Marine benthic microalgae are a promising bioindicator of contamination. To date, however, investigations on the microbenthic communities subjected to multiple stressors in natural environments are still very rare. To assess whether the benthic processes of primary production and oxygen consumption, and the structure of active and resting microbenthos, were affected by sediment contamination, seven stations were sampled in different zones of the port of Trieste, subjected to multiple and diffuse contamination, and a reference site in the Marine Reserve of Miramare. No major differences in total abundance of active microbenthos were observed among sites, but the dominance of stress-resistant species and the reduction of more sensitive ones, were registered nearby the main productive activities. The densities of resting microbenthos were higher in polluted areas, and represented by key dinoflagellate species that were clearly linked to contamination. The analysis of similarity applied to both active and resting communities significantly separated the most contaminated stations from the other ones. The photosynthetic capability of active microbenthos did not seem to be affected by contamination. The maximum oxygen consumption rates observed in sediments nearby the productive activities were likely ascribable to high organic C contents and the presence of metals in reduced chemical form.
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