Research on ecosystem functioning: are we building knowledge or increasing confusion? Fish richness, functional traits and food webs in two tropical estuaries
It is well accepted that assessing ecosystem functioning involves research crossing multiple biological organizational levels and disciplines. Ecosystem functioning is quantified by measuring the magnitude and dynamics of several ecosystem processes. This quantification is done using different tools and the resulting information is often interpreted separately. Our main aim was to evaluate if different approaches provide similar or complementary results and how do they contribute to have a better understanding of estuarine functioning. We used biological and environmental data from two Brazilian tropical estuaries with different levels and types of human disturbance – the heavily impacted Paraiba estuary and the protected Mamanguape estuary. Fish assemblages and their preys were collected along the salinity gradient, in the wet and dry season. We calculated species richness and we computed functional diversity of the fish community, based on effect traits defined mostly through the fish morphological measurements. To account for the seasonal and spatial trophic dynamics within the water column we analyzed the natural abundance of stable isotopes (carbon and nitrogen) in fish and possible food sources in each estuary.
Species diversity within each estuary was greater than the traits diversity. The species turnover along each estuary was relatively high, and this dissimilarity was considerably greater than the traits turnover (i.e. low functional β-diversity). These results highlight that although habitat selection was an important driver of species coexistence due to the spatial environmental gradients, the competition for resources might be considerably high because species tend to use the system in a similar way. The relatively narrow range of both 13C and 15N values among fish species confirmed that the consumers are competing for the same resources. Moreover, the 15N values indicated anthropogenic inputs of N (e.g. sewage discharges, agriculture) in both estuaries, regardless their protection framework. Consumers and sources from upstream areas showed depleted 13C values - terrestrial inputs, and enriched 15N signatures, being these patterns most pronounced during the dry season. The richness and functional diversity analyses also revealed that the decrease of the freshwater runoff during the dry season might be potentializing the disturbance effect in the Mamanguape system. Still, a higher percentage of commercial fish was produced in this system, highlighting an important function directly related to the ecosystem provisioning services. Our results suggest that different approaches provided complementary information and we discuss how each of them contribute for a more integrated understanding of functioning in the two tropical estuaries.