How Marine Heatwaves are Changing Ocean Ecosystems
What we learned from a massive seabird die-off in the north Pacific
The nutrient-rich northern oceans of Alaska support an abundance of life. Nutrients promote phytoplankton growth that ultimately supports the entire marine food web. Seabirds, both resident and migrants, take advantage of the abundant, energy-rich food to feed their young or support adult migration. Cold-adapted northern species also depend on this energy-rich food to make it through the harsh winter. But the ocean is changing and recent spikes in the ocean temperature during marine heatwaves are causing ripple effects throughout the marine ecosystem.
The following story is based on a published article by John Piatt at the U.S. Geological Survey Alaska Science Center in collaboration with coauthors. The work presented here is the result of partnerships across the U.S. Geological Survey, U.S. Fish and Wildlife Service, State of Alaska Department of Fish and Game, the National Park Service, the National Oceanic and Atmospheric Administration, Kawerak, the Aleut Community of St. Paul Ecosystem Conservation Office, and university and nonprofit researchers and citizen scientists through the Coastal Observation and Seabird Survey Team (COASST).
The North Pacific is Getting Warmer
Just as air temperatures around the globe are warming, so too, are ocean temperatures. Historically, El Niño Southern Oscillation (ENSO) events have resulted in warmer-than-normal coastal waters from California to Peru, but more recently, a new warming phenomenon has been identified across the world's oceans. Marine heatwaves are created by a regional build-up of warm water.
Marine heatwaves are prolonged periods of time when ocean temperatures are much warmer than usual. An extreme marine heatwave in the north Pacific Ocean began in late 2013 and lasted through the winter of 2016, with the most intense period from August 2014 to July 2016. It was the largest heatwave on record, a record that spans more than 150 years, due to: its spatial extent (the large area impacted: from southern California to the Gulf of Alaska), magnitude (how unusually warm it was: 3-6 degrees Celsius or between 5 and 11 degrees Fahrenheit above normal), and duration (how long it lasted).
Northeast Pacific Ocean sea surface temperatures (left) and sea surface temperature anomalies (right; how much sea surface temperatures are different than normal) from January 2015 to February 2016. Notice on the right how unusually warm water (in red) builds along the coast of North America, especially around Alaska and Baja California, Mexico. [Sea Surface Temperature & Sea Surface Temperature Anomaly - Audio Described, NASA]
While marine heatwaves have occurred in the past, they have become more prevalent and intense over the last century. In the northeast Pacific Ocean, the 2014-2016 heatwave is linked to an estimated 0.5-1 million common murres dying of starvation in the Gulf of Alaska and hundreds of thousands more in the California Current. Several other species of seabirds experienced large die-offs in the region of the northeast Pacific Ocean during this time, too. The seabird die-offs were one very noticeable result, but this ocean warming event sent shock waves through the entire north Pacific ecosystem. Every level of the marine food chain was altered.
Seabird Die-offs Have Happened Before, but Never Like This
About three quarters of all US seabirds nest in Alaska. That equates to approximately 150 million seabirds, across 38 species in 1,300 colonies—some individual colonies have over a million birds. They are here for the abundant food sources of zooplankton and small fish that thrive in the rich, cold waters. This rich food supply provides nourishment for chicks and builds strength for the long migrations, both of which take a lot of energy. But the warming ocean created widespread ecosystem changes that led to a crash.
The 2014-2016 marine heatwave resulted in a wide-spread common murre die-off and repeated nesting failure of breeding colonies from the southern Bering Sea to the south coast of California. At least 500,000 and perhaps as many as 1 million seabirds, most of them murres, died in the Gulf of Alaska and all along the west coast of the US.
Seabird die-off events are not unusual. Die-offs happen on an irregular basis throughout the world's oceans when food supplies are depleted or otherwise unavailable. But what made this die-off globally unprecedented was the extreme extent—the large geographic area that was impacted, how long it lasted, and the vast number of dead birds.
To put this die-off event into context, it was:
- Two to three times the number of seabirds that died in the Exxon Valdez oil spill of 1989.
- Two-and-one-half to five times the number of dead seabirds from the marine heatwave in the Tasman Sea off the west coast of New Zealand in 2011.
