Environmental Drivers and Multi-Trophic Assemblages in the Romanian Black Sea: Insights into Food-Web Structure

This study investigates how environmental gradients shape the structure and interactions of coastal biotic assemblages in the Romanian Black Sea. A multi-trophic analysis was conducted across a network of stations in June 2023, integrating phytoplankton, microzooplankton, mesozooplankton, macrozooplankton, ichthyoplankton, and macrozoobenthos with key physico-chemical parameters. Principal component analysis revealed strong north–south contrasts: the northern sector was characterized by nutrient enrichment (nitrate, ammonium, and silicate) supporting phytoplankton blooms and microzooplankton peaks, while the southern sector showed more saline conditions and extended trophic coupling from phytoplankton through mesozooplankton to ichthyoplankton. The central sector appeared transitional, with community structure more closely related to oxygen and phosphate levels. In the north and south, plankton dynamics were strongly linked to nutrient availability, while macrozooplankton consistently aligned with salinity and silicate, reflecting their preference for more marine waters and partial decoupling from nutrient-driven pathways. Fuzzy Cognitive Map analysis indicated combined bottom-up and top-down control, with phytoplankton supporting mesozooplankton and macrozooplankton exerting strong negative pressure. Phytoplankton functioned as the main network driver, with mesozooplankton as the central mediator, and the persistent negative macrozooplankton effect suggests direct biological regulation beyond salinity influence. These findings highlight the dual structuring of Black Sea food webs and provide an integrative, multi-trophic baseline for ecosystem-based management and Marine Strategy Framework Directive Descriptor 4 (Food webs) implementation.