Water quantity and quality as the factors driving the Serengeti ecosystem, Tanzania

Abstract

Thirty nine years of rainfall data from 232 sites, 5 years of river discharge data from 3 rivers, 4 years of animal migration data and 4 years of water quality data at 60 sites were explored to quantify the role of water in the Serengeti ecosystem. Seasonal variations in rainfall are largely predictable; interannual fluctuations are huge and not predictable solely from the Southern Oscillation Index. The wildebeest and zebras start their annual migration at the end of the wet season well before surface water runs out, however these waters are very saline (salinity ≈ 5–17 psu). The timing of the migration appears predictable from a salinity model. Salinity is also important for the vegetation because high salinity coincides with the transition between wooded savanna and grassland. This transition has moved markedly southward in the last 30 years, this change may be due to decadal changes in annual rainfall. Most rivers are commonly ponded, with ponds having a flushing rate of 1 month in the wet season and zero flushing in the dry season. These ponds form the only source of water for wildlife for several months a year. The water quality varies spatially and temporally. pH values vary between 5.9 and 10 and are correlated with salinity. Surface waters are heavily eutrophicated from animal dung. As a result, the dissolved oxygen concentration near the surface fluctuates widely between 1 and 200% of saturation. Direct solar heating is restricted to the top few cm because of low visibility. A strong thermal stratification in temperature (2 °C/m) results and inhibits aeration. Bottom waters can be anoxic and are aerated only when hippopotamus stir the water. Poor water quality may affect wildlife health and production.