Coastal upwelling may strengthen the controls of herbivory and light over the population dynamics of Hedophyllum sessile in the Oregon rocky intertidal

Survival of early life history stages is critical to the successful establishment of benthic populations. Although light availability and herbivory are likely to influence the passage of marine macroalgae through a recruitment bottleneck at the sporeling stage, the interactive effect of these factors on subsequent community patterns of macroalgae is not well studied. We experimentally tested the effect of light and grazing on sporelings of the common intertidal kelp Hedophyllum sessile. Studies were conducted at two sites along the Oregon coast: Strawberry Hill (an intermittent upwelling region) and Cape Blanco North (a persistent upwelling region). Herbivory and light availability were manipulated and kelp performance metrics (density and length) were measured monthly from May to November 2019. We found that the effects of herbivory and light availability were pronounced at Cape Blanco North but negligible at Strawberry Hill. At Cape Blanco North, herbivory had strong but opposing effects on the density and length of H. sessile. Kelp density was higher in treatments without herbivores while kelp length was greater in treatments with herbivores. Responses also differed with the life history stage. Herbivory had negative effects on juvenile kelp but positive effects on adult kelp while light availability had opposing effects on the length of juvenile and adult kelps. The length of juvenile kelps was higher in shaded treatments while the length of adult kelps was higher in unshaded treatments. Our study highlights the potential importance of coastal geophysical processes (and subsequently, nutrients) in modifying herbivore and light effects on population dynamics of H. sessile, and how these dynamics may be further influenced by different characteristics of the kelp (i.e., demographic traits and life history stages).

Automated summary

Survival of early life history stages is crucial for benthic populations. The interactive effects of light and herbivory on macroalgae community patterns are not well understood. This study investigated the effects of light and grazing on sporelings of a common intertidal kelp species and found significant effects varying with site and life history stage.