Importance of teleost macrograzers to seagrass composition in a subtropical ecosystem with abundant populations of megagrazers and predators

Herbivores can play an important role in structuring seagrass communities, yet understanding the relative importance of megagrazers (dugongs and sea turtles) and macrograzers (fishes) has been hampered by a lack of studies in ecosystems with healthy predator populations. This study used a series of nested exclosure-transplant experiments to investigate the impacts of fishes on seagrass species composition in a subtropical ecosystem (Shark Bay, Australia) with abundant populations of megagrazers and predators. Three species of fast-growing seagrasses (Cymodocea angustata, Halodule uninervis, and Halophila ovalis) were transplanted into exclosure cages that were located in shallow seagrass beds and systematically excluded grazers. Experiments were conducted during an entire warm season, and during both a warm and cold period. Fish trapping and belt transects were used to determine the relative abundance of herbivorous fishes, megaherbivores, and air-breathing piscivores. Mainly fishes, rather than megagrazers, affected the establishment and persistence of transplanted seagrasses during the warm season, dominating the top-down effects on both H. uninervis and H. ovalis. Grazer impacts were minimal during the cold period, except for fish grazing on H. ovalis, with dugongs abandoning the study area. Fish grazer density during the cold period trial did not differ from that of the warm period, but predator density (cormorants) was significantly greater. These data suggest that fish can play an important role in structuring subtropical seagrass systems, that herbivore impacts are seagrass-species dependent, and that seagrass beds may be shaped by herbivore responses to their predators.