Physical rather than biotic factors set the lower limit of mussel beds in a horizontal rocky intertidal platform

Abstract

Lower limits in the vertical distribution of rocky shore species have traditionally been considered as set by biotic factors, such as predation, grazing, or space competition. Yet, evidence in this regard comes mostly from sloping shores, i.e continuous surfaces without a noticeable topographic break. Here, we illustrate that this is not necessarily so in the case of horizontal shore platforms, i.e., those characterized by the presence of a sharp seaward scarp that delimits an upper and a lower platform level. Preliminarily, we observed that the lower limit of mussel (Brachidontes rodriguezii) beds in a horizontal shore platform of the Argentinean Patagonia occur in the scarp and that the surface of the lower platform immediately below is essentially bare, to the extent that not even biofilms or encrusting algae develop there. A priori, this lower mussel bed limit cannot be explained by competition or predators, as other forms of biotic cover should develop in the lower platform in both cases. To test whether physical forces – such as wave energy and sediment scour – set the lower mussel bed limit at this site, we compared water movement and abrasion in the upper and lower platforms, we evaluated losses of mussel cover and mussel attachment strength in boulders transplanted to both platform levels, and we manipulated predator access to transplanted mussel-covered boulders. Our findings show (a) that water motion and abrasion are higher in the lower than the upper platform, (b) that mussels transplanted to the lower platform for relatively short periods (2–3 days) show decreased attachment strength and increased detachment rates relative to those transplanted within the upper platform and, (c) that predators do not explain the higher mussel detachment rates observed in the lower platform. Collectively, these findings point to the harsh physical conditions at lower platform as the cause of bare surfaces and lower mussel bed limits at this site. They also suggest that our current general model of species zonation in rocky intertidal shores may need to be refined and/or expanded to account for species zonation limits in horizontal platforms.