SEDIMENT BIOGEOCHEMISTRY IN AN EAST-AFRICAN MANGROVE FOREST (GAZI BAY, KENYA)

The biogeochemistry of mangrove sediments was investigated in several mangrove forest communities in Gazi Bay, a coastal lagoon in Kenya, Af rica. Carbon dioxide fluxes, sediment median grain sizes, sedimentary organic carbon, nitrogen and phosphorus contents and pore-water charac teristics (ammonium, nitrate, sulfate and chloride) could be related t o forest type. Mangrove sediments have pH values that range from 3.5 t o 8.3 due to the limited buffer capacity of these sediments and intens e acidifying processes such as aerobic degradation of organic matter, oxidation of reduced components, ammonium uptake by roots and root res piration. The mangrove sediments are nitrogen-rich compared to mangrov e litter, as a result of microbial nitrogen retention, uptake and fixa tion, and import of nitrogen-rich material. It appears that mangrove s ediments in Gazi Bay act as a nutrient and carbon sink rather than as a source for adjacent seagrass and reef ecosystems.