ABSTRACT: The fate of subtidally drifting macrophytal detritus after its deposition ashore was studied based on short-term mass loss effects and species composition of beach-cast detritus. Different species of macroalgae and seagrass varied in both physical and microbial decay, as well as faunal decomposition rates. Their preferred status as food for detritivorous amphipods also varied. Thus, beach-cast detritus changed in species composition during detritus aging. Estimated turnover rates, based on daily input rates and mass loss rates, ranged from <1 d for Nereocystis luetkeana, Macrocystis integrifolia and Ulva spp. to roughly 30 d for Fucus spp. and Phyllospadix spp. Thus, the dynamics of nutrient fluxes within the marine–terrestrial ecotone depends not only on the spatial distribution and amount of beach-cast detritus, but also on its species composition.