Case studies: Eelgrass Lommabukten Skåne

Seagrass meadows effect on wave and currents

Seagrass meadows are highly productive ecosystems that perform numerous functions, such as providing food, habitat and nursery areas for numerous marine species. The most common seagrass in Sweden is eelgrass (Zosteramarina) which inhabits the shallow areas in both high-salinity waters as well as brackish waters. The eelgrass forms meadows which are highly important coastal ecosystems. Eelgrass may also perform additional ecosystems functions such as reducing the force of winds and waves. The root system of eelgrass binds the sediment and the plant shootsslow down the water movement.

In Skåne, the most common beach type consists of sand and gravel, where erosion and accumulation of sand occur alternately. Significant erosion occurs on smaller parts of the coastal stretch. It is not well understood and not studied along the Swedish coast, if eelgrass meadows function as a natural costal protection in coastal areas against erosion.

Method

The project aims to assess if eelgrass meadows attenuate wave conditions in Skåne. It also aims to increase the knowledge of eelgrass meadows in high energy environmentswhich are understudied today. We address these aims through 3 main objectives:

  • Measure waves and currents in an eelgrass meadow using field measurements inLommabay in Skåne.
  • Measure the effect of plant densities on wave damping.
  • Calculate the potential wave damping effect of eelgrass beds.

Results

Wave gauges and a current flow meter were deployed in Lomma bay during 3 months (2-Aug to 5-Nov) in two sites with high and low plant density. In addition, eelgrass cover was measured in the field using an underwater drop-video camera. Three storms were recorded during the deployment where the sea level also raised above average.

Results show that wave heights decrease as it propagates through the meadow. The wave reduction during storm events was on average 35-40% at the eelgrass dense site,and 5-10 %at the eelgrass sparse site. A simple model was built to calculate the wave decay over the meadow, suggesting that the main cause for wave reduction was the different drag effects caused by the dense and sparse vegetation.

The results also showed that sand from the artificial beach located in the south of the Lomma port seems to have been transported 300 m to deeper water (3m depth). In this section of Lomma Bay, wave decay was also lesssince the eelgrass density was low. This sand could potentially affect the eelgrass shallow limit even further by burying the plants.

Local context

The eelgrass morphologies vary from shallow to deep waters along the coastTherefore, eelgrass cover, leaf lengths, leaf width and number of leaves per shoot needs to be measured at every site. Wave attenuation by seagrass canopies has been measured at coastal systems where canopies occupy a large fraction of the water column with a ratio of 1:1 to 1:3 seagrass length to water depth. In this study, the seagrass canopy occupied a small fraction of the water column with a ratio of 1:6 to 1:12, showing that wave decay due to vegetation is still possible at these ratios.

Future research directions

  • More studies in other eelgrass meadows along the coast of Skåne
  • Studies throughout the year as seasonal change in shoot density could impact wave propagation.
  • Assess the effect of the artificial beach on the shallow limit of eelgrass.

Documents

All documents of the BwN project can be found at publications,  below you find the documents belonging to this specific case study:

Upcoming report: Promising areas for restoration of eelgrass along the coast of Skåne.