Saline gradient drives functional nestedness of polychaete communities in tropical estuaries

Abstract Estuarine environmental gradients are strong drivers structuring aquatic metacommunities and may cause an orderly loss or gain of species across sites. Locations with milder environmental conditions may allow the establishment of different species with different functional traits, while stressful sites may restrict the occurrence of a reduced set of species with specific functional traits. This can generate a functional nestedness pattern, where the sets of functional traits present at sites with low functional diversity are subsets of the functional traits present at sites with higher functional diversity. In this study, we evaluated the functional nestedness of polychaete communities along salinity gradients of tropical estuaries, and the effects of seasonal periods on the distribution pattern. Two tropical estuarine ecosystems (Mamanguape and Paraiba do Norte) were sampled for two years. Three functional traits were considered due to their potential to reflect the species responses to the salinity gradient: maximum body size, life span and fecundity. Functional nestedness was estimated with the traitNODF metric. Subsequently, we used a permutation test to assess whether the nestedness observed along the salinity gradient was distinct from the expected at random. Our results demonstrated that the polychaete communities presented a tendency to functional nestedness along the salinity gradient of estuaries, with observed statistics higher than expected in six of the eight tests performed (2 estuaries * 2 years * 2 seasons). Therefore, the functional diversity of polychaete genera present in the low salinity sites in general were nested subsets of the diversity at high salinity sites. Our results suggest that salinity acted as a strong environmental filter in estuarine ecosystems, causing an orderly loss in the functional traits of polychaete genera along the longitudinal extension of the estuaries. Our results advance in the understanding on how environmental gradients structure polychaete communities and can support future studies aimed to develop concrete conservation strategies for estuarine ecosystems located throughout the world.