Sediment properties and aquatic vegetation affect resuspension from boats in shallow coastal bays.

Resuspension of sediment and nutrients induced by boating may degrade shallow aquatic ecosystems but the effect differs greatly across habitats and biogeographic regions and the overall impact remains contested. Systematic studies across a range of environmental settings are a critical priority to identify particularly vulnerable ecosystems and support the development of adequate boating regulations that minimize the impact on the aquatic environment. We investigated sediment and nutrient resuspension induced by a standardised boat disturbance (acceleration by medium-sized day-cruiser boats) across habitats with different sediment characteristics and macrophyte vegetation volume (vegetation cover * height) in shallow coastal bays of the Baltic Sea. Sediment characteristics was the most important predictor for boat-driven resuspension, with the largest sediment resuspension recorded at stations with high sediment organic matter and mud content. At the peak of vegetation biomass in late summer and autumn, less sediment was resuspended in areas with greater vegetation volume, indicating that aquatic vegetation can reduce boat-induced resuspension to some degree. Despite the extensive disturbance generated by the experiment, resuspension of dissolved nutrients was limited and restricted to a small release of ammonium at sites with organic-rich sediment. Results suggest that the impact of small boats on aquatic ecosystems can be mitigated by restricting access to shallow areas that are protected from natural waves and currents and are characterised by disturbance-sensitive sediments, or by promoting cautious driving of motor crafts within these environments. To prevent negative effects on habitats by boat wake and resuspension, measures to reduce boating disturbance should be taken already at early signs of reduced vegetation density.