Reef Fish Assemblage Through Space and Time in a Thermal Transition Zone in the Southwestern Atlantic

Abstract

Aim: Assessing the structure of reef fish assemblages and their spatiotemporal changes (β-diversity) in seven reef locations across a thermal transition zone in the southwestern Atlantic. Location: Eastern Brazilian coast, from the northern Abrolhos Bank (17° S; 26.37°C) to the southern Espírito Santo state (20° S; 21.13°C). Methods: We used four- and 6-year datasets from underwater visual censuses to assess spatial patterns and temporal trends in reef fish density, biomass, diversity, trophic groups, and β-diversity. Results: We identified two main clusters of reef locations along the latitudinal gradient. Reefs from the Abrolhos Bank formed a distinct group characterised by higher fish density, biomass, and diversity. Fish assemblages in Espírito Santo exhibited a high species balance component relative to the Abrolhos Bank, with a turnover rate of ~63% between regions. Most trophic groups were more abundant in the Abrolhos Bank, except for planktivores and sessile invertivores. Spatial β-diversity decreased between regions over time, with species nestedness and turnover contributing variably to dissimilarity by year. Temporal changes in β-diversity were mostly driven by both extirpation-resultant homogenisation and colonisation-resultant heterogenisation. The former was mainly influenced by a decline of mobile invertivores and macrocarnivores with a tendency toward cooler thermal affinities, while the latter was influenced by an increase in species from the same trophic groups with a tendency toward warmer thermal affinities. Conclusions: Our results may be linked to both rising temperatures and overfishing. Despite the ecological relevance of the eastern Brazilian coast regarding marine biodiversity, only 19% of our sampled sites are fully protected (mostly within the Abrolhos Bank), while the remaining sites have been subjected not only to global stressors but also to local impacts, including overfishing. We emphasise the need to monitor and protect transition zones, such as the eastern Brazilian coast, amid habitat changes driven by warming oceans and local impacts.