The importance of different functional traits of macrobenthos in benthic processes of the Southern Bight of the North Sea was investigated in order to estimate the effects of density declines and species loss on benthic ecosystem functioning. Two laboratory experiments were performed: before (winter, t =10 ºC) and after (summer, t =18 ºC) sedimentation of the spring phytoplankton bloom. Single–species treatments of key-species (Abra alba, Lanice conchilega and Nephtys sp.) with different functional traits were added to microcosms at three density levels (natural - lower - very low) to account for possible density declines. Sediment-water exchanges of oxygen and nutrients, denitrification and bioturbation were measured. In absence of fauna, benthic mineralization in the summer experiment was 2.0 times higher than in winter. Fauna stimulated microbial respiration more in summer (up to 100% in L. conchilega treatments) than in winter (negligible fauna effect). As chl-a concentrations were similar in both seasons, the stronger fluxes in summer must be explained by a higher macrobenthic activity owing to the elevated temperature and better condition of the animals. Stimulation of mineralization by the three species in the microcosms was different, being behavior-related. Owing to its irrigation activity, the tube dweller L. conchilega had more pronounced influences on benthic respiration, nutrient release and denitrification than the biodiffusers A. alba and Nephtys sp.. Abra alba appeared to be a more effective bioturbator than Nephtys sp.. Processes such as benthic respiration, nutrient fluxes, denitrification and bioturbation seem to be related to animal densities and therefore decreases in densities can possibly have implications for ecosystem functioning.