Sand capping to promote eelgrass restoration

Eduardo Infantes1, Hanna Corell2, Louise Eriander1, Erik Kock Rasmussen3, Henrik Möller1, Matilda Rasmusen1, Lea
Stolpman1, Per-Olav Moksnes1,
1 Department of Marine Sciences, University of Gothenburg, Sweden
2 DHI Sverige, Stockholm Sweden
3 DHI Denmark, Hörsholm Denmark

Recent studies along the Swedish NW coast suggest that eelgrass recovery and restoration are challenged by local
regime shifts following the loss of eelgrass and their stabilizing effect on the sediment. This shift has increased
sediment resuspension and turbidity that prevent growth of eelgrass. Monitoring shows a decrease in water clarity of
~2 m at historic eelgrass sites, and test-planting demonstrated that poor water quality prevented eelgrass survival at
over 90% of the assessed historic sites. Moreover, losses of eelgrass continue today as the turbid water from historic
eelgrass sites appear to spread to neighboring areas causing decreased water quality and collapse of the meadows
there, in a chain reaction1.
Since 2018 we have assessed if sand capping, i.e. placing a 10 cm thick layer of sand and gravel on top of the
sediment, could stabilize the bottom, decrease sediment resuspension and improve the conditions enough to allow
eelgrass growth at historic sites. The goal of the study was to assess if sand capping could be used together with
eelgrass restoration to facilitate the return of eelgrass to historic sites, and stop the ongoing losses of eelgrass.
To assess if eelgrass could grow on sediment from a natural gravel pit on land (grain size 0.25-4 mm), if any negative
effect of sand capping could be found on infauna, and to select the best site for a large-scale study, a pilot study was
carried out where replicate 1 m2 sand plots were placed at two different depths in four historic eelgrass sites along the
Swedish NW coast and planted with eelgrass. The results showed higher growth rate of eelgrass, and similar or
higher abundance of infauna on sand plots compared to natural sediment at all sites. Erosion and sedimentation on
the sand plots was a problem at several sites where all planted eelgrass died within a year. High survival and growth
of eelgrass was only found at one site (Askerön), where continuing studies were carried out.
To identify the necessary size and optimal 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). Consistent with
field measurement, 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 showed further that sand capping could
decrease sediment resuspension in the bay and identified the optimal location to place a one-hectare large sand
capped area to reach a tipping-point where eelgrass could show positive growth if planted on the sand.
To test if sand capping could reduce resuspension and promote eelgrass growth, a large-scale study was carried out in
the March of 2021, where 1800 ton of sand and gravel were placed in a 10 cm thick layer at the selected area in the
bay, covering a one-hectare large area at 1.3-1.9 m depth. The sand was transported to the site in shallow barges and
spread over the area using an excavator placed on a barge and fitted with a high-precision GPS. The average
thickness of the sand capped area was impressively even (9.3±1.3 cm) and monitoring showed that resulting
sediment plumes from the work did not cause turbidity or sedimentation rates passed background levels further than
50 m from the capped area. Small patched of mussels (Mytulis edulis) and oysters (Crassostrea gigas) where found
on top of the sand after the capping was terminated, indicating that they have moved up through the sand and
apparently not harmed by the capping. The community of infauna within the sand capped and in adjacent areas will
be compared in the fall of 2021 to assess any possible negative impacts.
In May to July 2021, 80 000 shoots of eelgrass were successfully planted on top of the sand capped area in 1x1 m
checker pattern (16 shoots m-2 within planted squares). The survival and growth of eelgrass, wave attenuation, flow
changes, turbidity, sedimentation rate and light conditions will be carefully monitored within the planted and
adjacent areas in the years to come to assess the effect of the sand capping and eelgrass restoration.
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.

References
1. Moksnes P-O, Eriander L, Infantes E, Holmer M. 2018. Local regime shifts prevent natural recovery and restoration of lost

eelgrass beds along the Swedish west coast. Estuaries and Coasts. 41:1712–1731. DOI: 10.1007/s12237-018-0382-y