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

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 closely adjacent coral- versus macroalgal-dominated areas, we tested effects of macroalgal competition on the Indo-Pacific corals Acropora millepora and Porites cylindrica. 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 raised corals above the benthos in each area and exposed them to differing densities of the dominant macroalga Sargassum polycystum. Coral survivorship was high (≥ 93% after 3 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 two months, there was no legacy effect of macroalgal density on coral growth over the next seven 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.

Keywords: Fiji; coral reef; coral-algal competition; macroalgae.

Fig. 1

(Top) Percentage change in coral mass (mean ± SE) during a 36-week period (January–October 2014) for Acropora millepora (A) and Porites cylindrica (B) originally from the coral- or macroalgal-dominated area that were embedded within coral- or macroalgal-dominated area plots (with natural algal assemblages either left in place or physically removed within the fished area location). Growth differences among conspecifics were analyzed using the “compareGrowthCurves” function in the R package “statmod.” Letters to the right of lines indicate significant groupings via Hommel’s method (p < 0.05). (Bottom) The number of Acropora (C) and Porites (D) that survived throughout the duration of the experiment. Survival did not differ significantly as a function of treatment for either species.

Fig. 2

(Top) Percentage change in mass (mean ± SE) for the corals Acropora millepora (A) and Porites cylindrica (B) over two months (January–March 2014) of contact with differing densities of Sargassum polycystum. (Middle) Percentage change in mass (mean ± SE) during March–September 2014 for Acropora (C) and Porites (D) previously exposed to different densities of surrounding Sargassum, but with no Sargassum present during this period of growth assessment. (Bottom) Total mass change (g) (mean ± SE) during January–September 2014 for Acropora (E) and Porites (F) initially exposed to different densities of Sargassum for three months (December-March 2014), but with Sargassum then removed and absent for the next 7 months (March–September 2014). For all graphs, data for each species were analyzed using generalized least-squares (GLS) models. Letters denote significant differences (p < 0.05) among algal density treatments via Tukey tests. Numbers within bars indicate sample size.

Fig. 3

(Top) Percentage change in mass (mean ± SE) for the corals Acropora millepora (A) and Porites cylindrica (B) over two months (January–March 2014) of contact by differing densities of Sargassum polycystum. (Middle) Percentage change in mass (mean ± SE) during March–September 2014 for Acropora (C) and Porites (D) previously exposed to different densities of surrounding Sargassum, but with no Sargassum present during this period of growth assessment. (Bottom) Total mass change (g) (mean ± SE) during January–September 2014 for Acropora (E) and Porites (F) initially exposed to different densities of Sargassum for two months (January–March 2014), but with Sargassum then removed and absent for the next 7 months (March–September 2014). For all graphs, data for each species were analyzed by sequentially testing nested GLS models via likelihood ratio tests to obtain the optimal fixed structure for each model. P-values from these analyses are presented above each figure. D = inclusion of Sargassum density as a fixed term; A = inclusion of area (coral- or macroalgal-dominated area) as a fixed term. Numbers within bars indicate sample size.