CAGE FISH FARMING IN THE TROPICAL LAKE-KARIBA, ZIMBABWE - IMPACT AND BIOGEOCHEMICAL CHANGES IN SEDIMENT

Accumulation of nutrients in the sediment under a tilapia cage farm (2 .8-4.4% C, 0.26-0.49% N and 0.04-0.26% P) seemed to follow a seasonal pattern, with highest concentrations prior to winter water turnover, H owever, in April 1994 (and for P also in April 1992) the surface sedim ent contained significantly higher nutrient concentrations compared to controls. Generally, significantly higher pore water concentrations w ere found under the cages compared to controls. However, only in April 1992 were these concentrations of the same magnitude as those found i n temperate studies. The average flux of particulate material under th e cages, 20-49 g m(-2) per day, was up to 22 times greater compared to controls, Carbon accumulated only in April 1994, implying rapid decom position. This was supported by a 4-25-fold higher outward nux of ammo nium and phosphate from the farm sediment but sediment O-2 consumption was only 29-40% higher compared to control sites. It was concluded th at intensive fish farming in the tropics can generate similar eutrophi cation effects that are observed in temperate regions, However, the re sults also indicated that a tropical lake system may be able to proces s local deposition of organic wastes better than a temperate one, sugg esting that microbial decomposition may be a rapid and prominent proce ss.