The potential contributions of different microbial components (<20μm) and metabolisms to ammonium cycling were assessed during non-upwelling conditions in a coastal area off Concepción (~36.5°S). Assays with specific inhibitors to estimate rates of ammonium consumption and production, and carbon assimilation associated with photolithotrophic and chemolithoautotrophic (nitrification) metabolisms in the water column were performed. Despite low water column concentrations of ammonium in wintertime, intense ammonium transformations were registered. Prokaryotes (or bacterioplankton) contributed most to ammonium generation rates over the entire water column; these rates increased with depth (0.4-3.1μMd -1). In surface waters (10m depth), aerobic ammonium oxidation (potentially by Bacteria and Archaea) was the dominant consumption process (average 0.7μMd -1) whereas in the subsurface layer (20 and 50m depth), unexpectedly, eukaryotes accounted for most of its consumption (average 2.1μMd -1). Nitrification oxidized an important proportion of the ammonium in both layers (from 25% to 100%) and provided regenerated nitrate. The integrated water column rates of chemosynthesis (0.005gCm -2d -1) represented a large proportion (51%) of the total dark carbon fixation during the non-upwelling season when integrated rates of photosynthesis are relatively low (0.42gCm -2d -1) and microbial food webs dominate the transfer of carbon within this coastal system. © 2011 Elsevier Ltd.