Marine heatwaves: Rising ocean temperatures reduce the natural carbon storage capacity of seagrass meadows
A new study published in Communications Earth & Environment with contributions by researchers from the Leibniz Centre for Tropical Marine Research (ZMT) shows that higher seawater temperatures could significantly reduce the long-term ability of underwater plants to store carbon.
Scientists from Spain and Germany investigated how higher water temperatures affect the dissolved organic carbon released by seagrasses and marine algae. The results show that as temperatures rise, the proportion of dissolved organic carbon that persists in the ocean over long periods and therefore contributes to long-term carbon storage declines significantly.
The empirical part of the study included seagrasses and macroalgae. In ZMT’s aquarium facility in Bremen, Alba Yamuza-Magdaleno (University of Cádiz), Pedro Beca- Carretero and Hauke Reuter (Spatial Ecology and Interactions Research Group at ZMT) simulated different temperature conditions. Their findings show that even a modest increase in water temperature alters the composition and characteristics of the dissolved carbon released by the investigated seagrasses.
Less long-term carbon storage in the ocean
Seagrasses and algae absorb carbon dioxide from the atmosphere to support growth and metabolism. Some of this carbon is fixed into structural components, such as leaves and rhizomes. Part is stored in the seafloor for centuries or millennia, while another portion is released into the water as dissolved organic carbon. Some of these substances are quickly broken down by microorganisms. Others are far more resistant to degradation and can remain in the ocean for weeks, or years or even centuries.
It was precisely this long-lasting form of carbon that decreased significantly in the experiments conducted at higher temperatures. With a temperature increase of four degrees Celsius, the proportion of carbon that is difficult to break down decreased by an average of about 28 percent. At the same time, the proportion of carbon compounds that are easily broken down by microorganisms increased significantly. After 60 days, much of this more readily degradable carbon had already been decomposed.
“Seagrass beds and kelp forests are considered natural carbon sinks. If the ocean continues to warm, they could sequester significantly less carbon over the long term than previously thought,” explains lead author Alba Yamuza-Magdaleno of the University of Cádiz. “This is important because many climate models have so far assumed that these ecosystems permanently absorb large amounts of carbon dioxide from the atmosphere.”
The researchers also compared the amount of persistent dissolved organic carbon with carbon storage in the seafloor. Their analysis revealed that both processes could be similarly important, with storage rates of the same order of magnitude. To date, most studies have focused primarily on carbon stored permanently in marine sediments. The new study shows that dissolved organic carbon also plays a major role.
ZMT experimental facility MAREE provides new insights
For the study, the team used so-called mesocosms, large seawater tanks in which environmental conditions can be controlled and varied. Over a period of 40 days, the researchers tested three temperature levels ranging from 24 to 28 degrees Celsius.
The experiments at ZMT were carried out in 27 mesocosms containing different combinations of seagrass species. The study included three native species from the Bay of Cadiz, the seagrass Cymodocea nodosa and Zostera noltei, as well as the macroalgae Caulerpa prolifera. The researchers also studied the seagrass Halophila stipulacea, an invasive species originating from the Indian Ocean.
The scientists measured oxygen production, carbon turnover, and the chemical composition of the substances released. They then observed over a period of 60 days how quickly microorganisms broke down the various carbon compounds.
Surprisingly, the invasive seagrass species had very little impact on the carbon balance. Instead, temperature proved to be the dominant factor.
“It was particularly striking that the long-lasting fraction of dissolved organic carbon – the portion that normally persists in the ocean for extended periods – declined the most as temperature increased,” says Pedro Beca-Carretero of ZMT and the IIM-CSIC marine research institute in Vigo, Spain. “As a result, some of the carbon absorbed by these ecosystems can re-enter the carbon cycle more quickly.”
Significance for climate models and coastal protection
Coastal ecosystems such as seagrass beds, mangrove forests, and salt marshes are considered important natural carbon sinks and are often referred to as “blue carbon” ecosystems. They play a key role in removing carbon dioxide from the atmosphere and storing it over the long term.
The authors of the study emphasize that existing climate models may underestimate the effects of rising temperatures on dissolved carbon compounds. As a result, the actual carbon sequestration capacity of many coastal ecosystems could be lower in the future than previously assumed.
At the same time, the researchers highlight the limitations of their study. The experiments were conducted under controlled conditions and only provide a simplified representation of natural coastal regions. Factors such as strong currents, storms, or long-term changes in species composition could not be fully accounted for. Future field studies will investigate whether these findings also hold under natural conditions.
“We now need long-term observations in natural coastal areas to better assess the actual extent to which ocean warming affects carbon storage,” summarizes ZMT researcher Hauke Reuter. “Such data are essential for incorporating coastal ecosystems more accurately into future climate mitigation strategies.”
Publication:
Yamuza-Magdaleno, A., Azcárate-García, T., Egea, L.G., Álvarez-Salgado, X.-A., Reuter, H., Brun, F.G., & Beca-Carretero, P. (2026) Temperature-driven decline in recalcitrant dissolved organic carbon weakens coastal macrophytes’ blue carbon storage potential. Commun Earth Environ 7, 362. https://doi.org/10.1038/s43247-026-03417-y
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