Key takeaways
- Kelp forests play an important role in healthy coastal ecosystems, but they prefer cold water and die off if the water gets too warm, such as during marine heat waves.
- UCLA geographers using 40 years of satellite data have found that kelp within marine protected areas off the California coast rebounded more quickly than other kelp after marine heat waves in 2014 and 2016.
- Fish that prey on other animals that eat immature kelp are protected in these areas and likely created a trophic cascade that helped the kelp population bounce back.
UCLA geographers using satellite data have discovered that kelp forests recover better after marine heat waves in marine protected areas than in unprotected areas. The researchers believe that the protection put in place within MPAs may support kelp against multiple environmental stressors. For example, fish species protected by the conservation zones include ones that prey on organisms, like sea urchins, that graze on kelp. These types of fish and invertebrates are less abundant outside of the protected areas where fishing is allowed. By keeping the grazers in check, the fish serve as apex predators, helping reestablish kelp populations that die back when the water gets too warm.
This effect, known as a trophic cascade, has been documented in other ecosystems where apex predators are protected. The finding is published in the Journal of Applied Ecology.
"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 first author and UCLA doctoral student Emelly Ortiz-Villa.
Kelp is a kind of algae that grows from the ocean floor on thick, round stems. It can grow up to 100 feet and features leaf-like fronds that extend as much as a foot above the ocean's surface. Kelp forests support abundant marine life, including many kinds of fish, shellfish and mammals like seals and sea otters, and play an important role in creating healthy coastal ecosystems. It grows best in 42–72 F water.
Marine heat waves happen when elevated water temperatures persist for a long time. This is usually due to fluctuations in ocean currents caused by natural phenomena such as El Niño or La Niña. However, climate change is causing higher overall ocean temperatures, which are bad for kelp and make it even more vulnerable to heat waves.
Marine protected areas are ocean regions managed for the long term conservation of marine resources, ecosystems or cultural heritage. Fishing and other types of human disturbances are limited in these areas, though the extent of the restrictions can vary. Most of California's marine protected areas were established in the early 2000s.
The UCLA researchers were able to distinguish kelp patches in high-resolution images of the entire California coastline taken by Landsat satellites since 1984. Landsat is a joint NASA/USGS program that provides the longest continuous record of satellite imagery of the Earth's surface.
"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," said Ortiz-Villa.
The rebound effect was most pronounced in Southern California, where sea urchins that eat kelp are kept in check by California sheephead fish and California spiny lobsters, which may not be fished in marine protected areas. These species are absent from central and northern California, where sea stars and sea otters are the main predators of sea urchins. Sea stars have been decimated by a wasting disease, leading to the proliferation of sea urchins and consequent damage to kelp forests in these regions. Sea otters are not fished and therefore aren't affected by the fishing restrictions of marine protected areas.
Based on this information, the researchers concluded that the greater abundance of the predators of kelp grazers in marine protected areas causes reduced grazing pressure that helps kelp recover more quickly after a die-off.
"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 co-author Kyle Cavanaugh, who is a UCLA professor of geography and at the UCLA Institute of the Environment and Sustainability.
The research highlights the usefulness of setting aside restricted marine areas as a tool for helping ecosystems stay healthy as climate change pushes global ocean temperatures upward.
"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," said Cavanaugh.
