Beneath the sun-drenched surface of the Mediterranean Sea, a profound transformation is unfolding—one visible only through microscopes yet reverberating through entire food webs. Scientists have documented the first comprehensive evidence that the ancient sea's microscopic plankton communities are undergoing "tropicalization," fundamentally reshaping marine ecosystems that have remained stable for millennia.
Researchers from the <org>Institute of Environmental Science and Technology</org> at <org>Universitat Autònoma de Barcelona</org> analyzed marine sediment records spanning two thousand years, revealing dramatic shifts in plankton populations linked directly to rising ocean temperatures. The findings, published in Global and Planetary Change, illuminate how climate change is rewriting the rules of life at the foundation of marine ecosystems.
"Rising sea surface temperatures have already altered the base of marine food webs," explained lead researcher Arturo Lucas, whose team uncovered contrasting responses between two critical plankton groups that together form the invisible architecture supporting Mediterranean marine life.
The Microscopic Invasion
At the heart of this transformation lies Gephyrocapsa oceanica, a warm-water coccolithophore—microscopic photosynthetic algae encased in intricate calcium carbonate plates—typically found in tropical Atlantic waters. This species has become increasingly abundant in the western Mediterranean since the Industrial Era, serving as a living thermometer documenting the sea's warming.
The research revealed that coccolithophore diversity has increased rapidly over the past century and a half, as warm-water species migrate northward into historically cooler waters. Meanwhile, foraminifera—tiny zooplankton that occupy a different ecological niche—experienced declining diversity during the same period, suggesting the transformation favors some life forms while disadvantaging others.
