What Happens When Northern Oceans Get Too Warm: Lessons From the Pacific Marine Heatwave

Oceans surround most of Alaska and have shaped its people, cultures and economies for thousands of years. They feed rich commercial fisheries and support marine mammals, seabirds and much more. This review, by Park Service scientists Heather Coletti, Nina Chambers, Jamie Womble and Christine Gabriele, pulls together what Alaska park monitoring revealed when those waters got too warm.

A murre carcass on the beach. A massive seabird die-off accompanied the Pacific Marine Heatwave. USGS / Sarah Schoen.

The heatwave

Marine heatwaves — abrupt, extreme spells of unusually warm ocean — have increased worldwide and are likely to become more frequent and widespread as the climate warms. In 2013, researchers saw temperatures rising across the North Pacific: the start of the Pacific Marine Heatwave (PMH). It lasted more than three years, with water 5–11°F (3–6°C) above average from Southern California to the Gulf of Alaska, and it was the most consequential heatwave on record for its size, strength and duration. Its effects reached from plankton to whale watching in Hawaii to trawl fisheries in northern Alaska.

Out at sea: a food web breaks down

Plankton shrank. Everything from small fish to whales depends on plankton. During the heatwave, warm-water copepods — small, lean species — replaced the larger, fatty copepods of cold water, leaving poorer food for animals that eat zooplankton.

Forage fish are food for many species. Their staggered life cycles usually keep some populations available year-round, but during the heatwave several collapsed at once. Credit: NOAA.

Forage fish collapsed. Small plankton-eating fish normally spawn and grow on different schedules, so some are always available to predators. The heatwave was big enough that several species collapsed at the same time. The survivors were smaller and in worse condition, while warmer — and therefore hungrier — predatory fish and seabirds needed more of them. Neither could meet its energy needs.

Seabirds starved. From the southern Bering Sea to Southern California, common murres died in huge numbers and breeding colonies failed again and again. At least 500,000 and perhaps just over a million seabirds, mostly murres, died in the Gulf of Alaska and along the U.S. West Coast. Seabird die-offs happen from time to time, but this one was unprecedented worldwide in its area, duration and death toll.

With forage fish gone, seabirds — especially common murres — starved. USGS / Tony DeGange.

Marine mammals

Whales and seals across the Gulf of Alaska showed population declines, lower survival and unusual die-offs.

• Steller sea lions declined at Glacier Bay's main haul-out, South Marble Island, and across the Gulf of Alaska showed lower numbers, changed diets and reduced survival.
• Harbor seals declined in Johns Hopkins Inlet, though ice and changing tidewater glaciers also affect seal numbers there.

Steller sea lions declined, changed their diets and survived less well after the heatwave. NPS / Jamie Womble.

Even long-lived humpback whales were hit: poor body condition and fewer calves from stress and scarce, low-quality food. NPS photo under National Marine Fisheries Service permit 21059.

Along the shore: a mixed picture

The nearshore — the shallow coastal waters, limited in the North Pacific by how far light reaches — runs on kelp and seagrass. Mussels, clams and sea urchins pass that energy up to sea stars and sea otters, keystone predators that shape the whole system, and to birds such as the black oystercatcher. The intertidal zone, exposed at low tide, is a place of extremes: drying, freezing, shifting salinity and temperature, and fierce competition for space.

• Shores became more alike. Rocky intertidal sites from Prince William Sound west to Kenai Fjords, Kachemak Bay and Katmai, once distinct, converged on similar communities: kelp and seaweed gave way to mussels and barnacles, signaling a drop in the nearshore's primary producers and freeing space for new settlers.
• Sea stars declined, mussels boomed. Sea stars suffered heavy losses, probably from sea star wasting, which warm water is thought to worsen. With more space and fewer predators, mussels flourished. Other invertebrate prey stayed fairly stable.
• Predators responded differently. Sea otter numbers held steady, though their diet shifted toward mussels. Black oystercatchers increased sharply in the eastern and western Gulf, without any change in breeding success. Nearshore birds that eat bottom-dwelling prey showed little response — in sharp contrast to fish-eating murres.

After the heatwave, intertidal communities shifted from kelp and seaweed toward mussels and barnacles. Image from the National Park Service's article on the Pacific Marine Heatwave.

The contrast suggests a large disturbance can hit different food-web pathways in very different ways.

Lasting effects

Many changes — from offshore productivity to rocky shore communities — lasted at least five years after the heatwave began, often overriding the local forces that normally drive these systems. Without long-term monitoring data, none of these responses could have been documented. With more heatwaves expected, alongside ocean acidification, oxygen loss, changing stratification and falling productivity, tracking how species respond will help scientists anticipate future shocks and watch for recovery.

Sources

Based on Coletti H, Chambers N, Womble J, Gabriele C, "What Happens When Northern Oceans Get Too Warm," Alaska Park Science, National Park Service; a work of the United States government in the public domain.