Compound Extreme Events Induce Rapid Mortality in a Tropical Sea Urchin

The frequency, magnitude, and duration of marine heatwaves and deoxygenation events are increasing globally. Recent research suggests that their co-occurrence is more common than previously thought and that their combination can have rapid, dire biological impacts. We used the sea urchin Echinometra lucunter to determine whether mortality occurs faster when deoxygenation events are combined with extreme heating (compound events), compared to deoxygenation events alone. We also tested whether prior exposure to local heatwave conditions accentuates the impacts of compound events. Animals were first exposed for five days to either ambient temperature (28 7 degrees C) or a warmer temperature that met the minimum criteria for a local heatwave (30.5 7 degrees C). Animals were then exposed to hypoxia, defined as oxygen levels 35% below their average critical oxygen limit, combined with ambient or extreme field temperatures (28 7 degrees C, 32 7 degrees C). Subsets of animals were removed from the hypoxic treatments every 3 hours for 24 hours to determine how long they could survive. Prior exposure to heatwave conditions did not help or hinder survival under hypoxic conditions, and animals exposed to hypoxia under ambient temperatures experienced little mortality. However, when hypoxia was coupled with extreme temperatures (32 7 degrees C), 55% of the animals died within 24 hours. On the reefs at our Panama study site, we found that extreme hypoxic conditions only ever occurred during marine heatwave events, with four compound events occurring in 2018. These results show that short durations (similar to 1 day) of compound events can be catastrophic and that increases in their duration will severely threaten sea urchin populations.