Dynamics of macrofaunal communities of floating seaweed clumps off western Iceland: a study of patches on the surface of the sea

Abstract

The dynamics of macrofaunal communities of floating clumps of detached intertidal algae were studied by experiments and surveys. The density of some intertidal species tended to decrease slowly with time on experimental fronds of the brown seaweed Ascophyllum nodosum that were put afloat. Many planktonic and benthic species quickly colonized such fronds. Plastic seaweed mimics were also effective in attracting colonizers and it appeared to matter little whether the A. nodosum fronds carried a complement of intertidal animals or had been rinsed of animals before being put afloat. Colonization of experimental fronds was to some extent mediated through transfer from small algal pieces caught by the fronds. Floating algal clumps in coastal areas occur in a large range of sizes. Number of species present on such clumps was closely related to clump size. The density (numbers/100 g algae) of many intertidal species was positively linked with clump size. Distance from nearest shore was a poor predictor of species composition. The clumps form dynamic patches on the surface of the sea that often coalesce or break up. These patches greatly enhance the complexity of the neustonic environment and their species richness is much greater than in the surroundings.