MIGHT SHELL PRODUCTION BY “INVASIVE” AMPHISTEGINA LOBIFERA LOCALLY AMELIORATE CONSEQUENCES OF SEA-LEVEL RISE IN WARMING MEDITERRANEAN COASTAL WATERS?

Amphistegina lobifera is a relatively large (adult diameter ~1-2 mm), robust, algal-symbiont-bearing foraminiferal species that thrives in clear, nutrient-poor coastal waters throughout most of the subtropical/tropical Indo-Pacific. Drilling at Funafuti Atoll, reported in 1900, noted that Amphistegina shells were the primary constituents of sand deposits associated with reef formation. Studies in the mid-20^th century further noted the predominance of the shells of Amphistegina and Calcarina in Holocene reef sediments, though their predominance in beach sands was recognized as enhanced by hydrodynamic sorting and by the resistance of these shells to abrasion compared to most other carbonate constituents. Likely introduced through the Suez Canal, in recent decades A. lobifera have proliferated in the eastern Mediterranean Sea and are expanding westward. As Mediterranean waters warm, these foraminifers are thriving in oligotrophic, shallow-water habitats. In some areas their shells are reported to have accumulated to depths of a half meter or more in coastal sediments. The success of A. lobifera has raised concerns about its proliferation as an invasive species. However, assessments of foraminiferal assemblages along nutrient gradients have shown dominance only distant from nutrient sources, whether natural or anthropogenic. The genus Amphistegina appeared in the Eocene under higher atmospheric CO₂ concentrations than present. Experimental studies have found that these protists have retained the ability to sustain shell growth rates under elevated pCO₂, possibly as a consequence of enhancement of photosynthesis rates of their diatom symbionts. Warming temperatures also increase metabolic rates, increasing the energetic advantages of algal symbiosis in oligotrophic environments. As surface waters continue to warm, the already elevated salinity and alkalinity in the eastern and central Mediterranean will further enhance carbonate production by A. lobifera and possibly by other large foraminifers. Might hydrodynamic concentration of their shells by nearshore wave action actually provide some amelioration of the consequences of sea-level rise? This hypothesis can certainly be explored in future studies of coastal dynamics in this unique region.