Spatial variation in intertidal food web structures, Barkley Sound, British Columbia: Comparison using bulk and compound-specific isotope analysis

This thesis examines spatial variation in food web structure and trophic position of four intertidal invertebrates, Mytilus californianus (California mussel), Katharina tunicata (black katy chiton), Anthopleura elegantissima (aggregate green anemone) and Pisaster ochraceus (ochre seastar) in Barkley Sound, British Columbia. More specifically, using bulk delta13C and delta 15N stable isotopic analysis, the hypothesis that intertidal food webs have measurable spatial variation when examined at regional and site-level scales was tested. Because bulk analysis may confound results, compound specific isotopic analysis (CSIA) was used to resolve more precisely the base of the food chain by focussing on delta13C values of fatty acids. Mytilus californianus was found to occupy a trophic level of a strict herbivore (TL = 2.0) with virtually no variation between sites and CSIA suggested a diet of predominantly diatoms. One site was exceptional, having quite different delta 13C and delta15N isotopic compositions, which was attributed to this site being the most marine in character. Using bulk delta 15N K. tunicata, a grazer, was found to vary spatially in delta15N which in turn was believed to reflect spatial variation in the delta15N of macroalgae and diatoms. Anthopleura elegantissima, which is generally considered an omnivore, was found to have the largest between-site variation in trophic level, ranging from 2.5 to 3.3. Its diet appears to be principally based on zooplankton. Pisaster ochraceus, a known predator, was found to be strictly carnivorous with some spatial variations in trophic level. CSIA analysis suggests a diet predominantly consisting of M. californianus. Overall, these results show that trophic structures and feeding relationships in the intertidal zone of the Pacific Northwest have varying degrees of geographic variation. Further, this study is one of the first to demonstrate the power of CSIA in resolving food webs in intertidal ecosystems.