The effect of structurally complex corals and herbivory on the dynamics of Halimeda
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The calcareous green alga Halimeda is a key contributor to carbonate sediment production on coral reefs. As herbivores have a direct negative effect on the abundance of Halimeda, protection from herbivory is critical for Halimeda growth. Branching corals such as Acropora are likely to provide refugia for Halimeda from grazers, yet studies are scarce. Here, we investigated the vulnerability of two Halimeda species to herbivory using fish exclusion cages and assessed the contribution of coral structural complexity to seasonal changes in Halimeda biomass and morphometrics. While up to 50 % Halimeda abundance was depleted outside cages due to herbivory and the exclusion of large herbivores resulted in an increase in net growth up to threefold, Halimeda recruitment was positively affected by herbivory, more than two times greater outside cages. However, these responses differed between species and seasons; only one species was affected in winter but not summer. Coral structural complexity facilitated an increase of total algal biomass particularly in summer. At the individual level, thalli growing inside the Acropora canopy were always significantly larger (thallus biomass, volume and height) than those growing in exposed areas. We estimated that the carbonate production of Halimeda was nearly three times greater inside refuges provided by Acropora. Because Halimeda species differ in growth rates and susceptibility to grazing, we predict that the ongoing degradation of the habitat complexity provided by branching corals will alter Halimeda community structure and its contribution to local sediment budgets.
KeywordsCalcified macroalgae Branching Acropora Habitat complexity Parrotfish Carbonate production
We thank the PADI Foundation and the Australian Coral Reef Society for funding to CCS and an Australian Research Council Laureate Fellowship to PJM. Special thanks to field assistants Candice Rempel, Bar Ayalon, Henrique Bravo, Aaron Chai and Giovanni Bernal and to the staff of Heron Island Research Station and MSEL colleagues for their helpful advice and support in the field. We also thank Simon Blomberg for his invaluable advice on statistics and two anonymous reviewers for their constructive comments. Research was conducted under GBRMPA permit #G13/36037.1.
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