Lost water filters – historic and current contributions of freshwater pearl mussels (Margaritifera margaritifera) to the natural-filtration capacity of a river catchment

Abstract The freshwater pearl mussel ( Margaritifera margaritifera , FPM) was 200 years ago widespread in the upper Weiße Elster catchment (Vogtland, Germany). Due to population collapse, only a few hundred individuals remain today, primarily from captive breeding activities. This decline significantly reduced the species’ filtering capacity and its contribution to the river’s natural-filtration capacity. To assess this contribution, we measured FPM clearance rates and estimated the historical population sizes based on historical records. We also examined how shell length, water temperature, and food quality affect the clearance rates. At 15–19 °C (typical summer conditions), FPM showed an average clearance rate of 1.95 L h −1 mussel −1 . Rates varied depending on shell length and food quality. Based on historical data, the population size around 1800 likely ranged from minimum 250,000 to 3,125,000 individuals. Relating these estimates to the summer discharge of the river catchment, FPM may have filtered 2.7–33.6% of the flow around 1800. In contrast, today’s remaining population filters just 0.027%. It can therefore be concluded that FPM once played a substantial role in maintaining water quality in the Weiße Elster catchment, a role that has become negligible today.