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Temperatures around the world continue to rise—and the North Sea is no exception. Yet, in addition to this gradual warming, increasingly frequent and intense heat events also have consequences for marine organisms.

Researchers at the Marine Station Helgoland, a research facility of the Alfred Wegener Institute, have quantified the frequency and intensity of these heat waves along with their repercussions for plankton.

The researchers found that gradual warming causes significant shifts in the species spectrum. When heat waves are added, however, these alterations are amplified. The results have been published in three publications, the in Limnology and Oceanography. An earlier study was published in the same journal and appeared in the Journal of Animal Ecology.

From rising sea levels and new introductions of species to a temperature increase of 1.9°C since 1962, there is clear evidence that the North Sea is changing more rapidly at present than it has for millennia.

Yet, given that climate change takes place over long timescales—from a human perspective, at least—these changes can seem slow and gradual, perhaps not so serious, because many organisms might adapt gradually. "This puzzle, however, misses a key piece," says Dr. Cédric Meunier. "And that's marine heat waves."

Meunier researches the ecology of shelf sea systems at the Marine Station Helgoland (BAH), which became part of the Alfred Wegener Institute Helmholtz Center for Polar and Marine Research (AWI) in 1998. In collaboration with researchers from various disciplines, Meunier has focused on marine heat waves and their potential impact on plankton.

During these periods of intense heat, water temperatures rise suddenly—usually as a result of a preceding atmospheric heat wave—and remain well above average temperatures for several days. Given their abruptness and intensity, researchers aimed to determine how these heat waves impact organisms.

They have analyzed historical measurement data, examined the frequency and intensity of heat waves over recent decades through to the present day, and conducted an experiment looking at the future of the North Sea ecosystem. The 2024 papers focus on the past and the present. The third paper completes the cycle by focusing on the future impact of these heat waves.

The researchers' historical analysis was made possible by Helgoland Roads, one of the world's longest and most detailed ecological time series. Thanks to the tireless efforts of numerous researchers, the Helgoland Roads time series seamlessly documents the North Sea pelagic ecosystem since 1962.

In their analysis of temperature data, ecologist Dr. Luis Gimenez and his colleagues identified that marine heat waves have occurred more frequently in the German Bight over recent decades, through to the present day—and last longer, too. These heat waves are more common in late summer, when average temperatures in the annual cycle are at their highest.

A second study showed that heat waves have already exerted a significant impact on organisms. "We have identified substantial changes in medium-sized zooplankton, including copepods, as a result of temporary temperature increases," explains Margot Deschamps, another BAH researcher. "Some groups were decimated after a heat wave, at least temporarily, while others appeared to benefit and proliferate."

In the third research article, the team investigate future potential changes. They relied on one of the world's most advanced mesocosm facilities at the AWI Wadden Sea Station on Sylt. Each of the 30 cylindrical tanks (mesocosms) is 85 centimeters tall, 170 centimeters wide and holds about 1,800 liters of sea water.

By making targeted adjustments to the water's temperature, pH value and nutrient content, researchers are able to simulate the conditions in the world of tomorrow. As such, the mesocosms act as time machines, transporting the plankton within them into the future.

In this experiment, the research team exposed plankton from the North Sea to both current conditions and the conditions projected by RCP 8.5—a "business as usual" scenario defined by the Intergovernmental Panel on Climate Change (IPCC).

In this scenario, the global community's CO₂ emissions would continue to rise to over 100 gigatons by 2100, which could lead to a global temperature increase of 4°C to 5°C. Current conditions and "business as usual" RCP 8.5 conditions were simulated both without and with five-days-long heat waves which warmed the water by 2°C above average.

"The results indicate that climate change may cause shifts at many levels within the plankton community, which are further amplified or modified by heat waves," explains Meunier. "Certain bacterial groups benefit from these environmental changes, including some bacteria from the genus Vibrio, which are potentially dangerous to humans.

"While the total biomass of phytoplankton remains constant in the RCP 8.5 scenario, the composition of species shifts in favor of smaller species. The additional impact of heat waves particularly benefits coccolithophores, which feature a calcium carbonate shell, and phytoflagellates. Even stronger shifts occur in zooplankton communities.

"We observe a reduction in total biomass under warmer conditions, especially regarding medium-sized mesozooplankton. Additional heat waves have a particularly negative impact on Noctiluca scintillans, a species known for its bioluminescence."

Taken as a whole, the three research papers emphatically demonstrate the vast changes in species communities of North Sea plankton resulting from continued global temperature increases. However, focusing solely on average temperatures is not sufficient for ecological forecasts. This is because short-term events, such as heat waves, can modify, and potentially further amplify these effects.

"With this in mind, it is important to keep an eye not only on long-term climate trends but also on short-term events, like heat waves," says Cédric Meunier. "Marine heat waves impact organisms at the basis of the food web. This could have consequences for higher levels, such as fish."