Seasonal Changes in Nutrients in a Tropical Swamp: North Swamp, Lake Naivasha, Kenya

SUMMARY (1) North Swamp, a papyrus swamp at the north end of Lake Naivasha, Kenya, was assayed monthly during 1975-76 for dissolved (0-45-,um-filtered) nutrients: NO2-N, NH3- N, organic N, total P, P04 - P, S04 -S, Fe, Mn, 02, pH, conductivity and organic matter. (2) Particulate matter (0-45-100 ,um) was analysed occasionally for total C, N, S, Fe, Mn, cations, Cl, silica and organic matter. (3) Sediment traps under the floating papyrus mat were used to obtain an estimate of the rate of sedimentation inside the swamp. (4) All of the dissolved nutrients (N, P, S, Fe and Mn) generally showed a common pattern in vertical profiles through the swamp, with minimum values at the swamp surface and maximum values on the bottom. (5) The prevailing pH, 02 and redox potentials in the swamp indicate that at the surface much of the iron would be present as ferric-Fe, while ferrous-Fe predominates on the bottom; P04 - P may be adsorbed to Fe-organic complexes at the swamp surface; Mn would be in a soluble form; S04 - S is low and may be adsorbed or otherwise bound; H2S is in very low concentration; methane is present; and O2 is generally low or lacking. The annual output of organic N is much larger than the total input from the river, suggesting that nitrogen is fixed in appreciable quantities. (6) The maximum insoluble material (detritus and particulate matter) exceeds 27 000 mg l-1. Most of the nutrients were in this organic sediment, which represents a large portion of the annual nutrient input to the swamp. (7) It is concluded that papyrus swamps effectively extract dissolved nutrients from tropical river systems, but such nutrients are later exported as organic particulate matter or adsorbed to particles which are carried into the lake by through-flow. (8) It is also concluded that papyrus swamps contribute appreciable amounts of fixed nitrogen to lake-swamp systems.