Few reservoirs have received thorough investigations for algae types, but good accounts are available from studies of Lakes Volta and Kariba. Following filling up of the dams, colonization by algae was dominated by chlorophytes later succeeded by diatoms, desmids, cyanophytes and euglenophytes. The density rises after closure of the dam and fluctuates around a peak. The growth of phytoplankton is reflected by a rise in pH, a decline in phosphate during initial stages and subsequent rise in silicate concentration. Seasonal periodicity, primary production, and community structure are influenced by depth, latitude, nutrient inflow, temperature and sediment type. Eutrophication is common in small dams, which are close to sewerage systems or big towns and are usually dominated by Chlorocystis and cyanophytes.
Aquatic macrophyte communities range from small plants like Azolla, Lemna, Spirodela and Salvi-nia, to large floating Pistia, Elodea and the nuisance unintentionally introduced water hyacinth Echhornia crassipes. Ceratophyllum and Utricularia represent submerged aquatic forms. Emergent plants occur on the fringes of the reservoirs and are represented by Jussiaea, Echinochloa, and Alternanthera. The marginal elephant grass Vossia cuspidata colonizes dams within one year of filling while semi-aquatic
vegetation includes Typha, Phragmites, Echinochloa, Ficus, Scirpus and Leersia. Generally, weed infestation is related to the annual water level changes and influx of nutrients, and has created problems for transport, fishing and water supply.
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