Herbivores can drastically alter the morphology of macroalgae by directly consuming tissue and by inflicting structural wounds.  Wounds, in particular, can result in large amounts of tissue breaking away from macroalgae, amplifying the damage initially caused by herbivores.  Herbivores that commonly wound macroalgae often only occur over a portion of a macroalga’s lifespan or geographic range.  However, we know little about the influence of these periodic or regional occurrences of herbivores on the large-scale seasonal and geographical patterns of macroalgal morphology.  We used the intertidal kelp Egregia menziesii to investigate how the kelp’s morphology and the prevalence of two prominent kelp-wounding herbivores (limpets and gammarid amphipods) changed over two seasons (spring and summer) and over the northern extent of the kelp’s geographic range (six sites from central California to northern Washington).  Wounds from limpets and amphipods often result in the kelp’s fronds being pruned (intercalary meristem broken away), so kelp morphology was quantified as size (combined length of all fronds) and pruning (proportion of broken fronds).  In both seasons, limpets were the dominant herbivore at southern sites while amphipods were dominant at northern sites.  This pattern was likely driven by each herbivore species’ unique response to local wave action, temperature, and tidal regimes.  Within each season, most kelp had a similar morphology and collective herbivore prevalence (limpets and/or amphipods).  Our results suggest that large-scale geographic similarities in macroalgal-wounding, despite regional variation in the herbivores inflicting the wounds, can maintain similar macroalgal morphologies over large geographic areas.

We surveyed five sites over the northern distribution of Egregia menziesii (from central California to Washington) in spring and summer 2016, with an additional site added in the summer.  Survey sites, dates, and sample sizes are listed in Table 1.  At each site, we haphazardly placed a 20-m transect parallel to the shoreline within the E. menziesii zone.  Along the transect, we selected every third kelp that was a mature sporophyte (sensu Henkel and Murray, 2007) and measured two morphological features: (1) size - the combined length of all fronds on a kelp, measured to the nearest 1 cm, and (2) pruning - the fraction of fronds that were missing their intercalary meristem (i.e., the region of the rachis where the fastest growth occurs).  Only fronds that were longer than 10 cm were included in these measurements because fronds <10 cm made up a negligible portion of a kelp’s total size and could not always be reliably counted.  By this measurement, kelp can be large by having numerous short fronds or a few long fronds (Burnett and Koehl, 2020).  We also scored whether the kelp showed signs of herbivory from limpets and amphipods, which included the presence of the organisms on the kelp or their distinctive wounds (i.e., home scars on rachis caused by limpets, burrows in rachis caused by amphipods).  We measured as many kelp as possible within a single low tide, using multiple transects when necessary (samples sizes per site ranged from 25 to 52 kelp).