Physiological responses of a South African high-latitude coral community to global warming.

Abstract

The health of the world’s coral reefs is deteriorating rapidly due to global climate change and increasing localised anthropogenic stressors. The substantial benefits resulting from coral reef ecosystems, economically and ecologically, requires that research be conducted on their responses to rising sea temperatures driven by climate change. Millions of people depend on the natural resources that coral reefs provide, whether for food or eco-tourism, trade and other indirect sources of income. Although South African coral reef communities lie within a long-established marine protected area, the iSimangaliso Wetland Park World Heritage Site, this status does not preclude them from being affected by the potential effects of global warming. Therefore, the objective of this study was to quantify several physiological parameters, including net community calcification (total alkalinity), growth (dimension and weight) and photosynthetic efficiency in the hard coral Acropora appressa, the soft coral Sinularia brassica and ‘live rock’ under local historical-average (24.4°C), future (26.9°C) and bleaching-threshold (28.8°C) temperatures indicative of climate change conditions projected at Sodwana Bay.

Corals and live rock were exposed to the three different temperature treatments during a 10-week long mesocosm experiment that consisted of three phases: the initial phase during which temperatures were increased from 24.4°C over four weeks to reach setpoints of 26.9°C (future) and 28.8°C (bleaching threshold), respectively; the middle phase during which temperatures were held stable at each treatment’s setpoint for the proceeding four weeks; and the final phase during which a further 1°C increase was done over two weeks in the bleaching-threshold treatment to simulate an extreme warming scenario.

An initial increase in size was evident in both coral species exposed to the historical-average control temperature and the future temperature projected for Sodwana Bay in 2100 by the representative concentration pathway (RCP) 4.5 climate change scenario. Although the growth trends of both species persisted in the control treatment, the overall linear growth of A. appressa was lower under the RCP 4.5 climate change temperature scenario and bleaching-threshold temperature relative to the control temperature. While no significant treatment effects were found, a decrease in the linear extension of A. appressa was evident at the end of the experiment at the bleaching-threshold temperature relative to the control temperature. Continuous growth trends were evident in the control and RCP 4.5 climate change scenario for S. brassica, however a reduction in diameter after 5 weeks was apparent in the bleaching-threshold treatment. A gradual increase in buoyant weight of A. appressa was evident across all treatments and experimental phases, with a slower growth rate only apparent towards the end of the experiment in the bleaching-threshold treatment. The buoyant weight of S. brassica decreased up until the start of the middle phase in the control treatment and RCP 4.5 treatment. However, an increasing trend in the weight of S. brassica was measured in the same two treatments from the end of the middle phase until the experiment concluded. Contrastingly, the weight of S. brassica in the bleaching-threshold treatment continued to decrease throughout the course of the experiment. Pulse-amplitude modulated fluorometry measurements of the photosynthetic efficiency of both A. appressa and S. brassica were lower under the temperature conditions projected by the RCP 4.5 scenario and by the bleaching-threshold temperature, relative to the historical-average control temperature. Contrastingly, live rock showed no significant differences in photosynthetic efficiency among the different temperature treatments. On average, total alkalinity levels were higher under future temperature conditions projected by the RCP 4.5 temperature scenario and by the bleaching-threshold temperature, relative to the control temperature, indicating suppressed net community calcification. Suppressed net community calcification was particularly evident during (week 6) and at the end (week 8) of the middle phase of the experiment.

The experiment revealed that exposure to temperatures equivalent to those projected by the RCP 4.5 climate change scenario in 2100 and the local bleaching threshold are likely to be deleterious to high-latitude corals and coral reef communities in South Africa: buoyant weight and dimension, as well as photosynthetic efficiency were negatively affected in both species of coral and net community calcification was supressed under the two future climate scenarios of warming. Due to the location of Sodwana Bay reefs, the results indicate that calcification processes will be an essential physiological response to consider under global warming conditions. However, as high-latitude reef areas generally fared better during recent bleaching conditions, these reefs can be utilised to improve climate-change projection models. Such model improvements can guide climate policymakers in enhanced conservation efforts that will further stakeholder engagement and outreach. Accordingly, urgent action is needed to reduce greenhouse gas emissions to minimise the effects of global warming on coral reef communities as much as possible. Such efforts will further help to attain the 2°C Paris Climate Agreement and improve socioeconomic development for the management of reefs.