- Eight times the 1993 Gulf of Alaska murre die-off.
- Ten times the number of birds killed in a severe storm in the southeast Bering Sea in 1970.
Each murre represents 25,000 seabird deaths. Black murres represent lower number estimates and black and grey birds combined represent the upper number of estimates.
Seabirds are good indicators of marine ecosystem health because they spend most of their life on the ocean and depend entirely on it for food. Murres primarily eat forage fish (small, cold-water fish that are important in diets of larger fish and other marine animals as well, including many other seabirds). Common murres are deep divers, foraging for prey from the surface to the depth of the coastal shelf, more than 650 feet below. Across their range, they feed on a wide variety of prey: sand lance, capelin and other smelt, Pacific herring, Pacific sardine, northern anchovy, and krill. Because colonies are well situated in rich food areas, and murres are incredibly well adapted for foraging in the deep waters of the continental shelf, breeding failures and die-offs have historically been rare events. Until now.
Murres and other seabirds live in exposed, cold environments and must maintain high metabolic rates to thrive and reproduce. Common murres eat over 50% of their body mass every day, which equates to 60-120 fish. Ideally, they eat high-quality, lipid-rich prey—fatty and energy-rich species like herring or smelt. If their food source isn’t as energy-rich or they can't find enough fish, murres can’t meet their metabolic demands and quickly lose weight. If they can’t find any food for 3-5 days, they will die of starvation.
Robb Kaler discusses how common murres are highly capable dive feeders. [Audio Described - Robb Kaler discusses common murres]
“The die-off foretold a major disruption of energy flow through marine food webs and preceded alarming declines during 2016-2019 in reproductive capacity and population size of murres, other seabirds, commercial fish, and great whales.”
—John Piatt, U.S. Geological Survey
The North Pacific Low-fat Diet: How We Went from Energy Bars to Rice Cakes
The common murre die-off occurred because the birds starved to death. They starved because their food source changed—prey became less available and what was available was smaller, leaner, and less nutritious than usual. This pattern was repeated throughout the food web.
“They literally didn’t have enough to eat and became weak to the point of death.”
—Julia Parrish, University of Washington
Plankton Were Smaller and Less Nutritious
Plankton, tiny aquatic plants and animals, form the base of ocean food webs, supplying 90% of primary production. Everything from small fish to whales rely on plankton either directly or indirectly. But in 2014, phytoplankton biomass (microscopic plants) hit its lowest level since 1997 in the transition zone of the north Pacific Ocean.
Zooplankton (microscopic animals), such as copepods, were smaller and less nutritious during the heatwave. Copepods can vary in their size and lipid (fat) content. For example, warm-water copepods are generally smaller and lipid-poor (less fatty) than the larger, lipid-rich (fattier) copepods generally found in cold water. During the heatwave, there was a shift to more small, lipid-poor copepods and fewer large, lipid-rich copepods. So planktivorous seabirds (those that rely primarily on plankton), such as auklets and shearwaters, also experienced large die-offs in warmer ocean waters.
Some of the starved seabirds also had detectable levels of saxitoxin in their bodies. Saxitoxin is a poison produced by algae in blooms sometimes known as red tide that causes paralytic shellfish poisoning. Collectively, poisons produced by algae are known as phytotoxins. Warmer ocean waters are linked to the development of phytotoxins in harmful algal blooms. Harmful algal blooms were relatively rare in the cold northern ocean until the recent marine heatwaves. Now there is widespread evidence of them throughout the food chain, but little is known about the level of phytotoxins that species can tolerate.
Small Fish Declined as the Demand for them Increased
As early as 2014, scientists noted declines in forage fish. Fish species such as capelin experienced a sharp population decline and sand lance numbers were also low. Eulachon (pictured here), a kind of smelt also known as hooligan or candlefish, is one example of a forage fish.
In addition to reduced numbers of forage fish, a change in fish body condition was also noted. Sand lance were smaller and in poorer condition due to lack of nutrients and metabolic stress.
