Subtidal Coexistence: Storms, Grazing, Mutualism, and the Zonation of Kelps and Mussels

摘要: At exposed sites in the Gulf of Maine, USA, subtidal mussels (AModiolus modiolus) dominated space on upper rock surfaces at intermediate depths (1 1- 18 m), but not shallow (4-8 where dominants were kelps Laminaria digitata and L. saccharina. Observations experiments conducted to test following hypotheses about factors limiting vertical (depth) zonation kelp: (1) that upward distribution Modiolus is limited by interference effects kelp, or (2) it failure larval recruitment depths, (3) sea urchin grazing controls downward kelp. The hypothesis was rejected. Results indicated storm-generated dislodgement overgrown kelp mechanism reducing ability maintain hold zone. Dislodgement overgrowth most significant mortality source large mussels. Removal urchins, Strongylocentrotus droebachiensis, from lower edge zone resulted shift a 12.5 m depth, demonstrating depth limit set grazing. Storm disturbances varied frequency severity damage mussel populations, an important agent patch creation Small storms occurred monthly basis. Severe struck three times 1982 twice 1983; average 11 658 with attached dislodged cast ashore monitored beaches during such storms. synchronous between coastal offshore sites. mean density patches cleared beds (2.5 patches/0.25 i2) highest after fall Mean size ranged 91.0 cm2 (fall) 122.0 (winter). With exception fronts 10.5-12.0 m, densities signifi- cantly higher inside than outside. urchins increase .Modiolus survivorship off decreasing risk dis- lodgement tested urchin-removal experiment. led rapid recruitment, resulting 30-fold (via kelp-induced dislodgement) compared control resident urchins. Significantly more force required pull out remove them outside beds, suggesting are less susceptible predation dislodgement-caused while beds. Modiolus-Strongylocen- trotus interaction identified as facultative mutualism appears facilitate coexistence depths. recover disturbance examined clearing algal turf. Kelps recolonized all algal-turf 47% within 7 mo. In contrast, Alodiolus did close leaning response surrounding patch, over 3-yr period. This suggests bounce back may enhance their competitive superiority. For Modiolus, rate storm- generated exceeded recovery.