Abstract
Competitive interactions between corals and benthic algae are increasingly frequent on degrading coral reefs, but the processes and mechanisms surrounding the interactions, as well as the exacerbating effects of sediments trapped in turf algae, are poorly described. We surveyed the frequency, proportion, and outcomes of interactions between benthic algae (turf algae and macroalgae) and 631 corals (genera: Porites, Favites, Favia, Platygyra, and Pavona) on a degenerating reef in the northern South China Sea, with a specific focus on the negative effects of algal contact on corals. Our data indicated that turf algae were the main algal competitors for each surveyed coral genus and the proportion of algal contact along the coral edges varied significantly among the coral genera and the algal types. The proportions of algal wins between corals and turf algae or macroalgae differed significantly among coral genera. Compared to macroalgae, turf algae consistently yielded more algal wins and fewer coral wins on all coral genera. Amongst the coral genera, Porites was the most easily damaged by algal competition. The proportions of turf algal wins on the coral genera increased 1.1–1.9 times in the presence of sediments. Furthermore, the proportions of algal wins on massive and encrusting corals significantly increased with the combination of sediments and turf algae as the algal type. However, the variation in proportions of algal wins between massive and encrusting corals disappeared as sediments became trapped in turf algae. Sediments bound within turf algae further induced damage to corals and reduced the competitive advantage of the different coral growth forms in their competitive interactions with adjacent turf algae.
Highlights
Coral reefs around the world, including those of the South China Sea, have been undergoing rapid degradation due to anthropogenic stressors and changes in theirHow to cite this article Liao Z, Yu K, Wang Y, Huang X, Xu L. 2019
We carefully investigated and recorded features of the coral colonies, that is, coral genus, growth form, and perimeter; we visually examined the edges of the corals for the presence or absence of turf algae and macroalgae; and we quantified the proportion of coral-algal interactions
Benthic cover The benthos of Weizhou Island consisted of 7% hard coral, 31% benthic algae, 5% other biological substrates, and the residual 57% consisted of non-biological substrates (Fig. 1B)
Summary
Coral reefs around the world, including those of the South China Sea, have been undergoing rapid degradation due to anthropogenic stressors and changes in theirHow to cite this article Liao Z, Yu K, Wang Y, Huang X, Xu L. 2019. Macroalgae and turf algae generally tend to increase in abundance in degraded reefs (Nugues & Bak, 2006; Haas, El-Zibdah & Wild, 2010) and develop intense competitive interactions with the remaining corals (Hughes, 1994; McCook, Jompa & Diaz-Pulido, 2001; Bellwood et al, 2004). This is pronounced in reefs that experience frequent human activity (Barott et al, 2012; Brown et al, 2017). Phase shifts from coral to algal domination in reefs have been observed with increasing frequency as a result of overfishing (Hughes, 1994), eutrophication (Littler, Littler & Brooks, 2006), sediment deposition (Goatley & Bellwood, 2013; Goatley et al, 2016), and global climate change (Hoegh-Guldberg et al, 2007)
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