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Regional and context-specific approaches can improve study of marine heat waves

Marine heat waves can create chaos in ocean ecosystems and disrupt coastal economies. In recent years, they've made headlines for driving drastic changes in fish migration and more frequent occurrences of harmful algae blooms as well as for fueling storm systems. Yet in some situations, the broad definition of a marine heat wave—ocean temperatures exceeding a seasonal threshold for five consecutive days— leaves more questions than answers.

In a new commentary in Nature Climate Change, an interdisciplinary group of researchers from Woods Hole Oceanographic Institution (WHOI) and San Diego State University (SDSU) call on scientists to be more specific in their work. Different scientific disciplines prioritize varied aspects of marine heat waves, including what drives them and the regional impacts they may have.

"Ecologists examine biological effects, such as the migration of marine life and food sources. Meanwhile, physical oceanographers analyze underlying drivers from a statistical perspective, and climate scientists seek to attribute causes," explained Svenja Ryan, a WHOI physical oceanographer. "The detection, interpretation, and impact assessments of marine heat waves are heavily influenced by methodological choices, such as defining temperature baselines and selecting data sources."

"As scientists, we often aim for standardized methodologies to ensure comparability across studies. However, the drivers of anomalously warm waters and their ecological impacts can vary by region and species," said SDSU Postdoc Nima Farchadi, a co-lead author of the study and member of the Marine Predators Group.

"So while a 'one-size-fits-all' approach to identifying and characterizing marine heat waves may seem ideal, our goal as a community should be to determine the most suitable practices for specific regional oceanographic and ecosystem dynamics."

In 2012, a long-lasting marine heat wave wreaked havoc on Maine's marine life and fishing industry. At times, water temperatures were nearly 40°F above average. The state's staple lobster fishery suffered. According to the University of Maine study on the occurrence, lobster yields weren't low, but early molting resulted in a massive price drop. Due to pronounced ocean warming off the Northeast U.S. coast since 2012, fishers more frequently find warm-water fish and squid, not usually found in the region.

"Places like the Gulf of Maine are extremely dynamic, so the use of different methods to detect marine heat waves leads to strong differences in how we might characterize these events," said Camrin Braun, a WHOI biologist and member of the Marine Predators Group. "Conversely, when you look at an area like the Gulf of Mexico, a variety of methodologies for identifying marine heat waves lead to less noticeable differences in the resulting detection of these events."

"As we see marine heat waves happening more frequently, taking a regional approach to studying them will become more important. These extreme events can result in commercially important marine species losing suitable habitat in important areas they'd ordinarily occupy reliably," explained WHOI Postdoc Laura McDonnell, the study's co-lead author and member of the Marine Predators Group.

"Scientists need to take regional and context-specific approaches to studying marine heat waves so that we can support marine organisms and the lives and livelihoods of those who depend on ocean ecosystems."