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Publication Date

Fall 2008

Degree Type

Thesis - Campus Access Only

Degree Name

Master of Science (MS)


Moss Landing Marine Laboratories


Michael H. Graham

Subject Areas

Biology, Botany; Biology, Ecology


The abundance and composition of drift algae available for intertidal herbivore consumption, or the impact of drift algae on intertidal purple urchins is unknown. A caging experiment was conducted to manipulate the presence and absence of drift algae to intertidal Strongylocentotus purpuratus to test whether individuals denied drift algae would exhibit decreased survivorship, growth, or reproductive capabilities. Natural drift abundance consisting mostly of kelps and other brown algae increased during summer and decreased in winter. After seven months, individuals in drift-exclusion treatments had significantly smaller gonad indices and depleted delta 13C and delta15N isotope values compared to those in treatments with drift. Sea urchin test diameter, test height, wet weight, density, and jaw/test allometry did not differ among treatments. A short-term absence of drift larger than 1 cm2 did not negatively impact survivorship, growth, or community composition. Drift-deprived sea urchins persisted without increased grazing, suggesting that intertidal purple urchins are regulated by a different set of mechanisms than subtidal urchins.