The Mediterranean Sea, a region renowned for its exceptional biodiversity and ecological significance, is undergoing profound transformations driven by the relentless force of climate change. While the eastern Mediterranean basin has been the focus of numerous studies documenting tropicalization—the process by which warmer water species invade and alter the ecosystem—the western Mediterranean has historically shown more resilience to such changes. However, a groundbreaking study recently published in Global and Planetary Change by researchers from the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB) reveals compelling evidence that tropicalization is not only imminent but currently underway in the western Mediterranean as well, manifesting through shifts in microscopic marine life.
This pioneering research focuses on calcifying plankton, tiny yet ecologically crucial organisms that constitute the base of marine food webs. Specifically, the study analyzes two key groups: coccolithophores, photosynthetic microalgae that contribute significantly to carbon cycling through their production of calcium carbonate plates, and planktonic foraminifera, a group of zooplankton with complex calcium carbonate shells that serve as important indicators of oceanographic changes. These calcifiers also play a pivotal role in modulating seawater chemistry and carbon sequestration, making their study essential for understanding broader ecosystem dynamics in a warming sea.
Using sediment core samples from the Alboran Sea in the western basin and the Strait of Sicily in the central Mediterranean, the team reconstructed biodiversity patterns extending back two millennia. Sediment layers function as natural repositories, preserving calcified remnants of planktonic species and thereby enabling a chronological examination of marine biodiversity through time. This historical perspective allowed the researchers to detect subtle but meaningful shifts in plankton communities, likely driven by rising sea surface temperatures and changing nutrient profiles associated with ongoing climate change.
Results from these sediment records revealed a striking dichotomy between the two dominant planktonic groups. Coccolithophore diversity has exhibited a robust increase since the onset of the Industrial Era, whereas the diversity of planktonic foraminifera has diminished. This divergent trend is rooted in species-specific physiological and ecological traits that confer differential adaptability to warmer, more stratified, and nutrient-poor waters characteristic of the modern Mediterranean environment. Coccolithophores, with their adaptability and rapid reproductive cycles, seem to thrive under these conditions, while foraminifera, which have more restrictive ecological niches, suffer declines.
A particularly noteworthy finding is the marked proliferation of Gephyrocapsa oceanica, a coccolithophore species typically abundant in tropical Atlantic waters but previously limited in the Mediterranean to sporadic warm intervals in geological history. The current abundance of G. oceanica underscores the rapid pace of ocean warming and serves as a robust bioindicator of tropicalization, signaling that conditions once limited to warmer oceanic regions are now encroaching into the Mediterranean basin. This species’s expansion further underscores the connectivity between the Atlantic and Mediterranean via the Strait of Gibraltar, facilitating species migration.
Moreover, the study documents a gradual replacement of traditional Mediterranean plankton species by those better adapted to elevated temperatures and oligotrophic—nutrient-poor—conditions. This ongoing biodiversity restructuring is consistent with projections from advanced climate models and species distribution analyses, which forecast such ecological shifts in response to global warming. These results serve as an early warning, highlighting the susceptibility of marine base communities to environmental changes that could propagate through the entire trophic pyramid.
Understanding the implications of planktonic community shifts is paramount because these microscopic organisms form the foundation of marine ecosystems. Alterations at this base level can cascade to higher trophic levels, influencing fish stocks, marine mammals, and even human economies reliant on marine resources. Changes in primary producer and consumer dynamics might disrupt nutrient cycling, carbon sequestration, and overall ecosystem stability, thereby affecting the Mediterranean’s capacity to sustain its renowned biodiversity and productivity.
The research team emphasizes the urgency of integrating plankton studies into broader marine climate change research agendas. Historically underrepresented compared to studies on commercially important fish species, planktonic organisms offer a sensitive and early indication of ecosystem response to climate stressors. By leveraging sedimentary archives, this study bridges paleontological methods with contemporary ecological questions, offering a temporal depth to detect subtle but critical biotic shifts often missed in short-term observational studies.
Importantly, this investigation challenges the perception that tropicalization in the Mediterranean is primarily an eastern basin phenomenon fueled by the influx of tropical species through the Suez Canal. Instead, it highlights that internal ecological mechanisms, coupled with external influences like Atlantic species ingress, are driving tropicalization throughout the entire basin, including the western Mediterranean. This reorients conservation priorities and prompts a reevaluation of management strategies to encompass microscopic biodiversity and ecological processes foundational to marine ecosystem health.
The findings also raise critical questions about the future trajectory of the Mediterranean ecosystem as climate warming persists. The continued restructuring of planktonic communities may alter the biogeochemical cycling of carbon and nutrients, with potential feedbacks on atmospheric CO2 levels. Such feedbacks could influence the Mediterranean’s role in global climate regulation. Additionally, shifts in plankton communities may affect the marine food web’s complexity, resilience, and ability to support fisheries and other economically important services.
In conclusion, this study from ICTA-UAB provides a comprehensive, data-driven narrative of how the Mediterranean Sea’s microscopic life forms are rapidly reshaping under climate change pressures. The emergence of tropical species and the decline of others not only provide clear biological markers of warming but also signal fundamental transformations likely to ripple through the entire marine food web. Recognizing and addressing these changes is crucial for safeguarding one of Earth’s most sensitive and biodiverse marine regions amid the accelerating climate crisis.
Subject of Research: Not applicable
Article Title: Tropicalization and biodiversity restructuring of calcifying plankton in a rapidly warming Mediterranean Sea
News Publication Date: 27-Jan-2026
Web References: http://dx.doi.org/10.1016/j.gloplacha.2026.105314
Keywords: Mediterranean climate, Biodiversity, Biodiversity conservation, Biodiversity indicators, Biodiversity loss, Biodiversity threats, Marine biodiversity, Phytoplankton
