High Coral Recruitment Despite Coralline Algal Loss Under Extreme Environmental Conditions

Abstract

The crucial role of crustose coralline algae (CCA) in inducing hard coral larval settlement and ensuring the replenishment of coral reefs is widely accepted, and so are the negative effects of anthropogenic CO2 emissions on both CCA abundance and coral development. However, diversified and well-developed coral reef communities have been recently discovered in natural conditions where CCA and corals would not be expected to thrive. Back-reef pools, volcanic CO2 vents, mangrove estuaries, and semi-enclosed lagoons systems can present seawater pH, temperature, and dissolved oxygen values reaching or even exceeding the conditions currently predicted by the Inter Panel on Climate Change (IPCC) for 2100. In the semi-enclosed lagoon of Bouraké (New Caledonia, southwest Pacific Ocean), seawater pHT, dissolved oxygen, and temperatures regularly fluctuate with the tide reaching respectively minimum values of 7.23 pHT units, 2.28 mg O2 L-1, and maximum of 33.85°C. This study reports the effect of such extreme environmental conditions on hard coral recruitment and CCA originally settled at a forereef on artificial substrates that were transplanted over two years in two fringing reef and at the Bouraké lagoon. Our data emphasize the negative effects of the extreme conditions in our study sites on the CCA, which decreased in cover by ca. 80% and lost in the competition with turf algae, which, in turn, increased up to 162% at the end of the two years. Conversely, hard coral recruitment remained high at Bouraké throughout the study, three-fold higher than at two sites located outside Bouraké where environmental conditions were typical for coastal fringing reefs. Our findings show that while such extreme, climate change like-conditions have a direct and adverse effect on CCA abundance, and despite a certain persistence, coral larvae settlement was not affected. Based on previous findings from Bouraké, and the present observations, both coral recruits and adults seem to be unaffected despite the extreme environmental conditions. This study supports previous research illustrating how extreme natural and variable environments may reveal unexpected and positive insights on the processes underlying coral acclimatization and adaptation to global change.

Bleaching threatens positive carbonate budgets on Bahraini reefs

“…Following the bleaching event, the reef has experienced a change from CCA domination to turf algae which increased by 50% post-bleaching. This nding aligns with other studies in various locations such as Hawaii (Foo et al 2022) and New Caledonia (Tanvet et al 2022) all of which is attributed to SST warming (Cornwall et al 2019). Therefore, the high presence of CCA is evidence that this reef is in recovery but has been affected by this warming event which is impacting the recovery process (Teichert et al 2020;Roth et al 2018).…”

Physiological and morphological plasticity in Stylophora pistillata larvae from Eilat, Israel, to shallow and mesophotic light conditions

“… 92 Whilst the light intensity used in ‘shallow’ aquaria may be considered relatively low for 5 m depth at the planulae collection site (Eilat, Gulf of Aqaba), the intensity (together with the well-matched spectrum) may be considered as a simulation of a partially shaded ‘shallow’ reef which coral planulae under high light conditions typically seek. 93 , 94 , 95 , 96 , 97 Variable sample sizes were an unavoidable result of differences in the total number of larvae collected from each depth, resulting in an unbalanced design. Triplicate aquaria for were used for each light condition with planulae distributed as evenly as possible between aquaria.…”

Opinion: Missing half the picture, focus on stony corals during bleaching events has left critical knowledge gaps in our understanding of reef-scale bleaching and recovery

“…As such, understanding the dynamics of macroalgal responses to heat stress and their interac�ons with other benthic groups can help us understand not only grouplevel responses, but also make reef-scale mul�-species predic�ons. Finally, while some studies have found that coralline algae is crucial setlement media for coral larvae, and coralline algae can be killed by heat stress (Webster et al 2011;Tebben et al 2015;Tanvet et al 2022), we have litle informa�on on the dynamics of coralline algae through coral bleaching events. Understanding how sponges and algae respond to heat stress is crucial to understanding the full dynamics of heat stress on reefs and their role in inhibi�on or facilita�on of coral reef recovery.…”

“…Addi�onally, they provide food and habitat for many mobile species including specialist to non-stony coral habitat builders (Pratchet 2007;Epstein and Kingsford 2019). The majority of studies on reef impacts focus on how stony corals have been affected and o�en include other groups only when they are nega�vely impac�ng stony coral health, such as compe��on with or overgrown of corals by algae or sponges (Coll et al 1987;Lapointe et al 2007;Clements and Hay 2023), or recruitment of stony coral larvae to coralline algae (Webster et al 2011;Tebben et al 2015;Tanvet et al 2022). While it is important to understand how stony corals respond to stressors as they build the founda�on of reef structures, it is also important to understand how all habitat building species may be affected in order to disentangle reef-scale dynamics.…”