An investigation on the effect of a vegetable oil spill was conducted on the biological diversity of the Con Joubert Bird Sanctuary wetland in South Africa before and after biostimulation with different concentrations of fertilizer during 2008. Biostimulation responses were analyzed 30 days after different concentrations of fertilizer were applied to the freshwater wetland at three selected sampling sites. The Con Joubert Bird Sanctuary wetland showed a high degree of contamination after a vegetable oil spill, resulting in a large volume of vegetable oil in the sediment and water column, respectively. Vegetable oil contents differed at each sampling site before biostimulation and each site showed variable responses after biostimulation. In this study, biostimulation results displayed a high yield of microbial activity and vegetable oil degradation at site one and two respectively. However, the degradation of the high vegetable oil concentrations within the sediments at sampling site 3 may have been hampered or retarded by the polymerized state of the vegetable oil. The phytoplankton, protozoan, macroinvertebrates and microorganisms assemblage were affected and showed little improvement at site 3, even after biostimulation with the high fertilizer concentration of 800 g/m 2 , in comparison to sites 1 and 2 which showed greater biological activities and degradation of vegetable oil.