Chronic and punctuated changes in subtropical reef fish assemblages Ecosphere

Understanding temporal changes in marine biodiversity is crucial for managing and conserving reef ecosystems in the face of global environmental shifts. Although biodiversity changes at global and regional scales often differ, the role of subtropical systems in these patterns has been relatively understudied until recently. To better understand temporal dynamics in these lower latitude systems, we analyzed reef fish visual survey data collected from 2013 through 2023 on subtropical artificial and natural reefs in the eastern Gulf of America (internationally recognized as the Gulf of Mexico). We evaluated temporal changes in taxonomic composition and functional diversity, revealing a gradual increase in taxonomic richness (approximately one additional taxon every two years) accompanied by significant seasonal and depth‐related variability that was consistent between reef types. Beta‐diversity decomposition indicated increasing dissimilarity among assemblages over time. This was primarily influenced by the replacement of taxa (i.e., turnover) and by changes in taxon abundances, where decreases in some taxa were offset by increases in others (i.e., balanced variation). Additionally, we observed assemblage dynamics consistent with tropicalization, although a notably strong harmful algal bloom during the study period may have attenuated that signal on shallow reefs but not on deep ones. Specifically, shallow reefs exhibited abrupt compositional shifts, likely linked to that harmful algal bloom, whereas deep reefs showed gradual compositional changes resembling ecological drift. These findings build on previous work in this region that emphasize the importance of considering habitat type and depth in biodiversity assessments and contribute to a growing body of literature aimed at understanding the role of subtropical reefs in global biodiversity change.