LIFE AND DEATH BENEATH MACROPHYTE CANOPIES - EFFECTS OF UNDERSTORY KELPS ON GROWTH-RATES AND SURVIVAL OF MARINE, BENTHIC SUSPENSION FEEDERS

Experiments conducted on rocky bottoms at 7-11 m depth in the San Juan Archipelago, Washington assessed effects of canopies of understory kelps on growth of benthic suspension feeders, determined the mechanisms responsible for effects, and assessed the influence of kelp canopies on survivorship of benthic fauna. Kelp canopies influenced growth rates of diverse suspension feeders. At several sites the mussel Mytilus edulis, the barnacle Balanus glandula, and the serpulid polychaete Pseudochitinopoma occidentalis grew faster on the bottom beneath kelp canopies than on nearby exposed substrata. The cheilostome bryozoan Membranipora membranacea showed a mixed response to kelp canopies, growing faster in exposed regions at one site, but faster beneath canopies at another. There were no differences in growth of 2 other species (the cheilostome bryozoan Cheilopora praelonga and the sponge Myxilla incrustans) between kelp and no-kelp treatments; however, some processes influenced by plant canopies affected their growth. Specific mechanisms responsible for kelp effects on growth were assessed in a series of field experiments using Pseudochitinopoma, Membranipora, Cheilopora and Myxilla. Particulate deposition on the bottom, which is more intense beneath canopies, negatively affected growth of all 4 species. Kelps also reduced rates of flow and prevented development of microalgal turfs beneath the canopy. Pseudochitinopoma grew faster in the weaker flows below canopies and both Cheilopora and Myxilla grew faster where there were no microalgal turfs. These other effects of kelp canopies were at least as important to growth (in the cases of Cheilopora and Myxilla) or more important to growth (in the case of Pseudochitinopoma) than were the general, deleterious effects of higher sedimentation beneath canopies. The lower growth rates caused by higher sedimentation beneath kelp canopies did not result in higher rates of animal mortality. Surprisingly, kelp canopies typically did not influence mortality due to predation. For 7 of 12 taxa, mortality rates did not differ between kelp-covered and exposed treatments. Significantly higher mortality occurred outside canopies for only 4 of 12 taxa, and for at least 2 of these 4 differences probably were not related to predation. Mytilus, a species rare at these depths, exhibited higher mortality beneath kelp canopies due to predation by crabs. Other macrophytes in fresh and salt water, as well as some benthic animals that create complex, 3-dimensional habitats, should influence benthic organisms and assemblages in ways analogous to the kelps acting through their effects on flow, particle transport, and shading.