The Seasonal Growth and Succession of Plankton Algae in the White Nile

Abstract

A description is given of the seasonal growth and succession, over five years, of planktonic algae in a region of the White Nile affected by a reservoir. Dense populations develop during the period of water storage, and are dominated by the diatom Melosira granulata and the bluegreen alga Anabaena flos aquae var. intermedia f. spiroides. The sequence of their development is related to their abundance in the river water entering the reservoir basin. Reactions of algal development upon water characteristics include a considerable increase in pH and depletion of nitrate, phosphate, and probably silicon concentrations. Deficiency of inorganic nitrogen probably limits the maximum densities of Melosira. Maximum algal densities were in the range 20 ± 8 mm³ cell volume (or approximately mg dry weight) per L, and were associated with high rates of photosynthetic production, estimated in 1953 as approximately 2.2 g carbon/m²/day. The variation of photosynthetic rates with depth and population density is described for the decline of the 1953 population maximum.