Underwater valley at the continental shelf structures benthic and fish assemblages of biogenic reefs

Underwater relief and topography may influence currents, waves, temperature, depth and nutrient availability, factors that play important roles on structuring reef communities. The formation of valleys at the continental shelf, for instance, may result in unique geomorphologies that influences environmental factors on a local scale. Therefore, reefs within a valley and on its adjacencies are likely to experience different conditions despite the geographical proximity. An extensive biogenic reef formation at the Brazilian Equatorial shelf occurring within and around a valley was recently described from the geological perspective. Given the relative paucity of biogenic reefs along the Brazilian coast, understanding these biological communities is critical. We assessed benthic and fish communities in these reefs using photoquadrats and visual census, respectively. We studied reefs within the valley, on the West and East of the depression, to evaluate if the stronger tidal currents within the valley and its connection to a submarine canyon would differentially affect benthic and fish communities among these areas. We found higher coral cover and lower macroalgal cover on reefs located within the valley. Fish biomass was similar among areas, but biomass distribution among functional groups was variable. Invertivores and piscivores biomass was greater on reefs within the valley, potentially resulting from higher food availability and nutrient input. This work provides information on biological communities in these outer shelf biogenic reefs around valleys, a novel approach to this formation, showing that environmental factors associated to underwater valleys may structure reef communities even within small spatial scales. This effect may be due to local topography and current condition, to the nutrient-rich currents from the submarine canyon reaching the reefs through the valley and aiding primary productivity, or by a combination of these factors.