Eelgrass (Zostera marina) restoration on the west coast of Sweden using seeds

Along the northwest coast of Sweden, over 50% of Zostera marina L. (eelgrass) meadows have vanished since the 1980s. With the improvement of conditions, there is a growing interest to restore lost habitats, but methods are lacking for restoration of eelgrass beds at high latitudes where long winters create special challenges. We assessed if seed planting could be used for large-scale restoration, with the aim to identify the major causes of seed and seedling loss and to determine which planting method best increases restoration success. In the laboratory, we identified optimal conditions for long-term seed storage and demonstrated that eelgrass seeds can be successfully stored for 8 mo before being planted in the spring. However, field studies did not find an increased seedling establishment in seeds planted in the spring of 2013 compared to those planted in the fall of 2012. Field results suggest that the main causes for the seed loss were seed predation from crabs, seed transport by currents and bioturbation by lugworms, while the main processes affecting shoot development were light availability and physical disturbance. Covering the seeds with a layer of sand increased seedling establishment 2 to 6 times compared to uncovered seeds, probably through the reduction of seed predation and seed transport, and could potentially be developed as a method to reduce losses during restoration. In addition, positive feedback mechanisms (i.e. sediment resuspension and drifting macroalgae mats) may also prevent natural recovery and restoration success. However, high seed loss (on average 98.6%) and high shoot mortality pose a challenge that need to be addressed before restoration using seeds can be recommended for large-scale restoration.