Benthic Foraminifera Indicate Environmental Stress from River Discharge To Marine Ecosystems: Example from the Black Sea

Abstract

The focus of this research was to investigate how the Danube River discharge influences environmental conditions and benthic ecosystems on the Black Sea shelf using foraminifera to delineate affected areas. Specific goals included: (1) to examine the taxonomic composition, quantitative distributions, and test morphologies in foraminiferal assemblages; (2) to correlate them with environmental (oceanographic, geochemical, sedimentological) parameters; (3) to identify the main factors associated with changes in foraminiferal distributions; and (4) to identify assemblages and species that indicate environmental changes along the gradient from delta front to unaffected shelf. Foraminiferal assemblages revealed the influence on the benthic ecosystem of the discharge of water and sediments enriched with organic and inorganic compounds from the Danube delta. Three foraminiferal assemblages were recognized, indicating strong (assemblage Ammonia tepida ), weak (assemblage A. compacta ), and undetectable (assemblage A. ammoniformis ) deltaic influence as a function of distance from shore. Sites strongly influenced were characterized by lower diversity and species richness as well as dominance by A. tepida and Porosononion subgranosus mediterranicus . These species are tolerant of fluctuations in salinity and sedimentation, and thrive on labile organic carbon produced by abundant phytoplankton fertilized by nutrients in the fluvial discharge. The boundary between strong and weak influence on bottom ecosystems occurs at the 25 m isobath, which coincides with the distal zone of the delta front. The boundary between weak and no influence coincides with the frontal zone of the prodelta. No fluvial influence was detected on the outer shelf. The progression of foraminiferal assemblages, A. tepida to A. compacta to A. ammoniformis , and the increase of polyhaline Lagenida in the assemblages seawards, indicates that organic carbon flux is the dominant factor. Other factors, such as salinity and oxygen depletion, seem to play secondary roles, while the type of substrate likely contributes primarily to the diversity of microhabitats within the benthic ecosystems, with muddy delta front sediments providing fewer microhabitats than mixed mud and molluscan shells found on the outer shelf.