The subtidal zone on cold temperate rocky coasts is an environment exposed to much less physical disturbance than the intertidal, and sediment deposition is continuous. Removal of this sediment by gastropod grazers is therefore presumed to affect the structure of subtidal algal communities. This concept was investigated by evaluating the grazing effects of the dominant herbivorous gastropod Omphalius rusticus by exclusion experiments in the field. Settlement plates of both exclusion and control treatments were placed every month from November 2014, and immersed for 1 mo. Algae colonized from May and tended to increase in biomass toward summer. No marked differences were observed in the algal composition of exclusion and control. Sediment deposition showed no apparent seasonal changes. Cumulative successional plates of both exclusion and control treatments were placed in November 2014, and immersed for 1 to 9 mo. The colonization of algae started in February and the species number peaked earlier in the exclusion and later in the control. Sediment load and algal biomass were high from February to May in the exclusion, and from April to July in the control. Seedlings of Sargassum confusum were found in both plots starting in July. We conclude that O. rusticus constantly removed sediments by its grazing activity and had a large impact on the formation of the macroalgal community by controlling sediment deposition. Its presence delayed the colonization of early-successional turf algal species, but did not affect colonization of late-successional canopy-forming algae such as S. confusum.
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