There was likely more competition for these forage fish, too. Both seabirds and larger fish needed the smaller forage fish for food. When the water warmed, the fish warmed with it. A warmer fish is a more active fish—and a hungrier one. The increase in metabolism of large fish predators, like pollock, cod, and salmon put even more pressure on declining forage fish stocks. In the intensifying battle for food between large, predatory fish and murres, murres lost.
“Fish-eating birds, such as puffins, are going from Clif Bars to rice cakes. They have to work much harder to get the same energy content. And this is happening over thousands of square miles of ocean…”
—Julia Parrish, University of Washington
Marine Mammals also Suffered
Associated with the marine heatwaves were marine mammal unusual mortality events, defined as a significant die-off of any marine mammal population as described under the Marine Mammal Protection Act. Hundreds to thousands of sea lion pups died in 2015 off the California coast and fur seals died in large numbers and experienced reproductive failures in the Pacific. A record number of humpback and fin whales stranded in 2015-2016 along the Alaska and British Columbia coasts. Humpback whales showed evidence of malnutrition in Glacier Bay National Park and Preserve between 2014 and 2017 (Gabriele et al. 2017). The timing and location of these events coincided with warm-water conditions and forage depletion, further demonstrating the far-reaching impacts of the marine heatwave through all levels of the food chain (see more information on the NOAA 2020; list of Active and Closed Unusual Mortality Events ).
Marine Mammal Unusual Mortality Events
Between May 22 and December 31, 2015 in the Gulf of Alaska and between April 23, 2015 and April 16, 2016 in British Columbia, large whales experienced unusual mortality events (UMEs). A total of 46 whales were found dead: 34 in Alaska (12 fin whales and 22 humpback whales) and 12 in British Columbia (5 fin whales and 7 humpbacks).
Fin whales are second only in size to blue whales as the largest on the planet. Fin whales and humpback whales were impacted by the unusual mortality event.
Beginning in 2015, strandings of Guadalupe fur seals have occurred at a rate eight times the historic average. A higher-than-normal rate of stranding continues in California, Washington, and Oregon.
A Guadalupe fur seal pup, coast of California.
What are strandings? Marine mammals are considered stranded when they are found dead (on land or in the water) or are alive on land unable to return to the water. Learn more about marine wildlife stranding .
Since June 2018, occurrences of stranded ice seals have increased in the Bering and Chukchi seas. As of June 2020, a total of 278 seals (included bearded, ringed, and spotted) have been documented.
Spotted seal and her pup in the Bering Sea.
Since January 2019, throughout the Pacific coast from Mexico to Alaska, there has been an increase in gray whale strandings. As of June 2020, 125 strandings have been documented.
A gray whale and calf.
Seabird Mortality Events
The effects of extreme warming weren't confined to the marine heatwave of 2014-2016. Seabird die-offs in Alaska have occurred every year since 2015, impacting a variety of species in the Bering and Chukchi seas.
In 2013, crested auklets and murres died from avian cholera on St Lawrence Island.
A crested auklet and two whiskered auklets.
Cassin's auklets from Central California to British Columbia experienced a die-off at the beginning of the marine heatwave in late 2014 and early 2015.
Cassin's auklet.
Common murres in Alaska were the hardest hit by the marine heatwave in 2015-2016.
Common murre.
Rhinoceros auklets died from disease in 2016 in northern Washington.
Rhinoceros auklet.
Tufted puffins and some crested auklets experienced a die-off in 2016 linked to multiple environmental stressors.
Tufted puffins.
In 2017, northern fulmars and shearwaters were impacted in a die-off in the Bering and Chukchi seas.
Northern fulmar.
Again in 2018, a seabird die-off in the northern seas impacted murres, auklets, and other species.
Red-legged kittiwakes were impacted by the 2018 die-off in the Bering Sea.
In 2019, the seabird die-off consisted primarily of short-tailed shearwaters and extended from the southern Bering Sea to the Chukchi Sea.
Short-tailed shearwater.
Seabird Mortality Events, 2013-2019
The information presented here is a summarized view of mortality events documented by the Coastal Observation and Seabird Survey Team (COASST) and others. The coverage extends from northern California to Washington and throughout Alaska.
