Diversidade taxonômica e funcional da ictiofauna em zonas de arrebentação de praias oceânicas

Abstract

The surf zones are considered homogeneous coastal habitats, and the period of increased algae in this environment is the one that contributes the most to the increase in structural heterogeneity. These characteristics, therefore, are predictors that influence the composition and structure of the fish community of these coastal ecosystems. The present study aimed to verify the influence of the wrack volume and environmental variables (physical and chemical, hydrodynamic and nutrients) on the composition and abundance of fish, besides analyzing the functional and taxonomic diversity in sandy beaches. The hypothesis tested was that on the beaches where the highest amounts of algae and a lower hydrodynamism of the waves are recorded, there is an increase in the functional and taxonomic richness of fish, as well as its influence on the size and total biomass. The study was carried out in four beaches classified according to morphodynamics (wave dominated beaches: Cabo Branco and Intermares; Tide modified beaches: Bessa and Formosa) in periods of dry and rain. The fish were caught using a "beach seine" net hauled for three minutes at each site. The environmental variables were measured before each haul, being characterized as: habitat characteristics (wave height, wave frequency, wave time and sediment types), nutrients (ammonia concentration, phosphorus concentration, chlorophyll, NOx and algae) and physially-chemical (salinity, depth, transparency, pH and temperature). The wrack volume in each haul was also verified. At each site, three repetitions were performed for the hauls and the measurement of the environmental parameters. A total of 14 functional traits related to locomotion and feeding acquisition of fish were selected. The comparison of community attributes in the different beaches and periods of the year were verified using multivariate permutation analysis (PERMANOVA). For functional diversity, functional richness, functional uniformity, functional redundancy and CWM ("Community Weighted Mean" (CWM) were calculated. A total of 97 fish species were sampled on the beaches, with higher values richness, numerical abundance and biomass recorded on tide modified beaches and with higher volumes of algae. Temporally, the highest values of community descriptors were observed during the rainy season, accompanied by the record of the smallest individuals on both beaches, thus suggesting the recruitment period during this season. The highest values of functional richness were observed for the beaches with lower volume of algae and lower hydrodynamism, while functional uniformity showed higher values in beaches with higher stress. Our results also pointed to clear evidence that beaches play the role of spatial connectivity by proximity between different habitats. Habitat characteristics (hydrodynamism and presence of arribed algae) influenced the structure and composition of fish assemblages. The presence of arribed macroalgae and morphodynamics were considered important "drivers" for fish assemblies of the surf zones, which contributes to the increase of refuge sites against predators for juvenile fish in addition to increasing the supply of food resources in these coastal environments.