| Summary: | Abstract The unique aquatic Pontocaspian (PC) biota of the Black Sea Basin (BSB) is in decline. The lack of detailed knowledge on the status and trends of species, populations, and communities hampers a thorough risk assessment and precludes effective conservation. This paper reviews PC biodiversity trends in the BSB (Bulgaria, Romania, Moldova, Ukraine, and Russia) using endemic mollusks as a model group. We aim to assess changes in PC habitats, community structure, and species distribution over the past century and to identify direct anthropogenic threats. The presence/absence data of target mollusk species were assembled from literature, reports, and personal observations. Pontocaspian biodiversity trends in the northwestern BSB coastal regions were established by comparing 20th‐ and 21st‐century occurrences. The direct drivers of habitat and biodiversity change were identified and documented. We found that a pronounced decline of PC species and communities is driven by (a) damming of rivers, (b) habitat modifications that disturbed previous natural salinity gradients and settings in the studied area, (c) pollution and eutrophication, (d) invasive alien species, and (e) climate change. Four out of the 10 studied regions, namely, the Danube Delta–Razim Lake system, Dniester Liman, Dnieper–Bug estuary, and Taganrog Bay–Don Delta, contain favorable ecological conditions for PC communities and still host threatened endemic PC mollusk species. Distribution data are incomplete, but the scale of deterioration of PC species and communities is evident from the assembled data, as are major direct threats. Pontocaspian biodiversity in the BSB is profoundly affected by human activities. Standardized observation and collection data as well as precise definition of PC biota and habitats are necessary for targeted conservation actions. This study will help to set the research and policy agenda required to improve data collection to accommodate effective conservation of the unique PC biota.
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