Wrack deposition on different beach types: Spatial and temporal variation in the pattern of subsidy

The onshore deposition of macroalgal and macrophyte wrack provides a potentially significant marine subsidy to intertidal and supratidal herbivore and decomposer communities. Based on the study of daily input loads to beaches, we estimated summer wrack deposition of up to 140 Mg (dry mass)/km shoreline in Barkley Sound, British Columbia. However, input rates were highly variable depending on beach type, nearshore hydrodynamics, and buoyancy characteristics of the wrack. Cobble beaches retained similar to 10 times and 30 times more wrack than did gravel and sand beaches, respectively. Cobble and gravel beaches also differed in species composition of new (fresh) wrack input, with Macrocystis integr folia being characteristic for the former and Nereocystis luetkeana for the latter, which we attribute to buoyancy characteristics of the floating debris. On sand beaches, Phyllospadix spp. and Enteromorpha spp. were the dominant wrack species. Species composition of freshly deposited wrack also depended on wave exposure, but predictability based on the species pool within a beach's catchment was restricted. Drift lines of aging wrack differed from freshly deposited wrack in species composition, probably due to wrack decomposition that results in fluxes of nutrients and energy between the adjacent marine and terrestrial habitats. We hold that the characteristics of a given beach, e.g., substratum and wave exposure, and their effects on wrack input, will have important ecological and biogeochemical implications for the marine-terrestrial ecotone.