The Mediterranean Sea is rapidly changing under ongoing climate change. In the eastern basin, tropicalization is already well documented and driven by a combination of strong warming and the influx of tropical species through the Suez Canal. In contrast, the western Mediterranean has, until now, shown fewer such signals. However, a recent study demonstrates that the expansion of microscopic warm-water species provides a clear and early indication of tropicalization impacts on marine ecosystems.
Led by the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB) and published in Global and Planetary Change , this research identifies for the first time a process of plankton tropicalization in the western Mediterranean.
The Mediterranean is one of the world’s major biodiversity hotspots and, at the same time, one of the most vulnerable regions to climate change. Until now, most studies on changes in marine biodiversity have focused mainly on organisms that humans directly interact with or consume, such as fish. In contrast, plankton has received little attention, despite its fundamental role as the base of marine food webs and in the functioning of marine ecosystems.
“This study is particularly timely and relevant because it approaches biodiversity change from the perspective of microscopic plankton, with important implications in the largest ecosystem on Earth: the surface ocean. The results show that rising sea surface temperatures have already altered the base of marine food webs, namely planktonic primary producers and primary consumers, which are essential for the functioning and stability of ocean ecosystems,” explains Arturo Lucas, ICTA-UAB researcher and lead author of the study.
To detect these changes, the research team analyzed marine sediment records from the Alboran Sea, in the western basin, and from the Strait of Sicily, in the central Mediterranean Sea. These sediments act as natural archives, preserving fossil remains of plankton accumulated over time. Using this record, the biodiversity patterns were reconstructed over the past two millennia.
The study focuses on two dominant groups of calcifying plankton: coccolithophores, which are photosynthetic microalgae, and planktonic foraminifera, which belong to zooplankton. Both groups play a key role in regulating the marine carbon cycle and seawater chemistry and serve as effective environmental indicators, recording early changes in ocean conditions.
The results reveal contrasting responses of these groups to ocean warming. While coccolithophore diversity has increased rapidly since the onset of the Industrial Era, foraminiferal diversity has declined. These opposing trends are explained by differences in physiological and ecological traits and reflect how an increasingly warm, stratified, and nutrient-depleted sea favors some species over others.
One of the most remarkable findings is the increase in Gephyrocapsa oceanica , a coccolithophore species more common in tropical Atlantic waters that has long been present in the Mediterranean and disperses through the Strait of Gibraltar. Until now, this species had only been abundant in the Mediterranean during past warm periods, reinforcing its value as an indicator of ongoing warming.
In addition, although some Mediterranean species remain common throughout the study period, the results show that others are progressively being replaced by species adapted to warmer, nutrient-poor waters. “These changes are consistent with projections from climate and species distribution models, and point to a reorganization of planktonic communities,” says Arturo Lucas.
Although microscopic plankton is almost invisible to the naked eye, researchers warn that these changes may have cascading effects on marine ecosystem functioning. “It is important to bear in mind that alterations at the base of the food web can propagate to higher trophic levels, affecting the overall balance of the marine ecosystem,” explains Patrizia Ziveri, ICTA-UAB researcher and co-author of the study.
The research highlights that tropicalization of the Mediterranean, particularly in the western basin, is no longer a future projection but an ongoing process, emphasizing the importance of plankton studies for understanding how climate change is transforming one of the most sensitive seas on the planet.
Global and Planetary Change
10.1016/j.gloplacha.2026.105314
Experimental study
Not applicable
Tropicalization and biodiversity restructuring of calcifying plankton in a rapidly warming Mediterranean Sea
27-Jan-2026