Effects of eutrophication, grazing, and algal blooms on rocky shores

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. Rocky shores are among the most dynamic and productive ecosystems on the planet. Biomass and primary productivity are typically dominated by canopy-forming perennial macroalgae such as fucoids and laminarians. Together with seagrasses on soft-bottom habitats, these algae generate up to 40% of the primary productivity of the coastal zone (Charpy-Roubaud and Sournia 1990) and a significant fraction of global marine plant biomass (Smith 1981). They also fulfill important ecosystem functions, including carbon storage, nutrient cycling, and the provision of food and habitat for a diverse invertebrate and fish fauna (Borg et al. 1997; Worm et al. 2000). Recently, perennial macroalgae and their associated communities have severely declined in abundance in regions such as the Baltic or the Adriatic Sea, where they have been replaced by few species of bloom-forming annual algae (Vogt and Schramm 1991; Munda 1993). These bloom-forming algae do not provide the same biogeochemical and habitat functions as perennial algae, and their mass occurrence often has strong negative effects on coastal ecosystems and their inhabitants, including humans (Valiela et al. 1997). Detailed observations and experiments have linked the increased occurrence of annual algal blooms to elevated nutrient loads from coastal eutrophication (Fong et al. 1993; Hauxwell et al. 1998; Lotze et al. 2000). In addition, it has been shown that grazers such as littorinid snails, isopods, and amphipods can reduce or even prevent algal blooms through selective feeding on their early life-history stages, 1 Corresponding author ([email protected]). Acknowledgments Special thanks to Inka Milewski for discussion, inspiration, and field support; to Ulrich Sommer for comments and suggestions; and to Wade Blanchard for statistical advice. This work was supported by the German Ministry of Science and Education, the German Research Council (DFG), and the Conservation Council of New Brunswick (CCNB). such as propagules and recruits (Lotze and Worm 2000; Lotze et al. 2000). The interplay of eutrophication and grazing may thus determine the occurrence of algal blooms and, on a larger scale, both structure and function of coastal ecosystems (Geertz-Hansen et al. 1993; Hauxwell et al. 1998;