Unusually high coral recruitment during the 2016 El Niño in Mo’orea, French Polynesia

Abstract

The negative implications of the thermal sensitivity of reef corals became clear with coral bleaching throughout the Caribbean in the 1980’s, and later globally, with the severe El Niño of 1998 and extensive seawater warming in 2005. These events have substantially contributed to declines in coral cover, and therefore the El Niño of 2016 raised concerns over the implications for coral reefs; on the Great Barrier Reef these concerns have been realized. A different outcome developed in Mo’orea, French Polynesia, where in situ seawater temperature from 15 March 2016 to 15 April 2016 was an average of 0.4°C above the upper 95% CI of the decadal mean temperature, and the NOAA Degree Heating Weeks (DHW) metric supported a Level 1 bleaching alert (DHW ≥ 4.0). Starting 1 September 2016 and for the rest of the year (122 d), in situ seawater temperature was an average of 0.4°C above the 95% CI of long-term values, although DHW remained at zero. Minor coral bleaching (0.2–2.6% of the coral) occurred on the outer reef (10-m and 17-m depth) in April 2016, by May 2016 it had intensified to affect 1.3–16.8% of the coral, but by August 2016, only 1.4–3.0% of the coral was bleached. Relative to the previous decade, recruitment of scleractinians to settlement tiles on the outer- (10 m) and back- (2 m) reef over 2016/17 was high, both from January 2016 to August 2016, and from August 2016 to January 2017, with increased relative abundances of pocilloporids on the outer reef, and acroporids in the back reef. The 2016 El Niño created a distinctive signature in seawater temperature for Mo’orea, but it did not cause widespread coral bleaching or mortality, rather, it was associated with high coral recruitment. While the 2016 El Niño has negatively affected other coral reefs in the Indo-Pacific, the coral communities of Mo’orea continue to show signs of resilience, thus cautioning against general statements regarding the effects of the 2015/16 El Niño on coral reefs in the region.

Density‐dependence mediates coral assemblage structure

“…This enhanced sensitivity of pocilloporid recruitment to a reduction in pocilloporid cover (cf. Bramanti and Edmunds ) is consistent with the unusual effects of the El Niño year in which the study was conducted (i.e., 2016), when the supply of pocilloporid recruits in Mo'orea was substantially enhanced compared to decadal averages from this location (Edmunds ).…”

Recovery of coral assemblages despite acute and recurrent disturbances on a South Central Pacific reef

“…Coral cover increased gradually thereafter, reaching ∼17% in 2014 (Fig. 2a ; Supplementary Table S1 ), and has since recovered to pre-disturbance levels (∼50%) at several reef slope locations in 2016 22 .

Figure 2 Impact of eight large-scale disturbances between 1991 and 2014 on ( a ) cover of corals (all 18 genera), macroalgae and turf algae, and ( b ) cover of the four dominant coral genera at 12 m depth at Tiahura reef, Moorea island.

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Implications of high rates of sexual recruitment in driving rapid reef recovery in Mo’orea, French Polynesia

“…This inference provides a mechanism by which ER for pocilloporids, and perhaps other corals, could be supported, although evidence that it has occurred – for example, through an increased frequency of resistant phenotypes – has not yet been sought. Perhaps, however, early signs of ER are the basis of acquired resistance to thermal bleaching among corals in Mo’orea 44 , 53 , as well as only a mild bleaching response by outer reef corals to the El Niño of 2016 54 . Testing for changing frequencies of coral genotypes and phenotypes across an ecological hiatus (e.g., 2010), and evaluating the trends as possible products of ER, is a critical research need for Mo’orea.…”