Clear-cut wave height thresholds reveal dominance shifts in assemblage patterns on rocky shores

Abstract

Physical constraints, and particularly hydrodynamics, are major drivers of macroalgal canopy-dominated rocky shore communities. The abundance of habitat-forming seaweeds decreases with increasing wave exposure, triggering cascading effects on their associated communities. This study aims at describing the structure of benthic communities along a hydrodynamic gradient and understanding the role of wave height in structuring the ecological transition from macroflora to macrofauna dominance. In situ wave height was measured together with biological communities (macroalgae, macrograzers and sessile fauna) along an exposure gradient on 3 rocky shores of western Brittany. Results showed that Fucales cover and the abundance of several understory organisms are negatively correlated to in situ wave height, except for barnacles and limpets which were positively correlated. Results revealed the occurrence of a wave height threshold at approximately 80 cm from high to mid-shore levels. Beyond this threshold, we observed a large dominance of barnacles, while below this threshold, Fucales and the associated organisms abounded. A similar threshold was observed on low shore habitats, for barnacle cover only. The results of this study challenge the commonly accepted paradigm of a gradual shift from seaweed-dominated to sessile fauna-dominated zones with increasing wave exposure on rocky shores. Finally, this study sheds new light onto interspecific interactions which drive the structure of rocky shore-associated communities.