Saltmarsh foraminifera in the subarctic White Sea: Thrive in summer, endure in winter

Abstract

Abundance and diversity of intertidal benthic foraminifera decreases towards the poles. Well studied in the temperate zone, saltmarsh foraminiferal assemblages have only recently received attention at higher latitudes. We report modern foraminifera, including the taxonomy, abundance, and zonation, from a pristine saltmarsh in the subarctic White Sea, northern Europe. We sampled 10 stations (surface sediment, two replicates) from two high-tide flats covered with lush halophytic plants. Nine foraminiferal species found exhibited distinct vertical zonation confined to certain elevation levels. The high marsh assemblage comprised arenaceous Balticammina pseudomacrescens, Trochammina inflata, Jadammina macrescens, and monothalamous Ovammina opaca. This is the northernmost location where the first two species have been encountered alive. The low marsh assemblage was dominated by Miliammina fusca and Elphidium williamsoni. Summer abundances of live foraminifera were surprisingly higher than on other subarctic saltmarshes studied, mostly 100–500 and up to 3000 per 10 cm³. In winter, under a 40 cm of ice, the abundances were only 15–20 ind./10 cm³, but the species richness was the same. Unlike summer specimens of calcareous foraminifera, which are always brightly colored, winter Elphidium williamsoni all had bleak colorless cytoplasm, suggesting they discard kleptoplasts and abandon algal diet. We conclude that the insulating blanket of fast ice, which hardly moves on saltmarshes, prevents elimination of foraminifera in the harshness of winter, whereas the intense continental heating in summer lets them thrive on White-Sea saltmarshes. The composition of foraminiferal assemblages under local climatic conditions may thus deviate substantially from the latitudinal trend.