Variation in species composition and evenness drive long-term changes in the structure of rocky shore communities

Macroalgae play an important structuring role in marine coastal ecosystems, which have been affected by several anthropogenic pressures such as fishing, pollution, species introductions, climate change, among others. We examined long-term changes in the structure of coastal macroalgal-dominated assemblages over a period of +20 years by using an asymmetrical design to compare two datasets: one historic 1993-1995 (1 location) and one contemporary 2016-2018 (3 locations). Results showed significant contemporary reductions in the biomass of certain macroalgal morpho-functional groups (corticated and corticated foliose) but not in others (articulated corallines, canopy algae, filamentous and foliose macroalgae), which led to an overall halving in macroalgal biomass, although the latter was not statistically significant. In contrast, the contemporary dataset exhibited twice the sample richness compared to the historic dataset. Partitioning of species turnover in its compositional and abundance components varied with depth. At 5 m, compositional changes dominated temporal variation in assemblage structure, whereas at 15 m, changes in composition and abundance had a similar and substantial contribution for species turnover between the two datasets. Overall, these findings reveal dramatic changes in the structure of these assemblages within a relatively short timeframe. Although causality cannot be inferred from such observational studies, we put these changes in context of current ecological knowledge and discuss their potential and wider implications.