Setting reference conditions to assess the ecological status of the sublittoral and bathyal benthic communities of the southern Gulf of Mexico

Abstract

The southern Gulf of Mexico (sGM) requires tools to accurately assess the quality of the heterogeneous seafloor in the sublittoral and deep benthic zones. In this context, multivariate AZTI's Marine Biotic Index (M-AMBI) is a suitable assessment method to evaluate the impact produced by human pressures. Here we argue that setting reference conditions enhance the comprehension of natural variability of key variables, helping us to identify the ecological status of the sGM seafloor. We aim to set reference conditions for M-AMBI and validate the application of AMBI and M-AMBI to assess the ecological status of the sublittoral and deep zones of the sGM. We analyzed environmental, contaminant, and macroinfauna data collected in 595 sampling sites across five annual surveys (rainy and dry seasons), from the sublittoral to the bathyal zones. Nine types of sedimentary environments were obtained based on depth, sediment texture, and organic carbon. Of the 917 taxa collected, 830 (90\%) were assigned to one of the five levels of sensitivity (from sensitive to opportunistic species) under local biogeographic conditions; this represents the first list of species recorded from coastal (2 m depth) to deep environments (3571 m depth) in the sGM. According to reference conditions and types of sedimentary environments, we observed a monotonous decreasing sequence of species richness and species diversity (i.g. carbonated > terrigenous > transitional > deep environments). Overall, the ecological status ranged from acceptable (e.g. carbonated and terrigenous environments > 67\%) to non-acceptable quality (e.g. transitional and deep environments > 50\%). Our validation of AMBI and M-AMBI, using 95\% confidence intervals, permitted to obtain significant relationships (r(2) > 0.36; P < 0.001) between benthic indices and redox potential, metals, and hydrocarbons. At sites near and within the oil production area, contaminant concentrations (e.g. total hydrocarbons, individual PAH congeners) exceeded the sediment quality guidelines, and deeper sites were contaminated mainly by metals (e.g. Cd, Cu). These results show that the transitional and deep environments present a detectable imprint of the oil industry and river discharges, respectively. In conclusion, AMBI and M-AMBI were able to detect the impacts of human pressures in the region, resulting in reduced sediments and an increase of metals and hydrocarbons. Their use permitted to predict changes of fauna attributes, obtaining confidence intervals that can be used in monitoring programs to identify human pressures on benthic communities.