Functional changes in polychaete and mollusc communities in two tropical estuaries

Abstract Currently, there is limited knowledge on how benthic communities function in relation to changing environmental conditions in tropical estuaries. This study contributes to filling this knowledge gap by analysing structural and functional community patterns within two tropical estuaries in the northeast of Brazil. Replicate macrofaunal samples were taken in different environmental zones along the estuary, together with physico-chemical measurements. The faunal analysis focussed on the trait analysis of polychaete and mollusc communities as both of these groups have an important role in regulating the functioning of aquatic systems. The separate analyses of polychaetes and molluscs allowed evaluating the way in which each group responded to the environmental gradients. The results demonstrate that community functioning for the polychaetes depended greatly on the spatial environmental gradients within these systems, with generally higher functional diversity in the lower zones suggesting increased diversity in resource use strategies, whereas the upper zones showed very little functional diversity due to the prevalence of environmental filtering. In the case of the molluscs, their functioning remained more similar along the estuarine gradient, with low taxonomic and functional diversity throughout the estuaries. In general, polychaetes displayed higher levels of taxonomic and functional diversity, suggesting that compared to the molluscs, they potentially utilised the available resources more efficiently, which may enhance the processes and overall functioning of these systems. We advocate that more studies are needed to obtain a generalised pattern of estuarine function within and across biogeographical regions.