Study reveals how kelp forests persisted during the Great Pacific Marine Heatwave of 2014-2016

New research from the Monterey Bay Aquarium and the University of California, Santa Cruz reveals that denser, more sheltered kelp forests can withstand severe stressors amid warming ocean temperatures. Published in Proceedings of the Royal Society BThe study also offers the first comprehensive assessment of how the decline in kelp abundance has affected marine algae, invertebrates and fish living in Monterey Bay.

The study comes after a years-long marine heatwave – the product of a 2014 warm water “drop” extended by a 2015-2016 El Niño event – ​​lapped America’s west coast of the North with stifling sea temperatures.

It all started a decade ago, when a triple stressor – a major marine heatwave, starfish die-offs and a sea urchin outbreak – led to a sharp decline in kelp abundance on the central coast of California.

Using a 14-year dataset, the researchers found that these events caused an average 51% drop in kelp forest density in the years following the heatwave (2017-2020). compared to 2007-2013). By 2020, the decline had increased to 72%. Some kelp forests, however, survived these extreme events.

“We found that larger populations of giant kelp prevented changes in sea urchin feeding behavior, and that these evergreen forests were more resilient to multiple stressors,” said Dr. Joshua Smith, lead author of the study. and Ocean Conservation Research Scientist at the Monterey Bay Aquarium.

“What surprised us was that the evergreen kelp forests were located in generally less productive areas. These evergreen forests had a gradual reef slope and protection from wave exposure, allowing them to grow densely populated with kelp before the marine heatwave.”

Although this study identified the importance of habitat characteristics in forest persistence, predators may also aid kelp. Another recent study from the Monterey Bay Aquarium reinforced the role sea otters play in preventing kelp declines by eating sea urchins.

Where kelp patches were more exposed and less dense, the sudden rise of urchins in 2014 led many kelp forests to become “barren zones,” areas roamed by hungry urchins along the rocky reef. Smith and his co-authors examined changes in species composition in the barren and evergreen forest mosaic to understand the impact of kelp loss on the ecosystem as a whole.

“Although some kelp forests have persisted, the structure of ecological communities in drylands has not yet returned to its pre-2013 state,” Smith added.

“Across the region, the number of species has not declined, but changes in their relative abundances have led to an overall decline in species diversity, particularly for marine algae and kelp-associated invertebrates.” With less kelp and other macroalgae, the study noted a disproportionate increase in the number of animals that eat plankton, such as barnacles, scallops, tube snails and planktivorous fish.

The study aimed to understand the effects and consequences of the marine heatwave and is part of the Monterey Bay Aquarium’s larger efforts to understand the mechanisms of kelp recovery and restoration. Globally, kelp has been declining for half a century, at an average rate of 1.8% per year.

With more than half of the ocean surface experiencing extreme marine heat each year since 2014, warming ocean temperatures present a serious threat to cold-water species like kelp. As kelp forests and other marine ecosystems around the world face increasing threats from climate change, conserving ecosystems, like kelp forests, and predators, like sea otters, can mitigate the impacts of extreme events.

“This study offers important information to help inform strategies for protecting areas where kelp may persist alone and choosing sites for kelp restoration efforts, here in California and around the world,” said Dr. Pete Raimondi, a marine ecologist at UC Santa Cruz. who was not a co-author of the study.