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

MARIA BAGUR; JORGE L. GUTIÉRREZ; JULIANA A. GONZALEZ; LORENA P. ARRIBAS; M. GABRIELA PALOMO

JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2022 vol. 548 p. 1 - 10

0022-0981

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