The Mediterranean is warming faster than the global ocean average. Models predict tropicalization — warm-water species displacing cold-adapted ones. But predictions are tested against observational records that rarely span more than a few decades. Without a long baseline, it is difficult to distinguish a trend from a fluctuation, or to know what “normal” looked like before the Industrial Era.
Published in Global and Planetary Change, Lucas and colleagues at ICTA-UAB analyzed sediment cores from the western and central Mediterranean spanning two thousand years. They tracked two groups of calcifying plankton: coccolithophores and planktonic foraminifera. Since the onset of the Industrial Era, coccolithophore diversity has expanded while foraminiferal diversity has declined — contrasting responses from organisms sharing the same water. The key signal is the increasing abundance of Gephyrocapsa oceanica, a tropical Atlantic coccolithophore that enters through the Strait of Gibraltar and has historically been abundant in the Mediterranean only during past warm periods.
The structural insight is about divergent fates in the same environment. Two groups of calcifying plankton, both at the base of the marine food web, both responding to the same warming — but one expands and the other contracts. The warming does not select uniformly. It selects based on physiological traits: thermal tolerance, nutrient requirements, reproductive strategies. The tropicalization is not a replacement of cold species by warm ones across the board. It is a restructuring that creates winners and losers within the same trophic level, at the same location, in the same water. The base of the food web is reorganizing, and the organisms above it cannot tell the difference between tropicalization and stability until the structure changes enough to propagate upward.