Effects of pCO2 on spatial competition between the corals Montipora aequituberculata and Porites lutea

Abstract

We tested the hypothesis that ocean acidification (OA) affects spatial competition among scleractinian corals. Competitive ability was evaluated indirectly by linear extension of Porites lutea and Montipora aequituberculata placed in intraspecific, interspecific, and control pairings (paired with dead coral skeleton) and exposed to ambient (~400 µatm) and elevated (~1000 µatm) pCO 2 in experiments conducted in Moorea, French Polynesia, and Okinawa, Japan. High pCO 2 had no effect on linear extension of M. aequituberculata in Moorea, but in Okinawa, it reduced linear extension 37%; high pCO 2 had no significant effect on linear extension of P. lutea in Okinawa. Much of the negative effect of high pCO 2 on linear extension for M. aequituberculata in Okinawa was due to reduced extension in control pairings, with corals engaged in intra-and interspecific competition unaffected by OA. Linear extension of M. aequituberculata and P. lutea in interspecific pairings decreased relative to control pairings at ambient pCO 2 by 39 and 71%, respectively, indicating a strong effect of competition on extension rates. These differences however disappeared at elevated pCO 2 when the linear extension of controls was depressed. Together, our results show that OA can negatively affect the linear extension of corals not engaged in competition, as shown in the control pairings, and suggest that OA does not directly affect the ability of corals to compete with one another for space.

Spatial competition dynamics between reef corals under ocean acidification

“…Conversely, Tanner (1997) showed that growth of Acropora hyacinthus and P. damicornis was suppressed under interspecific competition (again, consistent with our study), but did not quantify effects of intraspecific competition44. The only other studies, as far as we are aware, that have compared the effects of intra- and interspecific competition found comparable growth suppression under both types of competition instead of the much greater effect of interspecific competition observed here313245. We suggest these contrasting results are due to differences in study duration; our competition trials ran for 1 year compared with 3–4 weeks in Evensen et al 31.…”

“…The only other studies, as far as we are aware, that have compared the effects of intra- and interspecific competition found comparable growth suppression under both types of competition instead of the much greater effect of interspecific competition observed here313245. We suggest these contrasting results are due to differences in study duration; our competition trials ran for 1 year compared with 3–4 weeks in Evensen et al 31. and Evensen & Edmunds (2016)32.…”

“…Additionally, space suitable for colonization by hard corals can be very low on degraded reefs (e.g., due to macroalgal dominance) despite their low coral cover30. To date, only two short-term studies have investigated the effects of OA on competitive interactions, finding that competition had little to no effect on coral growth under acidified conditions over ~3–4 weeks3132. However, long-term experiments are required to quantify competition outcomes because many corals grow slowly (particularly massive coral species known for their aggressive nature12), coral growth rates vary on seasonal cycles (e.g., ref.…”

Overgrowth of Caribbean octocorals by milleporid hydrocorals

“…) that it is unclear to what extent long‐standing rules determining the outcome of spatial competition (e.g., Lang ; Buss & Jackson ) can inform projections of future community structure (Evensen et al. ). Spatial competition involving Caribbean octocorals is particularly interesting in this regard, because the ecology of some reefs appears to favor increased octocoral abundances (Ruzicka et al.…”

Conspecific aggregations mitigate the effects of ocean acidification on calcification of the coral Pocillopora verrucosa

“…Here we extend our analyses of the effects of OA and coral-coral interactions on coral growth (Evensen et al, 2015;Evensen and Edmunds, 2016) by testing the hypothesis that colony density affects the physiology of Pocillopora verrucosa (Ellis and Solander 1786) under OA conditions. Treatment effects were evaluated from the summed physiological performance of multiple colonies placed in discrete spatial arrangements within a flume (after Vogel and LaBarbera, 1978) held at a constant flow speed.…”