The phytoplankton and protozooplankton of the euphotic zone of Lake Tanganyika: Species composition, biomass, chlorophyll content, and spatio‐temporal distribution1

Abstract

The seasonal cycle of phytoplankton and protozooplankton biomass observed at two widely separated pelagic stations in the euphotic zone of Lake Tanganyika from February through November 1975 could be divided into three phases, based on algal abundance and species succession and coinciding with three phases of annual thermal stratification. Phytoplankton biomass was minimal (as low as 60 mg·m‒3) during the phase of stable stratification and maximal, as high as 930 mg·m‒3, at the end of the period of deep mixing. Diatoms, especially Nitzschia spp., were abundant only during the period of mixing, while filamentous blue‐greens, principally Anabaena sp., were prominent during a period of rapid surface warming which produced shallow mixed layers following the period of deepest mixing. The biomass of Strombidium cf. viride nearly equaled or exceeded the phytoplankton biomass during much of the stably stratified period; this protozoan probably has a symbiotic relationship with zoochlorellae, which were always present in it.Lake Tanganyika showed a greater relative annual range between maximum and minimum phytoplankton and chlorophyll a concentrations than other tropical lakes and the temperate Great Lakes. Chlorophyll a ranged from 0.1 to 4.5 mg·m‒3 in the euphotic zone and from 0.2 to 20.4 in surface waters; phytoplankton biomass ranged from 25 to 1,570 mg·m‒3 in the euphotic zone. Annual mean concentrations were among the lowest known, 140 mg·m‒3 for phytoplankton biomass and 1.2 mg·m‒3 for chlorophyll a. Explanations for both the low annual means and the high annual variabilities may lie in he lake’s steep morphometry, which affects nutrient regeneration, and in high loss which cause high algal growth rates but low algal biomass much of the year.