Rafting on abiotic substrata: Properties of floating items and their influence on community succession

A wide diversity of organisms is dispersed on abiotic floating substrata, but little is known about the succession of fouling communities on these items. The main abiotic floating items found in the oceans (volcanic pumice, plastics, and Styrofoam) differ in 3 principal characteristics, namely surface rugosity, buoyancy, and floating behaviour, confirmed herein by light and electron microscopy (SEM), measurements of buoyancy, and laboratory tests to determine floating stability. In order to examine how these properties affect colonization and com munity succession on these substrata, 3 experiments were conducted in the field for 14 wk: (1) substrata were permanently submerged to determine if colonization depends on substratum surface ('Rings' experiment); (2) substrata were tethered at the sea surface to examine the importance of buoyancy ('Lines' experiment); and (3) substrata were caged to test the role of their floating behaviour ('Cages' experiment). The most common colo nizers in all 3 experiments were diatoms, Ulvales, and clonal ascidians from the genus Diplosoma. Surface rugosity of the substrata had only minor effects on the taxonomic richness of the developing communities. Buoyancy caused some differences in taxon cover, but taxon richness was similar on all 3 substrata. Finally, the floating behaviour at the sea surface had a strong influence on the communities developing on the floating substrata. Plastics (fewest changes in position) presented a higher richness and cover of taxa, while Styrofoam (most positional changes) harboured very few taxa. We conclude that colonization and succession on floating substrata is strongly influenced by positional changes, while surface rugosity and buoyancy appear important only during initial stages of community succession.