Ecological status of the highly impacted Guanabara Bay assessed using macrofaunal indicators

Assemblage descriptors have been broadly used to assess benthic health and understand ecosystems functioning. In Guanabara Bay, a highly impacted estuary in Brazil, studies have emphasized more environmental contamination than its consequences on benthic assemblage structure. To investigate the benthic assemblage structure and habitats status in Guanabara Bay, we assessed several biotic indicators (abundance, biomass, richness, species diversity indices, trophic diversity, AZTI’s marine biotic index) and potential stressors (temperature, dissolved oxygen, organic matter content, microplastics) in four regions with distinct hydrodynamic and levels of contamination. Our findings identified the dominance, up to 99% contribution total benthic biomass, of a single mollusk species in the shallow and inner areas, determining low diversities of species and trophic groups. Biotic index classified these areas as poor ecological status, mainly determined by high organic matter content in the sediment and low oxygen levels near the bottom. Temporal variations of benthic assemblage in upper estuary (inner area) suggested that events of marine water intrusions seem to improve habitat quality and species richness, particularly in summer. Contrary, low estuarine region (Central Channel) was classified as good/high ecological status, with higher species diversity, some of them pollution sensitive. However, analyses of the ratio of benthos and microplastic abundances revealed more microplastics than annelids for all regions and similar abundances to mollusks and crustaceans in the Channel, increasing the chances of some species mistaken microplastic for food. Despite similar ratios of benthic groups and microplastics in the Central Channel, the higher species diversity in this region may be valuable to recolonize other areas and support conservation measures in Guanabara Bay.