Influência de um vale na plataforma continental sobre as comunidades de recifes biogênicos no litoral do Rio Grande do Norte

Abstract

Natural communities are structured by physical and biological factors that drive the local dynamics of species coexistence. On reef habitats, conditions such as currents, waves, temperature, depth and nutrient availability play important roles on structuring communities. Nutrient availability is a frequent limitation in marine environments. The island mass effect, for instance, when the allochthonous nutrient supply supports a diverse ecosystem, and the transformation of dissolved organic carbon into particulate matter by sponges (sponge-loop) have an important role on maintaining ecosystems on oligotrophic conditions. Most Brazilian reefs are rocky or sandstone formations with a great influence of continental run-off, being mostly dominated by macroalgae and presenting low coral cover. Biogenic reefs in Brazil occur in the Abrolhos bank and southern Bahia state, presenting high coral cover. Recently, biogenic reefs with high coral cover were geologically described on northern Brazilian coast. We described benthic and fish communities in these reefs using photoquadrats of benthic substrates and visual census of fish community. Three reef areas were chosen around a valley, a potential pathway for nutrient input to the reefs, exploring variations among reefs at east, west and at the center of the valley. We found higher coral cover and lower macroalgal cover on reefs located within the valley. Fish biomass was similar among areas, but biomass distribution differed among functional groups. Invertivores and piscivores biomass was greater on reefs within the valley, potentially resulting from higher food availability resulting from increasing nutrient input. Our results reveal that the valleys may structure reef communities similarly to what happen with the island mass effect. This work contributes to description of biological communities composing these outer shelf biogenic reefs, elucidates the potential effects of the valley on these reefs, and fills a gap on the knowledge on these kind of reef formation in Brazil, generating information that can be further used for guiding management and conservation of those unique ecosystems.