EUTROPHICATION AND ZOOBENTHOS DYNAMICS

The possible effects of eutrophication on benthic dynamics are discussed based on the Pearson-Rosenberg Model that describes the effects of organic enrichment on qualitative characteristics of benthic communities. Depending on the amount of organic matter reaching the sediments, the model describes three successive states: a) slight increases in biomass and few or no changes in species composition over the ''normal'' situation; b) strong increases of biomass and replacement of ''normal'' species by opportunistic species; c) disappearance of benthic animal species and azoic sediments. In the field, increase of benthic biomass and changes in species composition on decadal scales which are attributed to eutrophication have been documented in a few cases. Mesocosm studies tend to show much more rapid (weeks to months) but often incoherent responses after experimental nutrient and organic matter additions. The few studies of the ''opportunistic'' species that appear adapted to become the first recolonizers of azoic sediments show that there is no simple relationship between population characteristics and ability to colonize and that food quality and food requirements have to be better understood. The impact of benthic fauna on benthic-pelagic coupling (sedimentation, particle uptake), bioturbation, benthic mineralisation, nutrient and dissolved organic matter release in shallow water appears to be considerable and disappearance of fauna due to increased organic loading and/or anoxia events is bound to exert a significant influence on shallow water energy and matter cycles.