Diatom Diversity, Community Composition, and Ecological Status of Freshwater Ecosystems in the Lower Danube and Black Sea Coastal Region of Bulgaria and Romania
Introduction: Freshwater ecosystems of the Lower Danube and Black Sea coastal region are among the most valuable aquatic habitats in southeastern Europe, supporting exceptional biodiversity and providing essential ecosystem services. They comprise interconnected riverine, floodplain, and coastal freshwater ecosystems that differ in hydrological connectivity, physicochemical conditions, and ecological functioning. Many of these ecosystems belong to the Bulgarian and Romanian LTER/eLTER network, where long-term ecological observations improve our understanding of ecological processes and support biodiversity conservation, restoration, and sustainable management. However, hydrological alteration, floodplain regulation, eutrophication, habitat modification, and climate-related changes have increasingly affected these ecosystems, highlighting the need for ecological assessment. Diatoms are widely recognized as sensitive indicators of environmental conditions and ecological status and are routinely applied in biomonitoring under the European Water Framework Directive (WFD). Nevertheless, regional assessments of benthic diatom diversity and ecological status across these representative freshwater ecosystems of the Lower Danube and Black Sea coastal region remain limited. Scope: The study aimed to assess the ecological status, diatom diversity, and community composition of freshwater ecosystems in the Lower Danube and Black Sea coastal region of Bulgaria and Romania. Particular emphasis was placed on evaluating the relationships between benthic diatom assemblages, ecological status, and key environmental gradients, especially water depth and electrical conductivity. Materials and Methods: The study included representative freshwater ecosystems from the Bulgarian LTER network and the Romanian LTSER Bălțile Brăilei Research Platform, comprising two Lower Danube River sites (Bulgaria and Romania), Srebarna Lake, Brăila lakes (Blasova and Zaton), Carasuhat Canal, Razim Lake, Durankulak Lake, Shabla Lake, and Ezerets Lake. These sites represent complementary riverine, floodplain, and coastal freshwater ecosystems within a common Lower Danube–Black Sea biogeographical region. Srebarna is a UNESCO World Heritage Site, Ramsar Site, and Biosphere Reserve, while Blasova and Zaton are relict lakes of the historical Lower Danube floodplain. Razim Lake represents the freshwater component of the Razim–Sinoie lagoon system, and the Bulgarian coastal lakes are protected wetlands of international conservation importance. At each site, in situ measurements of water temperature, dissolved oxygen, electrical conductivity, pH, water depth, and other key physicochemical variables were performed. Benthic diatom samples were collected, processed, and identified according to the most recent European standards. Species richness, Shannon diversity (H′), community composition, and ecological status based on the Indice de Polluo-sensibilité (IPS), expressed as the Ecological Quality Ratio (EQR), were determined. Relationships between benthic diatom assemblages, ecological status, and environmental variables were evaluated. Results: A total of over 300 diatom taxa were identified across the investigated ecosystems, demonstrating considerable diversity and variation in community composition among riverine, floodplain, and coastal freshwater habitats. The analyses indicate differences in ecological status and benthic diatom assemblages among the investigated ecosystems, corresponding to variation in environmental conditions. Taxa indicative of elevated ionic conditions were particularly well represented in Brăila lakes, where halobious species comprised a high proportion of the recorded flora. Conclusions: The investigated freshwater ecosystems encompass a broad range of ecological conditions representative of the Lower Danube and Black Sea coastal region. Preliminary findings suggest that water depth and electrical conductivity are among the principal environmental gradients influencing benthic diatom assemblages and ecological status. This study provides a regional baseline for assessing freshwater biodiversity and ecological quality across Bulgaria and Romania and contributes to long-term ecological research of freshwater ecosystems.