A study confirms that ocean warming accelerates the deterioration of seagrass meadows

A new study conducted by researchers from the IFISC (CSIC–UIB) and CEAB–CSIC shows that prolonged warming degrades seagrass meadows even when temperatures remain below lethal limits.

The meadows of Posidonia oceanica, the endemic marine seagrass species that forms the backbone of Mediterranean coastal ecosystems, are experiencing a rapid decline. Until now, most quantitative studies have assessed the impact of climate change based on the crossing of temperature thresholds. However, a new study led by researchers from the Institute for Cross-Disciplinary Physics and Complex Systems (IFISC, CSIC–UIB), the Center for Advanced Studies of Blanes (CEAB–CSIC), and King Abdullah University of Science and Technology (KAUST) in Saudi Arabia shows that chronic and cumulative thermal stress can degrade and fragment these meadows even when temperatures remain below their critical survival limits.

To capture the subtle but potentially destructive effects of prolonged and fluctuating conditions that do not exceed critical thresholds, the research team introduced a novel physiologically based index called Stress Degree Days (SDD).

“Unlike traditional threshold-based approaches that focus exclusively on discrete heat peaks, our conceptual framework explicitly accounts for how thermal stress affects the organism over time under dynamic field conditions,” explains Àlex Giménez-Romero, researcher at CEAB–CSIC and lead author of the study.

By convolving historical daily sea surface temperature data from 2000 to 2020 with experimentally derived mortality-rate functions, the scientists quantified cumulative thermal exposure across the entire Mediterranean basin over the past two decades.

An important step in the research was linking this thermal index to the physical structure of the meadows at a basin-wide scale. To do so, the authors used CAMELE, an open-source AI framework capable of processing high-resolution satellite imagery to map underlying habitats of Posidonia oceanica.

After mapping more than 30 satellite images from different regions of the Mediterranean, the scientists determined seagrass cover and fragmentation indices. The results revealed an alarming pattern: areas of high accumulated thermal stress, concentrated along the southern and eastern coasts of the basin, showed a reduction in cover exceeding 40%, along with severe structural fragmentation. Crucially, these degradation patterns emerged even in locations where maximum monthly sea surface temperatures remained below the species’ absolute lethal limit, determined experimentally.

The study not only analyzes the recent past but also projects the trajectory of this vital Mediterranean ecosystem under different IPCC climate scenarios. Under a moderate emissions scenario, seagrass meadows are expected to face an average loss of 40% cover by the end of the century. However, under a high-emissions scenario, basin-wide regression could increase to approximately 80%, leading to an almost complete contraction of suitable thermal conditions in the southern Mediterranean regions.

“The loss of continuous seagrass landscapes severely degrades critical ecosystem services,” warns Manuel Matías, researcher at IFISC and co-author of the study. “A fragmented seagrass landscape transitions into isolated patches, which can negatively affect clonal connectivity, reduce sediment retention capacity, and diminish oxygen export and carbon sequestration capabilities,” the research team concludes.

By integrating plant physiology, large-scale remote sensing, and climate modeling, this new methodological framework stands out as a key risk-assessment tool to guide targeted marine conservation strategies in the face of unprecedented ocean warming.