Quantifying the loss of filtration services following mass mortality of invasive dreissenid mussels

Freshwater mussels are ecosystem engineers that provide important ecosystem functions and services, such as bio-filtration. A method for quantifying filtration capacity in rivers was developed and applied in a case study that estimated the loss in filtration services provided by invasive alien dreissenid mussels following a mass mortality event in the river Meuse in the Netherlands. Mass mortality resulted from a sudden water level drop caused by damage to a weir during severe winter conditions. The low water level allowed an assessment of dreissenid densities on river groynes. Imagery acquired by an unmanned aerial vehicle (UAV) was used to construct 3D images of groynes and to determine surface areas inhabited by mussels. A bootstrapping approach was applied to assess the loss of filtration capacity due to the mass mortality event. The filtration capacity of dreissenids on groynes was sufficient to filter 0.3% to 12.3% of the discharge in a 25 km impounded stretch of the river Meuse during high and low discharges, respectively. On average, 3.6% of the discharge could be filtered by dreissenids on the groynes. Mortality on air exposed groynes was 100% leading to a full loss of dreissenid filtration capacity at these locations. Initial recolonization of groynes formerly exposed to air was observed 17 months after the low water event, ultimately leading to a recovery of the filtration capacity.