Interactions Between Wave Action and Grazing Control the Distribution of Intertidal Macroalgae

Abstract

Canopy-forming macroalgae are key species on temperate rocky shores. However, there is a lack of understanding of how the relative balance of physical and biological factors controls the establishment and persistence of intertidal macroalgae. Here we present an integrated study of the relative importance of wave-induced forces and grazing for the recruitment and survival of the canopy-forming intertidal macroalgae Fucus vesiculosus and F. spiralis. A set of overtopped breakwaters provided a nearly unconfounded gradient in wave exposure between seaward and landward sides. A biomechanical analysis was performed based on empirical measurements of maximum drag forces in breaking waves, a model of long-term maximum wave height, and the breaking stress of Fucus spp. The estimated maximum flow speed (7-8 m/s) on the seaward side of the breakwaters was predicted to completely dislodge or prune Fucus spp. larger than approximately 10 cm, while dislodgment was highly unlikely on the landward side for all sizes. Experimental transplantation of Fucus spp. supported the biomechanical analysis but also suggested that mechanical abrasion may further limit survival in wave-exposed locations. Experimental removal of the limpet Patella vulgata, which was the principal grazer at this site, resulted in recruitment of Fucus spp. on the seaward side. We present a model of limpet grazing that indicates that limpet densities >5-20 individuals/m2 provide a proximate mechanism preventing establishment of Fucus spp., whereas wave action >2 m/s reduces persistence through dislodgment and battering. In a conceptual model we further propose that recruitment and survival of juvenile Fucus spp. are controlled indirectly by wave exposure through higher limpet densities at exposed locations. This model predicts that climate change, and in particular an increased frequency of storm events in the northeast Atlantic, will restrict fucoids to more sheltered locations.

Additive effects of physical stress and herbivores on intertidal seaweed biodiversity

“…Similarly, under low nitrogen availability, slow-growing species dominate, but as nitrogen availability increases, fast-growing species with higher nitrogen requirements can overgrow the assemblage, and diversity declines (Worm et al 2002, Bokn et al 2003. A similar pattern in seaweed diversity can result from physical disturbances such as from waves (Denny 1995, Jonsson et al 2006 or floating logs that remove patches of organisms and initiate a succession in which diversity often, but not always, peaks at intermediate levels of disturbance (Sousa 1979, MacQuaid and Branch 1984, Svensson et al 2007 for subtidal seaweeds).…”

Fucus vesiculosus and spiralis species complex: a nested model of local adaptation at the shore level

“…Stressors may be abiotic, such as desiccation and wave exposure, or they can be biotic, such as the presence of predators or competitors. The combination of these different factors strongly shapes species distribution along intertidal environmental gradients, as testified by the local distributions of intertidal organisms, which have been an important object of study for over a century (Jonsson et al 2006). Among these, the genus Fucus is composed by multiple species that are distributed within different but overlapping vertical limits all along northern Atlantic intertidal zones, from the mediolittoral (F. serratus, F. evanescens) to the upper shore (F. spiralis).…”

Use of the intertidal zone by mobile predators: influence of wave exposure, tidal phase and elevation on abundance and diet

“…Levels of exposure to wave action were confirmed using measures of mean flow conditions following Jonsson et al (2006), via the dissolution of gypsum (CaSO 4 ) discs. Sets of 12 discs were attached to the rock surface ~3 m above chart datum (CD) for a single high tide on each of 2 occasions at each location.…”