Effects of turbidity and depth on the bioconstruction of the Abrolhos reefs

Turbidity increase is one of the main stressors to coral reefs. It affects light availability and will act together with global sea level rise to reduce potential photosynthesis that is important to light-enhanced calcification in corals. Corals are the main contributors to the morphological complexity of reefs through skeletal calcification. Bioconstruction by corals is a multifactorial process that is controlled by physical (e.g., irradiance and turbidity) and biological factors such as photoacclimation process. In this paper, we intend to show how turbidity and photobiology might interplay to produce a coral species distribution that controls reef growth and structural complexity. The Abrolhos complex is composed by a group of reefs closer to the coastline characterized by a high light attenuation value (Kd490 = 0.11), and another one about 60 km far from the coast characterized by a light attenuation coefficient of Kd490 = 0.08. In these reefs, different coral communities produce bioconstruction potential accordingly. We used data collected for 7 yrs, with the AGRRA (Atlantic and Gulf rapid reef assessment) Protocol. Coral species cover and distribution data were used to estimate reef bioconstruction [sensu Done (1995)] rates. A field experiment examined the metabolic function of five species (Mussismilia braziliensis, Mussismilia hispida, Mussismilia harttii, Montastraea cavernosa and Siderastrea stellata) through respirometry and rapid light curves (RLC). Bioconstruction potential and primary gross productivity demonstrated a substantial variability between reefs and species. In combination, these functional responses meant that coral species distribution is controlled by local factors and acclimation process. Therefore, our results suggest that these functional responses are useful tools to understand photoacclimation process and its consequences, species distribution and the space occupation on Abrolhos bank. We highlight the deleterious effect of turbidity in bioconstruction. This suggests that the local processes that increase sediment runoff is an immediate impact and must be controlled.