Abstract
Understanding the effects of natural processes on coral–algal competition is an important step in identifying the role of macroalgae in perturbed coral reef ecosystems. However, studies investigating coral–algal interactions are often conducted in response to a disturbance, and rarely incorporate seasonal variability. Here, naturally occurring coral–algal interactions were assessed in situ four times a year over 2 years across eight sites spanning diverse benthic communities. In over 6500 recorded coral–algal interactions, cyanobacteria and turf algae were found to be the most damaging regardless of season, resulting in visible damage to coral in greater than 95% of interactions. Macroalgae that primarily compete using chemical mechanisms were found to be more damaging than those that compete using physical mechanisms (e.g. abrasion), with both groups demonstrating decreased competitive ability in summer. While crustose coralline algae were the least damaging to competing coral, during summer, it became three times more competitive. Our results demonstrate that the competitive ability of macroalgae and the outcomes of coral–algal competition can fluctuate in seasonal cycles that may be related to biomass, production of chemical defences and/or physical toughness. The results of this study have important implications for understanding the trajectory and resilience of coral reef ecosystems into the future.
Highlights
Macroalgae from coral reef ecosystems vary in space and time owing to a combination of biotic and abiotic whiplash, abrasion smothering overgrowth allelopathy royalsocietypublishing.org/journal/rsos R
The types of interacting corals and macroalgae were recorded to genus level, with the exception of cyanobacteria, turf algae, articulate and crustose coralline algae (CCA), which generally cannot be identified to genus level in situ [39]
Interactions with cyanobacteria resulted in visible damage to 100% of coral colonies involved in interactions, whereas interactions with turf algae resulted in visible damage to 95.4–99.6% of colonies
Summary
Macroalgae from coral reef ecosystems vary in space and time owing to a combination of biotic (e.g. herbivory) and abiotic whiplash, abrasion smothering overgrowth allelopathy royalsocietypublishing.org/journal/rsos R. (e.g. temperature, wave action) processes [1,2,3]. Anthropogenic activities, such as land-clearing and fertilization for agriculture, the loss of herbivores because of overexploitation or thermal stress, can disrupt the natural balance leading to reductions in reef-building coral and increases in macroalgal cover [4,5]. Given rapidly shifting coral reef ecosystem dynamics [9], macroalgae and their interactions with corals are more relevant than ever
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