Abstract
In 2016, global temperatures were the highest on record, and mass coral bleaching occurred world-wide. However, around Sesoko Island, Okinawa, southwestern Japan, the heat stress assessed by degree heating week (DHW) based on local temperature measurements was moderate in 2016; in 1998, DHW was three times higher than in 2016 (10.6 vs. 3.3 in September in respective years). On a reef flat of Sesoko Island where the effect of severe coral bleaching on coral assemblage was monitored in 1998, significant coral bleaching occurred in 2016. Bleaching of the heat stress sensitive Acropora corals began in July 2016 on the reef flat as seawater temperature rose. We observed the bleaching and post-bleaching mortality status of individual colonies of Acropora spp. in 2016 in fixed plots on the reef flat. In total, 123 Acropora colonies were followed for six months after seawater temperature became normal by multiple surveys. At the beginning of September 2016, 99.2% of colonies, were either completely (92.7%) or partially (6.5%) bleached. Of those, the dominant species or species groups were A. gemmifera (Ag), A. digitifera (Ad), and tabular Acropora (tA). For all Acropora colonies, the overall whole and partial mortality was 41.5% and 11.4%, respectively. Whole mortality rate differed significantly among species; 72.5%, 17.9%, and 27.8% in Ag, Ad, and tA, respectively. Mortality rates at the end of the surveys were similar in smaller (≤10 cm in diameter) and larger Ag, but the former suffered mortality earlier than the latter. Higher survival of smaller colonies was observed only in tA (100%), which may be associated with large morphological differences between smaller and larger colonies. Some of the dominant Acropora colonies had survived without partial mortality including 15.0% survival of the most vulnerable Ag at the end of the surveys. These results suggest that moderate heat stress may have a potential for selecting heat-tolerant genotypes. A longer period of mortality lasting for six months, was observed in Ag in addition to immediate whole mortality after bleaching, due to the continuous loss of living tissue by partial mortality. This highlights the need for multiple surveys at least during several months to accurately assess the impact of thermal stress event to corals. In contrast to DHW based on local measurements, DHW obtained from satellite data were similar between 1998 and 2016. Although satellite-based measurement of sea surface temperature is very useful to reveal variations in heat stress at a large spatial scale, temperature should be measured on site when variations at smaller spatial scales are of interest.
Highlights
Coral reefs develop in warm shallow seas, and support the most biodiverse communities in shallow marine ecosystems (e.g., Paulay, 1997)
We focus on the heat-stress sensitive Acropora spp., because Acropora corals are potentially the most dominant corals in terms of abundance and percentage cover in many Okinawan reefs, including the present study site (Japanese Ministry of the Environment & Japanese Coral Reef Society, 2004), and contain high species diversity within the genus (Wallace, 1999)
We report the effect of moderate heat stress on Acropora corals, which were found to be sensitive to heat stress on a reef flat in Sesoko Island where the mortality of corals after a mass-bleaching event was monitored in 1998 (Loya et al, 2001)
Summary
Coral reefs develop in warm shallow seas, and support the most biodiverse communities in shallow marine ecosystems (e.g., Paulay, 1997). The corals play a core role in coral reef ecosystems by providing habitats (e.g., Knowlton, 2001) and food sources for other organisms (e.g., Wild et al, 2004). The center of the distribution of corals is in warm seas, such as those in the tropics, corals are vulnerable to heat stress. When sea surface temperatures (SSTs) exceed the maximum values in ordinary years by 1 ◦C in the area where the corals live, the corals may lose their zooxanthellae and bleach (Hoegh-Guldberg, 1999). Bleached corals do not always die, but certain species or genotypes might suffer whole or partial mortality that could allow them to recover by regrowth (Gilmour et al, 2013)
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