Strong impacts of grazing amphipods on the organization of a benthic community

Large brown seaweeds dominate coastal hard substrata throughout many of the world's oceans. In coastal North Carolina, USA, this dominance by brown se aweeds is facilitated by omnivorous fishes, which feed both on red and gree n algae and on herbivorous amphipods that graze brown algae. When fish are removed in the field, brown seaweeds are replaced by red seaweeds, and herb ivorous amphipods are more abundant. Using an array of large (similar to 40 00 L) outdoor mesocosms, we tested three mechanistic hypotheses for this pa ttern: fish feeding facilitates brown algal dominance (1) by removing red a nd green algal competitors, (2) by removing amphipods and reducing their fe eding on brown seaweeds, or (3) through an interaction of these mechanisms. Our experiments revealed strong impacts of both fish and amphipods, and a key role for the interaction, in structuring this community. When both fish and amphipods were removed (the latter with dilute insecticide), space was rapidly dominated and held for 17 weeks by fast-growing, primarily filamen tous green algae. In contrast, when either fish, amphipods, or both were pr esent, green algae were cropped to a sparse turf, and space was more rapidl y dominated by larger macroalgae. The impacts of amphipods and fish on late -successional macroalgal assemblages were comparable in magnitude, but diff erent in sign: red seaweeds prevailed in the amphipod-dominated treatment, whereas browns dominated in the presence of fish. Laboratory feeding assays and amphipod densities in the tanks suggested that the significant effects of amphipods were attributable largely, if not exclusively, to the single amphipod species Ampithoe longimana, which fed heavily on brown macroalgae. Our experimental removal of red and green algae failed to enhance cover of brown algae significantly; however, the latter reached substantially lower cover in the grazer-removal treatment, where green algae were very abundan t, than in the fish-only treatment, where green algae were sparse. Thus, ou r results support the third hypothesis: fish-mediated dominance of brown al gae involves both suppression of grazing amphipods and removal of algal com petitors. Although collective impacts of fish and amphipods on this benthic community were generally comparable in magnitude, impacts normalized to ea ch grazer's aggregate biomass were consistently higher for amphipods than f or fish, sometimes by 1-2 orders of magnitude. Thus, the impacts of grazing amphipods (specifically A. longimana) on the benthic community were both s trong and disproportionate to their biomass. These experimental results imp ly that grazing amphipods, which are ubiquitous in marine vegetation but po orly understood ecologically, may play important roles in the organization of benthic communities, particularly where predation pressure is low.