Experimental assessment of benthic foraminifera as salinity bioindicators: Integrating morphological and eDNA approaches

Abstract. Benthic foraminifera are extensively used as bioindicators for paleoenvironmental reconstruction, yet environmental DNA (eDNA) analysis provides a powerful lens to uncover their community diversity and environmental responses. Currently, quantitative experimental studies on their responses to salinity gradients are scarce. Here, sediments from the intertidal zone of Qingdao Bay were subjected to a 10-week controlled culture across 13 salinity levels (0–60 psu), and community dynamics were analysed using both morphological and eDNA approaches. Foraminifera exhibited high tolerance to extreme low (0 psu) and high (60 psu) salinities, and community diversity (Margalef index) increased significantly with salinity (p < 0.05). The relative abundance of calcified Rotaliida declined with increasing salinity (p < 0.01), allowing for the establishment of a robust linear regression model. The eDNA analysis revealed that soft-bodied Monothalamiids – often overlooked morphologically – reached up to 76.2 % relative abundance (average 56.1 %). In contrast, the salinity-driven increase in high-Mg calcite-shelled Milioliida was observed only through morphological analysis. These results demonstrate the distinct but complementary nature of morphological and molecular methods. This study addresses the scarcity of experimental constraints on salinity responses, offering a calibrated reference for applying foraminifera in both ecological assessment and paleo-reconstruction.