Exploring ecosystem functioning in two Brazilian estuaries integrating fish diversity, species traits and food webs

Ecosystem functioning is assessed through research that spans multiple biological organisational levels and disciplines, and it is quantified by measuring the magnitude and dyna mics of several processes. This quantification is done using different tools, and the resulting information from each tool is often interpreted independently. Our main aim was to contribute to the understanding of the functioning of tropical estuarine ecosystems facing anthropogenic impact, using information on fish diversity, traits and food webs. We used biological and environmental data from 2 Brazilian estuaries with different levels and types of human disturbance: the heavily impacted Paraiba estuary and the environmentally protected Mamanguape estuary (IUCN, Protected Area Category V). Fish assemblages and their prey were collected along the salinity gradient in the dry and wet seasons, and we evaluated (1) species taxonomic diversity, (2) functional diversity based on effect traits and (3) food-web structure through analyses of the natural abundance of stable isotopes (carbon and nitrogen). Species turnover, i.e. the change in species identity, along each estuary was relatively high, and this dissimilarity was considerably greater than the trait turnover (i.e. low functional beta-diversity). These results highlight that, although habitat selection was an important driver of the coexistence of fish species due to spatial environmental gradients, the competition for resources might be high because species tended to use the system in a similar way. The relatively narrow range of both delta C-13 and delta N-15 values among fish species also implies competition for the same resources. Moreover, the delta N-15 values indicated anthropogenic inputs of nitrogen in both estuaries, regardless of their protection framework, which highlights the need for more effective conservation measures.