Impact of diurnal temperature fluctuations on larval settlement and growth of the reef coral &amp;lt;i&amp;gt;Pocillopora damicornis&amp;lt;/i&amp;gt;

Lei Jiang,Pei-Yuan Qian,Xiu-Bao Li,Lin Cai,You-Fang Sun,Xin-Ming Lei,Sheng Liu,Laurence J Mccook,Jian-Sheng Lian,Yu-Yang Zhang,Hui Huang,Guo-Wei Zhou

doi:10.5194/bg-14-5741-2017

Abstract

Abstract. Diurnal fluctuations in seawater temperature are ubiquitous on tropical reef flats. However, the effects of such dynamic temperature variations on the early stages of corals are poorly understood. In this study, we investigated the responses of larvae and new recruits of Pocillopora damicornis to two constant temperature treatments (29 and 31 °C) and two diurnally fluctuating treatments (28–31 and 30–33 °C with daily means of 29 and 31 °C, respectively) simulating the 3 °C diel oscillations at 3 m depth on the Luhuitou fringing reef (Sanya, China). Results showed that the thermal stress on settlement at 31 °C was almost negated by the fluctuating treatment. Further, neither elevated temperature nor temperature fluctuations caused bleaching responses in recruits, while the maximum excitation pressure over photosystem II (PSII) was reduced under fluctuating temperatures. Although early growth and development were highly stimulated at 31 °C, oscillations of 3 °C had little effects on budding and lateral growth at either mean temperature. Nevertheless, daytime encounters with the maximum temperature of 33 °C in fluctuating 31 °C elicited a notable reduction in calcification compared to constant 31 °C. These results underscore the complexity of the effects caused by diel temperature fluctuations on early stages of corals and suggest that ecologically relevant temperature variability could buffer warming stress on larval settlement and dampen the positive effects of increased temperatures on coral growth.

Highlights

Scleractinian corals and the reef ecosystems they construct are currently facing environmental changes at unprecedented rates of changes
This study was the first to examine the effects of both increased temperature and daily temperature variability on the early stages of a reef coral
We found that realistic diurnal temperature fluctuations considerably tempered thermal stress on larval settlement and had varied effects on the physiology and early development of P. damicornis

Summary

Introduction

Scleractinian corals and the reef ecosystems they construct are currently facing environmental changes at unprecedented rates of changes. Of these changes, rising seawater temperature is generally recognized as one of the most immediate and widespread threats (Hoegh-Guldberg, 1999; Hughes et al, 2003). The most conspicuous response of corals to elevated temperatures is to expel their endosymbiotic dinoflagellates and/or photosynthetic pigments, giving the affected colonies a pale appearance, a process known as coral bleaching (Hoegh-Guldberg, 1999). Sea surface temperatures have increased by approximately 0.7 ◦C since preindustrial times (Feely et al, 2013), and a further increase of 2–3 ◦C is expected by the end of this century (Bopp et al, 2013), giving rise to increased concerns about effects on corals.

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