MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps12410

Spatial and temporal limits of coral-macroalgal competition: the negative impacts of macroalgal density, proximity, and history of contact

Cody S. Clements, Douglas B. Rasher, Andrew S. Hoey, Victor E. Bonito, Mark E. Hay*

*Email: mark.hay@biology.gatech.edu

ABSTRACT: Tropical reefs are commonly transitioning from coral- to macroalgal-dominance, producing abrupt, and often lasting, shifts in community composition and ecosystem function. Although negative effects of macroalgae on corals are well documented, whether such effects vary with spatial scale or the density of macroalgae remains inadequately understood, as does the legacy of their impact on coral growth. Using adjacent coral- versus macroalgal-dominated areas, we tested effects of macroalgal competition on two common Indo-Pacific corals species. When corals were transplanted to areas of: i) macroalgal-dominance, ii) macroalgal-dominance but with nearby macroalgae removed, or iii) coral-dominance lacking macroalgae, coral growth was equivalently high in plots without macroalgae and low (62-90% less) in plots with macroalgae, regardless of location. In a separate experiment, we exposed corals to differing densities of the dominant macroalga Sargassum polycystum. Coral survivorship was high (≥ 93% after three months) and did not differ among treatments, whereas the growth of both coral species decreased as a function of Sargassum density. When Sargassum was removed after three months, there was no legacy effect of macroalgal density on coral growth over the next six months; however, there was no compensation for previously depressed growth. In sum, macroalgal impacts were density dependent, occurred only if macroalgae were in close contact, and coral growth was resilient to prior macroalgal contact. The temporal and spatial constraints of these interactions suggest that corals may be surprisingly resilient to periodic macroalgal competition, which could have important implications for ecosystem trajectories that lead to reef decline or recovery.