They discover heat waves at the bottom of the ocean, which threaten fishing and the ecosystem

Heat waves are not something exclusive to the atmosphere, but also occur under the sea and have equally devastating effects, as revealed by an investigation by the US National Oceanic and Atmospheric Administration (NOAA). The marine heat wave that took place in the period 2013-2016 known as ‘The Blob’ warmed a large expanse of surface waters in the Northeast Pacific, altering marine ecosystems on the West Coast, depressing salmon returns and damaging commercial fisheries.

In an article published in the magazine Nature Communicationsa team led by NOAA researchers used a combination of observations and computer modeling to generate the first comprehensive assessment of marine heat waves at the bottom of the productive waters of the continental shelf surrounding North America.

“Researchers have been investigating marine heat waves at the sea surface for more than a decade,” said lead author Dillon Amaya of NOAA. “But This is the first time we’ve been able to dive in as deep and assess how these extreme events play out along the shallow seabed.”

Marine heat waves dramatically impact the health of ocean ecosystems around the world, disrupting the productivity and distribution of organisms as small as plankton and as large as whales. Therefore, scientists try to study, track and predict the timing, intensity, duration and physical drivers of these events.

Most of that research has focused on extreme ocean surface temperatures, for which there are many more high-quality observations taken by satellites, ships, and buoys. Sea surface temperatures can also be indicators of many physical and biochemical characteristics of the oceans of sensitive marine ecosystems, making analyzes easier.

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Marine heat waves have increased by 50% in the last decade

Around 90% of the excess heat from global warming has been absorbed by the ocean, which has warmed by about 1.5 °C over the past century. Marine heat waves have become 50% more frequent over the past decade.

In recent years, scientists have intensified their efforts to investigate marine heat waves across the entire water column using the limited data available. But previous research did not account for extreme temperatures on the ocean floor along the continental shelveswhich provide critical habitat for important commercial species such as lobster, scallops, crabs, flounder, cod, and other groundfish.

Due to the relative paucity of bottom water temperature data sets, the scientists used a data product called ‘reanalysis’ to make their assessment, which started with available observations and employed computer models simulating ocean currents and the influence of the atmosphere to “fill in the blanks.” Using a similar technique, NOAA scientists have been able to reconstruct global climate since the early 19th century.

While ocean reanalyses have been around for a long time, it’s only recently that they’ve become accurate enough and high enough in resolution to examine ocean features, including bottom temperatures, near shore.

The research team, from NOAA, CIRES and NCAR, found that on the continental shelves around North America, deep-sea heat waves tend to persist longer than their surface counterparts and may have larger warming signals than the overlying surface waters.

Deep-sea and surface-sea heat waves can occur simultaneously in the same place, especially in shallower regions where surface and bottom waters mix.

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But seafloor heatwaves can also occur with little or no evidence of surface warming, which has important implications for the management of commercially important fisheries. “That means it may be happening without managers realizing it until the impacts start to be feltAmaya alerted.

Serious economic and ecological consequences

In 2015, a combination of harmful algal blooms and kelp forest habitat loss off the West Coast of the United States, both caused by The Blob, caused the closure of seafood restaurants that cost the economy more than 185 million dollars, according to a 2021 study.

The commercial Dungeness crab fishery in the three states recorded a loss of $97.5 million. Coastal communities in Washington and California lost a combined $84 million in tourism spending due to recreation closures.

In 2021, a groundfish survey published by NOAA Fisheries indicated that Gulf of Alaska cod had plunged during The Blob episode, experiencing a 71% decline in abundance between 2015 and 2017. Instead, young groundfish and other marine creatures in the Northern California Current thrived under these unprecedented ocean conditions, according to a 2019 paper from Oregon State University and NOAA Fisheries researchers.

Unusually warm bottom water temperatures they have also been linked to the spread of invasive lionfish throughout the southeastern US, coral bleaching and subsequent declines in reef fish, changes in the survival rates of juvenile cod in the Atlantic and the disappearance of lobster populations near the coast in southern New England.

The authors say it will be important to maintain existing continental shelf monitoring systems and develop new real-time monitoring capabilities to alert marine resource managers to bottom warming conditions.

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“We know that early detection of marine heat waves is needed for proactive management of ocean costro,” said co-author Michael Jacox, an oceanographer. “It is now clear that we need to pay more attention to the ocean floor, where some of the most valuable species live and can experience very different heat waves than on the surface,” he added.

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