundance and biomass in Shirati Bay, Lake Victoria, Tanzania

“…Thus, the eutrophic nature of Shirati Bay favoured the flourishing of cyanophytes and bacillariophytes. The existence of higher phytoplankton abundance and biomass for the month of March 2005 obtained in the present study was previously reported by Sekadende et al (2005a) in the satellite lakes of Lake Victoria, Regina et al (2012) for Kariba Lake, Sitoki et al (2012) for Nyanza Gulf in Lake Victoria and Darchambeau et al (2014) for Lake Kivu. During the study period, the months of March to May fall within the heavy rain season, a period characterized by water stratification in Lake Victoria (MacIntyre et al 2014).…”

“…(2005a) in the satellite lakes of Lake Victoria, Regina et al . () for Kariba Lake, Sitoki et al . () for Nyanza Gulf in Lake Victoria and Darchambeau et al .…”

The Potency of Phytoplankton in the Lakes of Universitas Indonesia (UI) as the “Carbon-Eater”: A Preliminary Study in Revealing the Lake as a Possibly Another Carbon Bank

“…The possibility of dominance of zooplankton in biomass is suggested as the cause of negative correlation [21]. Besides, certain species of phytoplankton has a very low ability in producing primary product because different phytoplankton species has different ability in producing primary product [23].…”

Lake Modeling Reveals Management Opportunities for Improving Water Quality Downstream of Transboundary Tropical Dams

“…To mimic the oxygen supply by primary production within the simplified oxygen module, we introduced a positive oxygen flux entering the uppermost layer of the reservoir. This oxygen flux remains constant throughout the year, and its value for Lake Kariba (17.2 mmol m −2 day −1 ) has been estimated by the stoichiometric conversion of the net primary production value (206 mg C m −2 day −1 ), calculated by Ndebele-Murisa (2011) and Ndebele-Murisa et al (2012).…”