Skip to content

Marine Heatwaves Threaten Ocean Ecosystems, Undermining Marine Protected Areas

Research reveals rising number of heatwaves killing off fish, corals, and kelp, with implications for climate and economy.
Getting your Trinity Audio player ready...

Heatwaves are sweeping the seas – in a marine version of wildfires, reveals new research.

And unlike their terrestrial ‘cousins’, periods of dramatic ocean warming can surge for months – or even years.

They have led to mass mortality across the world wiping out fish, corals, kelp, algae and seagrass – with alarming implications for the planet.

Other crises include displacement events, economic declines and habitat loss. Even marine protected areas (MPAs) which limit fishing are prone to the phenomenon.

Lead author Dr. Joshua Smith said: “MPAs in California and around the world have many benefits, such as increased fish abundance, biomass and diversity.

“But they were never designed to buffer the impacts of climate change or marine heatwaves.”

The number of heatwaves affecting the seas has increased sharply – killing swathes of sea-life.

Humanity relies on the oceans for oxygen, food, storm protection and the removal of climate-warming carbon dioxide from the atmosphere.

California Sheephead. Humanity relies on the oceans for oxygen, food, storm protection and the removal of climate-warming carbon dioxide from the atmosphere. PHOTO BY CHRIS HONEYMAN/SWNS 

The analysis is based on decades of long-term ecological monitoring data from California’s diverse ocean habitats.

It spanned the largest marine heatwave on record – which rolled through the Pacific toward California from 2014-2016.

The monster marine heatwave was formed from an environmental double-whammy – unusual ocean warming nicknamed ‘The Blob’ followed by a major El Nino event that prolonged the sweltering sea temperatures.

It blanketed the West Coast from Alaska to Baja and left a wake of altered food webs, collapsed fisheries – and shifted populations of marine life among various other consequences.

The team at the University of California, Santa Barbara, synthesized over a decade of data collected from 13 MPAs located in a variety of ecosystems along the Central Coast.

They included rocky intertidal zones, kelp forests and shallow and deep rocky reefs.

The researchers looked at fish, invertebrates and seaweed populations inside and outside these areas, using data from before, during and after the heatwave.

They also focused on two of these habitats, rocky intertidal and kelp forests, at 28 MPAs across the full statewide network to gauge whether these locations promoted one particular form of climate resilience — maintaining both population and community structure.

Dr Smith, now at Monterey Bay Aquarium, said: “We used no-take MPAs as a type of comparison to see whether the protected ecological communities fared better to the marine heatwave than places where fishing occurred.”

The results published in the journal Global Change Biology were described as “somewhat sobering” – but not entirely unexpected.

Co-author Professor Jenn Caselle said: “The MPAs did not facilitate resistance or recovery across habitats or across communities.

“In the face of this unprecedented marine heatwave, communities did change dramatically in most habitats.

“But, with one exception, the changes occurred similarly both inside and outside the MPAs.

“The novelty of this study was that we saw similar results across many different habitats and taxonomic groups, from deep water to shallow reefs and from fishes to algae.”

The implications are every part of the ocean is under threat from climate change.

Dr. Smith said: “MPAs are effective in many of the ways they were designed, but our findings suggest that MPAs alone are not sufficient to buffer the effects of climate change.”

The ecological communities have not returned to their former, pre-heatwave state – shifting toward a pronounced decline in the relative proportion of cold-water species and an increase in warm-water species.

For example, increases in the abundance of the senorita fish, a subtropical species and previously rare in central California, had an outsized influence on the shift of communities. Whether these species persist in their new locations remains to be seen.

Prof. Caselle said: “This study makes it clear why long-term monitoring of California’s MPAs is so critical.

“Some of these time series are longer than 25 years at this point and the data are critical to understanding and readying human communities for the changes occurring in our marine communities.”

Continued study will show if future shifts in marine communities occur at different rates or to different base states in MPAs compared to fished areas.

The MPAs present opportunities to study the response of marine ecosystems to shifting conditions and potentially tailor management techniques accordingly.

Dr. Smith said: “The ecological communities in MPAs are still being protected, even if they are different as a result of the heatwave.

“Given that marine heatwaves are anticipated to increase in frequency and magnitude into the future, swift climate action and nature-based solutions are needed as additional pathways to enhance the health of our oceans.”

Ocean heat waves are becoming more frequent, prolonged and severe. The number of heatwave days jumped by more than 50% in the 30 years to 2016, compared with the period of 1925 to 1954.

Co-author Professor Kerry Nickols, from Cal State University Northridge, added: “With the devastating impacts of climate change already apparent, it is very important that we are upfront about climate solutions – as long as we are burning fossil fuels and warming the globe marine ecosystems will be at risk, even if they are protected from fishing.”

Produced in association with SWNS Talker

Edited by and

“What’s the latest with Florida Man?”

Get news, handpicked just for you, in your box.

Check out our free email newsletters

Recommended from our partners