Meeting Abstract

P3.28  Friday, Jan. 6  Benthic community structure mediated by the red sea urchin Strongylocentrotus franciscanus in the San Juan Archipelago WHIPPO, R*; LOWE, AT; BRITTON-SIMMONS, KH; University of British Columbia; Friday Harbor Laboratories; Friday Harbor Laboratories

The red urchin Strongylocentrotus franciscanus is a ubiquitous member of the San Juan Archipelago marine community ranging from the shallow subtidal to depths greater than 100m. Despite the absence of attached algae on which to feed in the deep subtidal, red urchins are quite common in these habitats, subsisting primarily on detrital seaweeds produced in the shallow photic zone. By capturing these fluxes of organic material from the water column using their spines, they are able to remain attached to the substrate and do not actively seek food. Red urchins are also known to be strong interactors in shallow algal habitats, but very little is known about interactions between urchins and the surrounding community in the deep subtidal. These urchins are up to 20cm in diameter, and extensive field observations suggest that the presence of urchins dramatically alters localized benthic invertebrate abundance patterns. This study tested the hypothesis that red sea urchins alter benthic invertebrate community structure and abundance patterns in the deep subtidal zone. This was accomplished through the use of underwater photography pairing invertebrate communities underneath and adjacent to randomly selected urchins across three sites in the San Juan Channel. Analysis revealed that sea urchins are significantly altering abundance patterns of sessile and mobile fauna they are associated with. The influence of urchins on mobile community abundance varied widely between sampling depths, while sessile organismal abundance did not. Future work will be aimed at understanding the mechanism by which urchins alter these communities. This study increases our ecological understanding of deep subtidal environments in the Pacific Northwest and highlights the important role dominant grazers play in biological communities.