Contrasting benthic ecological quality within tropical estuaries in Southeastern Brazil based on biotic indices

Background Tropical estuaries are exposed to multiple sources of impact within a background of strong natural environmental gradients. Monitoring and managing these ecosystems is challenging because gradients in salinity and hydrodynamics overlap with intense, spatially localized anthropogenic pressures. Benthic biotic indices provide standardized tools for assessing the ecological health of estuaries and are sensitive to multiple impact sources, but as these indices are based on regional taxa assignments, they need testing to verify their capacity to indicate soft-bottom assemblages under stress. Although these indices have been applied across a broad latitudinal range, their performance remain less established in the tropics. Methods We quantified macroinfaunal abundance, richness, diversity and assemblage composition across 64 stations in two tropical estuaries of southeastern Brazil, calculated the AZTI Marine Biotic Index (AMBI) and Multivariate AZTI Marine Biotic Index (M-AMBI), and evaluated their associations with salinity, sediment grain size, organic-matter quality, fecal sterols and trace and major elements. We then quantified multivariate associations between macrofaunal assemblages, benthic ecological indices and environmental gradients. Results Benthic ecological status was spatially heterogeneous, with localized Poor/Bad and High/Good indices within estuarine sectors. Community composition was associated with salinity, sediment grain size, organic-matter quality and fecal sterols. Multivariate tests identified significant associations between community variation and salinity, biopolymeric carbon, sterols, total organic carbon, total nitrogen, fine sediments and selected metals. Overall, the indices captured pronounced station-level variation in both urban and less-developed estuaries, and reveal their value to identify stress within tropical estuaries of Brazil.