Recent scientific studies have revealed the challenges of eelgrass recovery and restoration due to local regime shifts that follow the loss of eelgrass and its stabilizing effect on sediment. These shifts result in increased sediment resuspension and turbidity, which prevent eelgrass growth in the West coast of Sweden. Monitoring data shows that water clarity has decreased by around 2 meters at historic eelgrass sites. Additionally, test-planting has demonstrated that poor water quality has prevented eelgrass growth at over 90% of assessed historic sites.

In an initial pilot study, we aimed to assess if eelgrass could grow on sediment from a natural gravel pit on land and if sand capping would negatively impact infauna. The study involved placing replicate 1 m² sand plots at two different depths in four historic eelgrass sites, which were then planted with eelgrass. The results showed a higher growth rate of eelgrass and similar or higher abundance of infauna on sand plots compared to natural sediment at all sites. However, erosion and sedimentation on the sand plots proved to be a problem at several sites, resulting in the death of all planted eelgrass within a year. High survival and growth of eelgrass was only observed at one site (Askerön), where the researchers continued their studies.

To determine the optimal size and location of a sand capped area to decrease sediment resuspension at Askerön, a high-resolution 3-D hydrodynamic model was created for the study area (MIKE 3, Flow Model FM), coupled with a biophysical module to simulate sediment resuspension and turbidity (ECOLab, DHI). The model showed that wave-driven sediment resuspension occurred mainly in the shallow part of the bay (<1.5 m), where turbidity was too high for eelgrass growth. It also demonstrated that sand capping could decrease sediment resuspension in the bay and identified the optimal location to place a one-hectare sand capped area to reach a tipping point where eelgrass could show positive growth if planted on the sand. This study is a promising step towards eelgrass restoration and combatting the ongoing losses of eelgrass in historic sites.

In March 2021, a one-hectare area at 1.3-1.9 m depth in the bay was selected for a sand capping experiment to test its potential to reduce resuspension and promote eelgrass growth. Approximately 1800 tons of sand and gravel were transported to the site via shallow barges and placed in a 10 cm thick layer using an excavator with a high-precision GPS. The resulting sand capped area had an impressively even average thickness of 9.3±1.3 cm, and monitoring indicated that sediment plumes did not cause turbidity or sedimentation rates beyond background levels further than 50 m from the capped area. After capping, small patches of blue mussels (Mytulis edulis) and oysters (Crassostrea gigas) were found on top of the sand, suggesting that they were not harmed by the capping and had moved up through the sand. However, the infauna community within the sand capped and adjacent areas will be compared in fall 2021 to assess any possible negative impacts.

Starting in May and continuing through July 2021, a successful planting of 80,000 eelgrass shoots was carried out in a checker pattern on top of the sand capped area, with a density of 16 shoots per square meter. The progress of the eelgrass growth, as well as changes in wave attenuation, flow, turbidity, sedimentation rate, and light conditions, will be closely monitored in both the planted and surrounding areas in the coming years to evaluate the impact of the sand capping and eelgrass restoration efforts.

During the summer of 2022, the eelgrass plants demonstrated resilience by surviving the harsh winter conditions of cold and darkness. Impressively, the 80,000 planted shoots grew to an astounding 860,000 shoots, marking a tenfold increase in just one year. However, the growth distribution of the plants was not uniform across the planting area, with certain sections of the plot exhibiting dense patches while others had small empty gaps.

Acknowledgement. We thank Mogens Flindt from the University of Southern Denmark for many helpful advice on sand capping, and Beatrice Allenius and Anders Olsson from the County Administrative Board of Västra Götaland for great collaboration within the project and providing all necessary permits. This study is supported by grants from the European Maritime and Fishery Fund and the Swedish Agency for Marine and Water Management.