Coupling of land and water: Phosphorus fluxes in the Upper Tana River catchment, Kenya.

Linkages between nutrient limitation in the Masinga reservoir in Kenya and nutrient supply from its catchment were approached by the study of phosphorus dynamics. The pattern of rainfall and the warm temperatures were characteristic of tropical climates and determined the state of phosphorus in the soil and its delivery to aquatic systems. Mechanism of phosphorus transport were related to adsorption/desorption equilibria between orthophosphate and metal (hydr)oxides fixed at the surface of clay particles. The monitoring of suspended solids, total and dissolved phosphorus, dissolved nitrogen, conductivity and silicate during a one and a half year field study in a selected subcatchment highlighted patterns of nutrient mobilization and transport from rivers to the reservoir. Yearly budgets of suspended sediment and total phosphorus export from the subcatchment yielded respectively 161 t/km2/ann and 109 kg/km2/ann for a yearly runoff of 346 mm. Phosphorus loading was contemporaneously monitored in the reservoir. The annual loading was 10.4 gP/m2 (1993) and did not result in the establishment of eutrophic conditions. Constant high temperatures, high concentrations of suspended solids and the occurrence of a deep water outlet combined to generate density flows. Part of the load was sedimented in well oxygenated, shallow, upper sections of the reservoir while the rest exited through the deep water uptake and was transported to other reservoirs downstream. The study of phytoplankton biomass and primary production in relation to nutrient availability revealed that periods of phosphorus limitation alternate with light limitation. The phytoplankton assemblage responds with an alternation of periods dominated by filamentous Cyanophytes (Cylindrospermopsis africana, Planktolyngbya limnetica) and pennate diatoms (Achnanthes sp., Synedra ulna). Gross primary production varied between 0.03 and 1.43 g C/m2/d. A survey of four other reservoirs in the cascade suggested that bottom sediments are a phosphorus sink.