A decline in phytoplankton, or a change in the phytoplankton species present, due to hotter sea temperatures could reduce zooplankton and the abundance of the forage fish that feed on them. Image by the National Oceanic and Atmospheric Administration (NOAA) (Public domain).

World - Record North Atlantic heat sees phytoplankton decline, fish shift to Arctic

Scientists warn that record-high sea surface temperatures in the North Atlantic Ocean this year are having consequences for sea life.

  • As marine heat waves there have worsened over the years, populations of phytoplankton, the base of the oceanic food chain, have declined in the Eastern North Atlantic.
  • With experts predicting more heat anomalies to come, North Atlantic fish species are moving northward into the Arctic Ocean in search of cooler waters, creating competition risks with Arctic endemic species and possibly destabilizing the entire marine food web in the region.
  • Lengthening and intensifying marine heat waves around the globe are becoming a major concern for scientists, who warn that the world will see even greater disruptions to ocean food chains and vital fisheries, unless fossil fuel burning is curtailed.

The verdict is in: Global sea temperatures shattered new heat records in 2023, with anomalous warmth erupting in the Northeast Atlantic Ocean in a year that also saw numerous record-setting extreme terrestrial heat waves, droughts, floods and wildfires on five continents.

According to the U.S. National Oceanic and Atmospheric Administration (NOAA), North Atlantic waters reached a high of 24.9° Celsius (76.82° Fahrenheit) in July — where an annual peak isn’t usually reached until early September, worrying the scientific community about repercussions for aquatic life. July’s record high was more than 1°C (1.8°F) warmer than the 30-year climatological normal for the North Atlantic from 1982 to 2011.

Researchers say rapid warming in the region is endangering phytoplankton — the base of the marine food chain, thus reducing food availability for species that depend on these microscopic algae to survive. A hotter North Atlantic could also be accelerating the “Atlantification” of the Arctic Ocean, as more temperate fish species move into Arctic waters to beat the heat.

All this comes as a new El Niño takes hold globally in 2023. El Niños — caused by the development of a band of warmer surface water in the central and eastern Pacific Ocean — recur naturally every few years. They have major impacts on the world’s weather, including elevated atmospheric temperatures, which could in turn increase ocean warming, lengthening and intensifying marine heat waves.

NOAA says this El Niño has a greater than 95% likelihood of lasting through January-March 2024, with a more than 70% chance of being a strong event. The consequences? A potentially hotter northern summer in 2024, triggering more disruptive anomalous extreme weather events around the world.

Warming effects spreading through the food web

Earlier this year, European scientists suggested that future climate change-driven warming could result in “widespread changes in opportunities for commercial fisheries” across the Northeast Atlantic shelf seas. Importantly, they found that within the area sampled, sea temperatures were the primary driver of fish community composition. And now with those temperatures soaring (2022 set the previous North Atlantic heat record), those fish communities are visibly altering.

The rising intensity and frequency of marine heat waves in the North Atlantic is also expected to disrupt the balance of marine ecosystems into the future — starting at the base of the oceanic food web with phytoplankton.

In the Eastern North Atlantic, experts have flagged the recent marine heat wave as a probable cause of a decline in microscopic algae, which many aquatic species depend on: Phytoplankton are eaten by zooplankton, which are in turn eaten by small fish, which are eaten by bigger fish, which are eaten by seals, and so on.

The phytoplankton reduction is thought to be triggered by warming waters, which alter ocean stratification, reducing the availability of nutrients and sunlight that the microorganisms need in order to photosynthesize and grow.

The North Atlantic typically hits the peak of its spring phytoplankton bloom in June, which is when the largest increase in phytoplankton biomass is generally detected. But that’s also when the intense warming occurred this year.

And with phytoplankton populations now decreasing, scientists are detecting impacts up the food chain.

Also, a reduction in phytoplankton populations could reduce carbon sequestration. That’s because these microorganisms in high numbers remove significant amounts of carbon from surface waters when they die and sink, storing carbon in the deep ocean and making a major contribution to long-term carbon sequestration. But fewer phytoplankton means less sequestration, and less curbing of global warming.

The organic carbon synthesized by phytoplankton during that June period, when photosynthesis is at its highest level, has long been crucial to providing the energy needed to sustain the North Atlantic region’s marine food web, triggering big losses up the food chain, Arthi Ramachandran, a polar expert with science popularization NGO Arctic Basecamp, wrote in an email to Mongabay.

Ramachandran cited similar observations during the anomalous 2015 Northeast Pacific marine heat wave, which caused comparable shifts in marine species distribution and abundance. “The increase of physical and chemical anomalies led to a cascade of effects that was seen throughout the food web,” she said.

Scientists continue to be concerned about the synergistic impacts of these compound events — as a combination of climate drivers widely impact other environmental factors, resulting, for example, in low-chlorophyll levels during high-temperature extremes.

“Right now, in the Eastern North Atlantic, a significant decrease in chlorophyll-a concentration — used as a proxy for phytoplankton abundance — has been observed,” Ramachandran noted. Scientists have shown that such a decline was closely related to the spatial distribution of marine heat waves.

A drop in the quantity of phytoplankton, or a change in the phytoplankton species present in the North Atlantic, could result in less zooplankton there and a switch toward smaller phytoplankton species in the region, Maxime Geoffroy, a research scientist at Canada’s Marine Institute of the Memorial University of Newfoundland, explained to Mongabay in an email.

This, in turn, could reduce the abundance of forage fish that feed on zooplankton, he added. “For instance, capelin and sand lance [which once inhabited North Atlantic waters,] are [now] known to be increasingly present in the Canadian Arctic where they increase competition for Arctic endemic species.”

Fish heading north to escape the heat

What happens in the North Atlantic Ocean isn’t likely to stay in the North Atlantic Ocean.

Scientists continue to observe the ongoing Atlantification of the Arctic Ocean, as it moves toward a warmer ecosystem where polar fish species are forced to compete with intruding North Atlantic fish.

“They are now replacing native species there,” Ramachandran said, as global warming weakens the barrier between the layers of a relatively fresh Arctic Ocean, allowing warmer, saltier North Atlantic waters to blend and, as a consequence, diminish sea ice resilience in the polar region.

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