THE EFFECTS OF GRAZING BY SEA URCHINS, STRONGYLOCENTROTUS SPP., ON BENTHIC ALGAL POPULATIONS1

Abstract

A series of shallow intertidal pools at Mukkaw Bay, Washington, ranging in height from −0.3 to +0.6 m had the urchin Strongylocentrotus purpuratus removed from them. Subtidal rocks at Friday Harbor, −7.3 to −8.2 m, were either caged or had Strongylocentrotus fransiscanus removed at monthly intervals. Observations of the rate and pattern of algal succession for periods of up to three years showed that following an initial establishment of new species, brown algae began to dominate. The rate of domination is related to the area’s tidal height, with succession most rapid in the lower intertidal areas or subtidally. After a variable period, the majority of the algal biomass was vested in a single perennial brown algal species, Hedophyllum sessile in the inter tidal and Laminaria complanata or Laminaria groenlandica subtidally. These plants existed neither in the control areas throughout the study, nor in the experimental pools and rocks before urchin removal. Intermittent urchin browsing could make a major contribution to the variety of algae coexisting within limited areas on these rocky shores.