Tropicalization and biodiversity restructuring of calcifying plankton in a rapidly warming Mediterranean Sea

Climate-driven shifts in species diversity, community composition and phenology can disrupt ecosystem functioning and compromise marine ecosystem stability. The Mediterranean Sea, a global biodiversity hotspot, is particularly vulnerable to anthropogenic climate change and is experiencing a growing biodiversity crisis especially driven by warming, habitat degradation, pollution and the introduction of new species. Yet, current knowledge of biodiversity changes in this region is largely limited to shallow shelf benthic communities and macroorganisms. Calcifying phytoplankton and zooplankton, however, offer a unique lens into pelagic ecosystem dynamics, as their fossil record preserves signals of biodiversity change in deep-sea sediments spanning the Industrial Era and the current phase of rapid warming. This study focuses on the reconstruction of diversity change of two dominant calcifying plankton groups: coccolithophores and foraminifera (primary and secondary producers). We examine two selected sedimentary records of the western and central Mediterranean—Alboran Sea and the Strait of Sicily—spanning the last ~1700 and 200 years of modern climate, respectively. By quantifying abundance, diversity and turnover of species composition we evaluate the potential response of calcifying plankton assemblages to oceanographic changes under anthropogenic climate change. The results revealed contrasting changes in the diversity of the two dominant calcifying plankton groups, with a rapid increase in coccolithophore diversity alongside a decrease in planktonic foraminiferal diversity during the Industrial Era. We attribute these group-specific responses to ecological and physiological differences, particularly in relation to life cycles and water column distribution, under increasingly stratified, nutrient-depleted surface waters driven by rising sea surface temperatures. In addition, this study provides first indication of tropicalization of the western Mediterranean plankton derived from the increasing intrusion and eastward expansion of the coccolithophore Gephyrocapsa oceanica from the Strait of Gibraltar into the Mediterranean Sea. The highest abundances of this species are restricted to the Equatorial Atlantic Divergence Zone and only found in higher proportions in the Mediterranean during past warm periods. Although the temperate-productivity foraminiferal species of Globigerina bulloides and Globorotalia inflata remain the dominant foraminiferal species, previous minor dominant species are being replaced by warm-oligotrophic species, such as Trilobatus sacculifer and Globigerinella spp. These align with model projections of tropical species migrating into the area, previously documented among benthic organisms and point to a broader restructuring of planktonic life. • First assessment of calcifying plankton diversity response to Mediterranean Sea anthropogenic climate change reveals group-specific response. • Evidence of plankton community tropicalization is observed in the western and central Mediterranean via the Strait of Gibraltar, inferred from species trends of abundance, ecological preferences, and paleorecord comparisons. • Sedimentary microfossil records provide essential baselines for biodiversity studies capturing long-term dynamics.