Effects of herbivores and water quality on Sargassum distribution on the central great barrier reef: Cross-shelf transplants

On the central Great Barrier Reef, there is a marked gradient in abundance of macroalgae across the continental shelf. Fleshy brown macroalgae such as Sargassum are abundant on nearshore reefs and virtually absent from mid-shelf reefs. This gradient correlates with differences in abundance of herbivorous fish and with differences in water quality. Inshore fringing reefs have fewer and less diverse herbivorous fish and higher inputs of terrestrial nutrients and sediments. This study used a combination of cross-shelf transplants and herbivore exclusion cages to test the importance of water quality and herbivores to the survival of adult Sargassum. Sargassum survived and even thrived for up to 2 mo on a mid-shelf reef, but only if protected from herbivores. Tissue nutrient analyses indicated that mid-shelf transplants were not limited by nitrogen or phosphorus supplies. Thus Sargassum can survive in the nutrient and sediment conditions on the mid-shelf reef. In this sense, the transplant serves as a partial test of water quality effects on Sargassum abundance. Herbivore exclusion was critical to survival of transplanted Sargassum on the mid-shelf reef but had no effect on survival in the nearshore fringing reef zone in which Sargassum normally occurs. There was significant spatial variation in survival of Sargassum, especially in the presence of herbivores.