Climate Change

Coming in hot! Marine heatwaves impact sea turtle nesting on Diego Garcia Island

Reviewing: Hays, G. C., Chivers, W. J., Laloë, J. O., Sheppard, C., & Esteban, N. (2021). Impact of marine heatwaves for sea turtle nest temperatures. Biology Letters17(5), 20210038.

What are marine heatwaves?

You have probably heard of a heatwave on land, especially during the summer, where there is a long period of unusually high temperatures. Similarly, a marine heatwave (MHW) is an extended period of extremely high temperatures (temperatures higher than the normal or mean for that time of the year) in a sea or ocean that can last for weeks. It is caused by various factors, including ocean currents which can build up areas of warm water or warming of the ocean surface from increased atmospheric temperatures. MHWs can occur both in the summer and the winter, and they can affect small areas of coastline or span multiple oceans. Impacts have been reported even in remote and relatively pristine areas far from human pressures. Wherever they occur, MHWs threaten marine biodiversity and ecosystems and negatively impact the fisheries, aquaculture, and tourism industries. In 2016, for instance, MHWs across northern Australia caused severe bleaching of the Great Barrier Reef and led to high levels of abalone mortality in Tasmania. The impacts have been well documented for coastal ecosystems like coral reefs, seagrasses, and kelp forests. However, it is not yet fully understood how such impacts extend to adjacent terrestrial systems like sandy beaches, which are important nesting sites for sea turtles.

Sea turtle’s nesting and gender: temperature matters

In most species, gender is determined during fertilization, but in most sea turtles, gender is determined after fertilization. The temperature of the developing egg during nesting determines whether the sea turtle will be male or female. Research has shown that if a turtle’s egg incubates below 26°C, there is a 99% chance that the turtle hatchling will be male while temperatures above 32°C will produce female hatchlings. At a temperature of 29°C, there is a 50:50 chance that the turtle hatchling will be male or female. With warmer temperatures worldwide, there are concerns that warming may result in a higher percentage of female sea turtles and ultimately lead to population decline. This study looked at the effects of a 2015/2016 marine heatwave event in the Chagos Archipelago (Fig. 1) on sea turtle incubation conditions and hence hatchling survival and sex ratios. The Chagos Archipelago, a remote island group in the Indian Ocean, is known to harbor some of the most pristine coral reefs in the world. The archipelago experienced a major coral bleaching event due to the heatwave. Beach and water temperatures were recorded before, during, and after the coral-bleaching event. This information was used to investigate the possible effects on sea turtle nesting. Long-term temperature records were also used to determine the likely impacts of the increasing frequency of marine heatwaves for sea turtle nests.

Figure 1. Map of the Indian Ocean, zooming in on the Chagos Archipelago and Diego Garcia Island.

How did the 2016 MHW affect sea turtle nesting?

Figure 2. (a) A Hawksbill turtle and a (b) green turtle. Images from creative commons.

The highest nesting density is observed on the southern coast of the island. Hence temperature loggers measured sand temperature in the south every 4 hours at the depths preferred by hawksbill (Fig. 2a) and green (Fig. 2b) sea turtles to nest. A total of 52 temperature loggers were deployed between October 2012 and August 2019. At this site, Hawksbill turtles nest during October-February, while green turtles nest year-round with higher nesting activity during June-October.

Figure 3. The predicted (a) hatchling sex ratio and (b) hatchling success on Diego Garcia. In both cases, extreme values were predicted for March 2016, which was the warmest month in the time series. Reproduced from Hays et al. (2021).

The patterns in the sand temperature matched the patterns observed in the air and water temperatures around Diego Garcia. This shows that the warm air and water conditions across the region were reflected in the sand on nesting beaches. Further analyses showed that a 1°C rise in seawater temperatures translated into a 1.07°C rise in sand temperatures, which means that marine heatwaves would translate into warmer conditions for turtle nesting. When the hatchling sex ratios and hatchling survival rate were modeled against mean monthly sand temperatures, a seasonal pattern was observed. 10-20% of female sea turtles hatched in the coolest months (July and August), and 80-90% of female sea turtles hatched in the warmest months (February-March). The highest female-biased sex ratio and the lowest hatchling survival rate was observed for March 2016, the warmest month in the multiyear time series (Fig. 3)

The future of marine heatwaves

Overall, these results highlight that marine heatwaves impact not only marine ecosystems but also adjacent terrestrial ecosystems like sandy beaches. However, the magnitude of the impact may differ. Globally, a higher percentage of female hatchlings dominate because the nesting temperatures are already higher than the average temperature of 29°C. This implies that in regions where nesting temperatures are already high, marine heatwaves are likely to be dire, reducing male production and increasing hatchling death. Given these concerns, it is important to consider how rising nest temperatures can be mitigated.

The Intergovernmental Panel on Climate Change (IPCC) predicts that the global ocean will continue to warm and that marine heatwaves have increased by 50% over the past decade, becoming more frequent and more severe. As marine heatwaves become more frequent, they push ecosystems beyond their resilience and cause extreme weather events such as tropical storms. Given the severe and long-lasting impacts of MHWs on marine life and human society, the best that can be done is significantly slowing down ocean warming and raising awareness of its effects across researchers, policymakers, conservationists, and the general public. By improving our scientific understanding of marine heatwaves and their ecological and economic impacts, we can better predict future conditions and protect marine and terrestrial habitats and resources that are most at risk. Building ocean resilience is key to limiting ecological and economic losses linked to marine heatwaves.

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