Behind the paper: climate-related hypoxia events erode tropical reef habitat

In this post, we speak to Noelle Lucey and Curtis Deutsch, authors of the recent PLOS Climate article “Climate warming erodes tropical reef habitat through frequency and intensity of episodic hypoxia”, about the story behind the research.

What led you to decide on this research question?

Temperature and oxygen conditions together determine how metabolically viable (or ‘breathable’) the ocean water is for most marine animals, and thus where they can live in the ocean. This is problematic because the ocean is losing oxygen and warming up. In doing so, it’s becoming more metabolically challenging for marine life.

This metabolic ‘breathability’ can dictate where species live on large oceanic scales under long-term average temperature and oxygen levels.  But we didn’t know whether those conditions would predict habitability on smaller, more regional scales that experience high environmental variability over time, from the day-night cycle to the erratic weekly and monthly fluctuations that are the ocean’s ‘weather’. So here, we tried to determine if this variability, restricted the metabolism of tropical species, leading to changes in where they ultimately live (Is marine biogeography influenced by ocean weather causing metabolic limitations in a high biodiversity tropical coral reef?).  We were especially interested in weather events when temperature is high and oxygen is low, what we now refer to as ‘metabolic storms’.

Could you talk us through how you designed your study?

Because we wanted to characterize short-term weather conditions on a tropical coral reef ecosystem, we focused on a reef on the Caribbean coast of Panama where a well-studied natural temperature and oxygen gradient occurs. Within this gradient, we measured how variable O2 and temperature conditions are through time.

We chose to work with two brittle stars species,  relatives of the common star fish, because they are very abundant and active on these reefs, and because they are easy to work with in the laboratory. We were able to physically challenge them to ascertain how much oxygen they need when they are most active, and we were also able to do underwater surveys to determine how abundant they are on different parts of the reef.

These two aspects of the study were then combined with an eco-physiological model to identify exactly when reefs become metabolically challenged and if it’s related to which reefs these animals live in.

What challenges did you encounter during your study?

Studying the distribution of animals and infrequent extreme conditions underwater presents a number of interesting challenges related to the logistics of marine science research. This type of data is much harder to get in the marine environment compared to on land because the observer has to be underwater to find the animals and collect the data, and ocean conditions can be difficult to work in, and for loggers/equipment to survive long-term deployments.

What did you find most striking about your results? What do they tell us about future prospects for reefs and their management?

Even though short-lived episodes of extreme hypoxia, i.e. metabolic storms, occur infrequently on this reef- around 5% of the time, they still explain the biogeography of these brittle star species. How? Because the environment becomes metabolically stressful during episodes which occur primarily at night when these brittle stars are most active. During this time, the species that can’t get enough oxygen from the water to maintain its active lifestyle, relocates to cooler, more oxygenated reefs.

This is concerning because we also found that over the last decade, both the frequency and intensity of low oxygen conditions increased as temperatures increased on the reef. With continued warming, these metabolic storms are likely to get worse and limit other species ability to get enough oxygen from the water. This is expected to push more species out of their current coral reef habitats. We hope that others interested in preserving coral reef diversity will consider this mechanism on coral reefs in other parts of the world.

What further research questions need to be addressed in this area?

How widespread are these findings? Do other coastal systems experience this same relationship between oxygen extremes and warming? Does climate warming cause an increase in metabolic storms? Are subtle biogeographic shifts based on oxygen driven metabolic limitation causing unknown losses of other tropical species?

Our work underscores the need for high-resolution joint oxygen-temperature measurements in many more coastal habitats around the world. It also sheds light on the need for further research in tropical oceans. Hypoxia is generally thought of as a relatively new risk tropical marine life, and the metabolic stress it causes with the current warming trajectory has important implications for biodiversity loss.

Why did you choose PLOS Climate as a venue for your article?

We chose PLOS Climate because of its commitment to Open Access and emphasis on publishing multidisciplinary/interdisciplinary research. We believe these two components are essential for truly advancing our understanding of future climate impacts.  

Read more about the research in this press release!