Abstract
Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy‐forming algae such as Fucus spp. with annual, bloom‐forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae’s susceptible recruits. We examine these generalizations across large scales. We use replicated “bioassay” experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalis propagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10‐fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant (;80%). Comparative field surveys showed that perennial algae decreased by 30–60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24–46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity.