Algal diversity in relation to physical disturbance: a mosaic of successional stages in a subtidal cobble habitat

Effects of physical disturbance on algal diversity and composition were investigated in a subtidal cobble habitat at Plum Cove, Cape Ann, Massachusetts, USA.Monthly collections of cobbles with attached algae were made for 1 yr using SCUBA at 4 to 5 m below MLW.Cobbles were segregated into 4 size categories representing a gradient of disturbance frequency.Regression analyses were used to determine the relationship between cobble size and diversity and life-form of attached algae.Small cobbles (Category 1; 2 cm diam.), frequently disturbed (100 % chance of turnover d-l), are dominated by crustose algae and juvenile uprights and exhibit low hversity (5.77 species cobble-'; total 31 species).Category 2 cobbles (4.5 cm diam.), dominated by crustose algae and annual uprights, are disturbed less frequently (10% chance of turnover d-') and have a higher diversity (10.35 species cobble-'; total 46 species).Category 3 cobbles (7 cm diam.), disturbed infrequently (50 % chance of turnover mo-l), exhibit the highest diversity (15.65 species cobble-'; total 56 species) with equal amounts of crustose algae, annual uprights, and perennial uprights.The largest cobbles (Category 4; 9.5 cm diam.), are the least disturbed (10 % chance of turnover mo-') and exhibit lower diversity than intermediate size cobbles (13.39 species cobble-'; total 37 species).These stable cobbles are dominated by perennial upright species, representing the climax condition in this habitat.The cobble habitat is a mosaic of successional stages with algal diversity and composition on each cobble determined by the f r e q u e ~c y of disturhsnce.These patterns support the ir,ter--cdiatc disturbancc hypothesis ar.d suggest that this community is maintained in a non-equilibrium state.