Light availability indirectly limits herbivore growth and abundance in a high rocky intertidal community during the winter

Light availability and consumer pressure are fundamental factors that structure aquatic communities, but their integrated effects are rarely studied in marine benthic systems. Using a factorial experiment, I investigated the effects of light availability and grazing by limpets (Lottia digitalis) to determine the relative influence of each on the growth and abundance of producers and consumers in a rocky intertidal community during the winter. Light reduction via shading reduced the abundance of filamentous algae and reduced the abundance of the herbivorous periwinkle Littorina sitkana. The effects of limpet grazing reduced the abundance of filamentous algae, Porphyra spp., and diatom mats. Higher limpet densities were also associated with significantly lower densities of adult, but not juvenile, Littorina spp. Light and limpet density interacted to determine Lottia growth, which was high in unshaded, single-limpet enclosures but was negligible in plots with two limpets, shades, or both. Variation in bottom-up (resource-driven) and top-down (consumer-driven) forces are important determinants of abundance at both trophic levels, but the effects are complex, taxon-specific, and, for littorine snails, size-specific. Because the study species accomplish most of their growth and reproduction during the winter, the interplay of light and herbivory during this season will likely influence long-term community dynamics.