Bioassay experiments were performed two times (with 2 years in between) in order to investigate if nitrogen(N, ammonium), phosphorus (P, phosphate) and carbon (C, glucose) additions would stimulate the growth ofbacteria and phytoplankton differently in three different tropical aquatic environments. The water and theirindigenous microbial communities were taken from a freshwater coastal lake (Cabiunas), a coastal (Anjos),and an offshore marine station (Sonar) in the Atlantic outside Cabo Frio, Rio de Janeiro State, Brazil. Ammonium,phosphate and glucose were added alone or in combination to triplicate bottles. In the lake, P seemedto be the primary limiting factor during the first experiment, since both bacterial production and phytoplanktongrowth was stimulated by the P addition. Two years later, however, addition of P inhibited phytoplanktongrowth. During both years, C was closely co-limiting for bacteria since CP additions increased the responseconsiderably. For both the coastal and offshore seawater stations, phytoplankton growth was clearly stimulatedby N addition in both years and the bacteria responded either to the P, N or C additions (alone or incombination). To conclude, the results from these tropical aquatic systems show that it is possible that phytoplanktonand bacteria may compete for a common resource (P) in lakes, but can be limited by different inorganicnutrients in marine waters as well as lakes, suggesting that phytoplankton and bacteria do notnecessarily compete for the same growth limiting nutrient in these environments.