Why Amphistegina lobifera, a tropical benthic foraminiferal species, is thriving at temperate latitudes in the Mediterranean Sea

Abstract

Amphistegina lobifera Larsen is a relatively large (adult diameter ∼1–3 mm), robust, diatom-symbiont-bearing benthic foraminiferal species that thrives in clear, nutrient-poor coastal waters throughout much of the subtropical/tropical Indo-Pacific. Studies in the late 19th to mid-20th century noted the predominance of the shells of Amphistegina and Calcarina in Holocene reef sediments, enhanced by hydrodynamic sorting and by the resistance of these shells to abrasion. Having migrated through the Suez Canal at least 60–80 years before present, A. lobifera populations have proliferated in the eastern Mediterranean Sea and have expanded their range westward. As Mediterranean waters warm, these foraminifers are thriving in oligotrophic, shallow-water habitats, and have produced shell accumulations of a half meter or more in some areas. While the success of A. lobifera has raised concerns about its proliferation as an invasive species, assessments of foraminiferal assemblages along nutrient gradients have shown dominance only distant from nutrient sources, whether natural or anthropogenic. The genus Amphistegina emerged in the Eocene under higher atmospheric CO2 concentrations than present and experimental studies have shown that these foraminifers can sustain shell-growth rates under elevated pCO2. Warming temperatures also increase metabolic rates, increasing the energetic advantages of algal symbiosis. As surface waters continue to warm, the already elevated salinity and alkalinity in the eastern and central Mediterranean should further enhance carbonate production by A. lobifera and possibly by other benthic foraminifers that host algal symbionts. Observed rates of accumulation by hydrodynamic concentration of dead shells by nearshore wave action are comparable to rates of ongoing sea-level rise. Might this return of prolific shallow-water carbonate production ultimately prove at least locally beneficial as climate change progresses?