UCLA researchers have found that kelp forests located within marine protected areas (MPAs) along the California coast recover more quickly from marine heat waves than those outside such zones. The study, published in the Journal of Applied Ecology, used 40 years of satellite data to track changes in kelp populations after significant warming events in 2014 and 2016.
Kelp forests are important for coastal ecosystems but are sensitive to warm water. Marine heat waves, which occur when ocean temperatures remain high for extended periods due to natural events like El Niño or La Niña and are intensified by climate change, can cause widespread die-offs of kelp.
The research team, led by UCLA doctoral student Emelly Ortiz-Villa, observed a clear difference between protected and unprotected areas. “We were initially using satellite data to study how marine protected area implementation affects the kelp population in general, but after looking at the data over time, we were surprised to see a big difference in how kelp populations in and outside of protected areas rebounded after marine heat waves in 2014 and 2016,” said Ortiz-Villa.
MPAs restrict fishing and other human activities to conserve marine resources. Most of California’s MPAs were established in the early 2000s. Within these zones, fish species that prey on organisms like sea urchins—which feed on young kelp—are more abundant because they are not fished. This creates what scientists call a trophic cascade: predators keep grazers under control, allowing kelp forests to recover more effectively after environmental stressors.
The rebound effect was especially strong in Southern California. There, California sheephead fish and spiny lobsters—both protected within MPAs—help control sea urchin populations. In central and northern California, where these predators are absent and sea stars have declined due to disease, unchecked sea urchin numbers have led to greater damage to kelp forests.
“We didn’t see a big increase in kelp forests immediately after the implementation of marine protected areas, but after something very severe, like a marine heat wave, we did see greater recovery in the protected area,” Ortiz-Villa said.
Co-author Kyle Cavanaugh emphasized that while MPAs cannot directly address ocean warming or prevent heat waves themselves, they can still support ecosystem resilience by reducing other pressures on kelp. “Marine protected areas typically cannot directly mitigate ocean warming or marine heat waves. However, this study shows that they can still enhance the resilience of kelp forests to climate change by reducing other stressors,” said Cavanaugh.
The findings suggest that spatial protection measures could be an effective strategy for helping vulnerable ecosystems cope with rising ocean temperatures driven by climate change. “These findings can inform decisions about where to establish new MPAs or implement other spatial protection measures. MPAs will be most effective when located in areas that are inherently more resilient to ocean warming, such as regions with localized upwelling or kelp populations with higher thermal tolerance,” Cavanaugh added.