Temperature maps are derived from the National Oceanic and Atmospheric Administration sea-surface temperature high-resolution dataset and represent the average sea surface temperature anomaly (SSTa) across the years 2013-2019. Graphics created by COASST.
A Window to the Future
Persistent shifts in ocean temperature could result in less food for seabirds, which means that the environment may no longer be able to support the population sizes it once could.
The effects of extreme warming weren't contained to the marine heatwave of 2014-2016. Northern Alaska continues to experience warm ocean waters connected to record loss of sea ice in the Arctic. Sea ice reflects the sun’s rays, preventing the dark water underneath from absorbing heat. But without ice, the water readily absorbs energy from the sun and warms. It is an example of a feedback loop—warmer water prevents ice from forming and the loss of sea ice results in warmer water.
Until recently, there was a thermal barrier in the Bering Sea that kept warm-water species from coming further north: the cold pool. Just as it sounds, the cold pool was a mass of deep, cold, salty water that extended along the continental shelf of the Bering Sea south of the Bering Strait. In normal years, the cold pool forms under the sea ice, sinking down to the sea floor where it lingers throughout the summer before chilling again with new sea ice formation in the fall. But in 2018, for the first time in nearly 40 years of monitoring, the cold pool disappeared. The water had warmed all the way to the seabed due to warmer-than-usual summer temperatures and multiple years of decreased sea ice (Gramling 2019, NOAA 2019).
Notice the extent of the cold pool (colored blue and purple) varies in size by year, but in 2018 it is gone completely. Also noticeable is the increasing warmth as time goes on (red and orange colors).
Animation of Bering Sea Trawl Survey Bottom Temps from 1982-2018. NOAA, Alaska Fisheries Science Center
Most fish species can tolerate a range of temperatures. However, as Alaskan ocean waters warm, colder-water fish move northward or deeper in the water column to find their preferred temperature habitat and warm-water fish species expand into the newly created warm-water territory. The loss of the cold pool opened the northern Bering Sea and Chukchi Sea to warm-water- tolerant fish and increased competition for forage fish.
Walleye pollock is one species that moved north in absence of the Bering Sea cold pool.
Many species depend on marine ecosystems—including us. These conditions may be giving us a glimpse—a warning—of what our northern oceans will look like in the future as the Earth warms and sea ice declines.
“We are just beginning to understand the many ways that the low ice extent in the northeastern Bering Sea in 2018 has affected this marine ecosystem.”
—Elizabeth Siddon, NOAA Alaska Fisheries Science Center
“We predict that changing environmental conditions will continue to affect the marine food web structure—and potentially the productivity of the northeastern Bering Sea shelf ecosystem.”
—Robert Foy, Director, NOAA Alaska Fisheries Science Center
Responding to Alaska's largest seabird die-off at Katmai National Park & Preserve - Audio Described
How We Know
A volunteer documents a gull carcass during a beach survey.
In Alaska, data collection is a group effort. Coastal communities throughout Alaska have a long history of gathering detailed information about the status and timing of marine resources. Fishers, indigenous communities, and tour operators have all contributed key observations to our collective knowledge of ecosystem change in the ocean. Federal, state, and tribal agencies are on the front lines of research, including the U.S. Geological Survey, U.S. Fish and Wildlife Service, State of Alaska Department of Fish and Game, the National Park Service, the National Oceanic and Atmospheric Administration, Kawerak, and the Aleut Community of St. Paul Ecosystem Conservation Office. University and nongovernmental organization (such as the Wildlife Conservation Society) researchers, and citizen scientists through the Coastal Observation and Seabird Survey Team (COASST) are vital partners. These collaborative efforts are critical to document and monitor novel events like the 2014-2016 marine heatwave and to understand what these changes could mean for the marine ecosystem in the future and how we might adapt to these new conditions.
