Heat waves are not limited to the atmosphere; they also appear beneath the seas and bring equally serious consequences. An investigation by the US National Oceanic and Atmospheric Administration reveals the sea heat wave event known as The Blob, which unfolded from 2013 to 2016, warming a broad swath of surface waters in the Northeast Pacific. This shift altered marine ecosystems along the West Coast, reduced salmon yields, and hurt commercial fisheries.
An article in Nature Communications documents a study led by NOAA researchers who combined direct observations with computer modeling to deliver the first comprehensive assessment of marine heat waves on the continental shelf floor around North America.
“Researchers have been examining marine heatwaves at the seafloor for more than a decade,” said Dillon Amaya, the study’s lead author. “This is the first time we could reach that depth and evaluate how these extreme events formed on the shallow seabed.”
Marine heat waves profoundly affect ocean ecosystems worldwide. They disrupt the productivity and distribution of organisms from tiny plankton to enormous whales. Consequently, scientists strive to study, monitor, and predict the timing, intensity, duration, and physical drivers of these events.
Much of this research has emphasized extreme ocean surface temperatures, with high-quality observations gathered from satellites, ships, and buoys. Sea surface temperatures also reflect many physical and biochemical properties of sensitive marine ecosystems, simplifying analysis.
Marine heatwaves have increased 50% in the last decade
About 90% of the excess heat from global warming has been absorbed by the ocean. It has warmed by roughly 1.5 °C over the last century, and marine heat waves have become 50% more frequent in the past ten years.
In recent years, scientists have intensified efforts to investigate marine heatwaves through the entire water column using available data. Prior work rarely accounted for deep-sea temperatures along continental shelves, places that host critical habitats for commercial species such as lobster, scallops, crab, flounder, cod, and other groundfish.
Given the limited bottom-water temperature datasets, researchers used a reanalysis data product to reconstruct past conditions. This approach starts with existing observations and employs computer models to simulate ocean currents and atmospheric impacts to fill gaps. Using a similar method, NOAA scientists have reconstructed global climate up to the present.
Ocean reanalyses have been around for a long time, but only recently have they achieved the accuracy and resolution needed to study near-shore bottom temperatures.
A research team from NOAA, CIRES, and NCAR notes that on the continental shelves around North America, deep-sea heat waves tend to persist longer than surface events and may show stronger warming signals than the waters above them.
Deep-sea and surface heat waves can occur in the same place, especially in shallower zones where surface and bottom waters mix.
Yet seafloor heat waves can occur with little or no surface warming, with important implications for managing commercially important fisheries. This means managers might miss warming events until their effects are felt, Amaya warned.
Serious economic and ecological consequences
In 2015, the Blob caused a combination of harmful algal blooms and kelp forest habitat loss off the U.S. West Coast, leading to seafood closures that cost the economy more than 185 million dollars, according to later research.
Commercial Dungeness crab fisheries in three states reported losses of 97.5 million dollars. Coastal communities in Washington and California lost 84 million dollars in tourism spending due to recreational closures.
A 2021 NOAA Fisheries groundfish survey showed Gulf of Alaska cod declined by 71% between 2015 and 2017 during The Blob. In contrast, juvenile groundfish and other marine life in the Northern California Current thrived under the unusual ocean conditions, as highlighted in a 2019 study by Oregon State University and NOAA Fisheries researchers.
Unusually hot bottom-water temperatures have also been linked to the spread of invasive lionfish in the southeastern United States, coral bleaching, declines in reef fish, altered survival rates of juvenile cod in the Atlantic, and declines in lobster populations near the coast of southern New England.
The authors emphasize the importance of maintaining existing continental shelf monitoring systems and developing new real-time monitoring capabilities to alert marine resource managers to bottom-warming conditions.
Early detection of marine heatwaves is essential for proactive ocean management, said oceanographer Michael Jacox. The evidence now clearly shows that attention must extend to the ocean floor, where key species live and can experience heat waves very differently from surface waters.
Attribution: Nature Communications 2023
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The environmental department notes that ongoing monitoring will support sustainable fisheries and healthier ecosystems across North American coasts.