Data are collected annually for selected species of marine birds at breeding colonies on the Alaska Maritime National Wildlife Refuge, and in other areas of Alaska, to monitor the condition of the marine ecosystem and to evaluate the conservation status of species managed by the U.S. Fish and Wildlife Service. The strategy for seabird colony monitoring includes estimating timing of nesting events, rates of reproductive success, and population trends of representative species of various foraging guilds (for example, offshore diving fish-feeders, diving plankton-feeders) at geographically dispersed breeding sites. This information helps us understand ecosystem processes and respond appropriately to emerging threats and species declines. It also provides a basis for researchers to test hypotheses about ecosystem change (Dragoo et al. 2019). The value of the marine bird monitoring program is enhanced by having sufficiently long time-series to describe patterns for these long-lived species.
Related Links More about seabird die-offs in Alaska Alaska Ocean Observing System North Pacific Pelagic Seabird Database , U.S. Geological Survey North Pacific Seabird Data Portal
The National Park Service Southwest Alaska Inventory & Monitoring Network , in partnership with Gulf Watch Alaska , monitors nearshore marine systems , including marine birds, marine water chemistry, intertidal invertebrates, kelp and seagrasses, and sea otters. The National Park Service nearshore long-term monitoring is integrated with other monitoring components of the marine environment through partnerships with federal and state agencies, universities, and private entities and with funding support from the Exxon Valdez Oil Spill Trustee Council. Together, we create an ecosystem approach to monitoring nearshore species, open-water ecosystems, and environmental indicators (such as climate and chemistry) as possible drivers of the marine system. Learning about each of these components in context of the marine food web, how they interact with one another, and what drives changes in the system increases our understanding of these important ecosystems, especially when an event like the extreme marine heatwave and seabird die-off occurs.
The Inventory & Monitoring Division of the National Park Service is like a physician for our parks. We track the health of key vital signs such as water, plants, and wildlife. Together we can use this information to take care of our national parks for this and future generations.
Alaska has four inventory and monitoring networks: Arctic Network , Central Alaska Network , Southeast Alaska Network , and the Southwest Alaska Network .
Developed by Nina Chambers, Jessica Weinberg, Heather Coletti, and Stacia Backensto, National Park Service (July 2020)
This story is based on:
Piatt, J. F., J. K. Parrish, H. M. Renner, S. K. Schoen, T. T. Jones, M. L. Arimitsu, K. J. Kuletz, B. Bodenstein, M. Garcia-Reyes, R. S. Duerr, R. M. Corcoran, R. S. A. Kaler, G. J. McChesney, R. T. Golightly, H. A. Coletti, R. M. Suryan, H. K. Burgess, J. Lindsey, K. Lindquist, P. M. Warzybok, J. Jahncke, J. Roletto, and W. J. Sydeman. 2020. Extreme mortality and reproductive failure of common murres resulting from the northeast Pacific marine heatwave of 2014-2016 . PLOS ONE 15(1): e0226087.
Also cited:
Dragoo, D. E., H. M. Renner, and R. S. A. Kaler. 2019. Breeding status and population trends of seabirds in Alaska, 2018 . U.S. Fish and Wildlife Service Report AMNWR 2019/03. Homer, Alaska.
Gabriele, C. M., J. L. Neilson, J. M. Straley, C. S. Baker, J. A. Cedarleaf, and J. F. Saracco. 2017. Natural history, population dynamics, and habitat use of humpback whales over 30 years on an Alaska feeding ground . Ecosphere 8(1): 10.1002/ecs2.1641
Gramling, C. 2019. What happens when the Bering Sea’s ice disappears? Record low sea ice in 2018 sent ripples through the entire Arctic ecosystem ScienceNews Available on line at: https://www.sciencenews.org/article/bering-sea-ice-disappearing-arctic-ecosystems (accessed 18 February 2020)
National Oceanic and Atmospheric Administration (NOAA) Fisheries. 2020. Active and closed unusual mortality events. Available at: https://www.fisheries.noaa.gov/national/marine-life-distress/active-and-closed-unusual-mortality-events (accessed 3/10/2020)
National Oceanic and Atmospheric Administration (NOAA) Fisheries. 2019. Scientific Teams Set Out to Track Unprecedented Changes in the Eastern Bering Sea, News April 18, 2019. Available at: https://www.fisheries.noaa.gov/feature-story/scientific-teams-set-out-track-unprecedented-changes-eastern-bering-sea (accessed 3/10/2020)
