Experiment with seeding of eelgrass

The first experiment with sowing of harvested Danish eelgrass seeds has been started in Odense Fjord. The seeds are harvested in Dalby Bay in July 2014 and kept in seawater for the time of sowing. The purpose of seed planting is to investigate the potential for restoring eelgrass beds in Danish waters.

The purpose of seeding of eelgrass seeds in Odense Fjord is to investigate the seeds’ ability to germinate and grow under different physiological conditions. Therefore, the seeds are sown on four different locations in Odense Fjord; two locations with exposed shores with sandy sediment and two protected shores with muddy sediment. Experimental sites are established near existing eelgrass beds, whereas other sites are selected, where eelgrass vegetation previously was observed on the location.

In studies from the US east coast, experiments with sowing with 30 seeds per m2 have been performed, whereas up to 300 seeds per m2 have been sown in Odense Fjord. The many sown seeds in the Danish experiment are supposed to ensure a high degree of replication, because it is expected that the unprotected seed will, due to wave friction, be forced out to deeper water, where the light cannot support the growth of the eelgrass. It is expected that approx. 30-50 % of the seeds will germinate on the protected locations. By having a short distance between the plants, they will protect each other against physical disturbance from waves, current and floating macro algae.

Approach for sowing

The eelgrass seeds are sown in different plots with three replicas for each plot at the four experimental sites as seen in figure 1:

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Figure 1. Experimental site with different plots. The presented experimental set-up is performed on a sandy bottom

In each experiment, it is clarified if it is possible to optimise the sediment conditions for the sprouting seeds and at the same time minimise physical stress from waves, current, floating microalgae and burrowing benthic fauna.

In each experiment, it is investigated how to minimise physical stress from waves, current, floating microal-gae and burrowing benthic fauna and thereby optimise the sediment conditions for the germinating seeds The first plot presented in figure 1 constitutes a testing where no seeds are sown, while the seeds in plot two are sown unprotected and directly into the existing bottom sediment. In plot 3, the seeds are protected against burrowing benthic fauna like lug worm, by the use of coconut fibre mat. The seeds are integrated in the fibre mat, before the mat is placed on the bottom of the fjord as seen in figure 2.
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Figure 2. The process of placing the coconut fibre mat with the purpose of creating a membrane against burrowing benthic fauna is shown. (A) The upper sediment layer is removed in a depth of 3-4 cm, (B) the fibre mat with eelgrass seeds is positioned and attached on the marked site and (C) the removed sediment layer is carried back on the fibre mat with a shovel.

In plot 4 the seeds are sown directly into the bottom sediment but physically protected by cylindrical tubes. This also applies to plot 5 where the seeds are even more protected against burrowing benthic fauna by the use of a fibre mat. Plot 6 is a large-scale experiment conducted with sowing of seeds from a machine.

The described experimental set-up applies for locations with sandy bottom. At the experimental sites on muddy bottom, additional sowing of eelgrass seeds is conducted with sand capping which improves the seeds’ anchoring capacity on muddy bottoms.

Future steps

In the coming year Associate Professor Mogens Flindt and postdoc Thomas Valdemarsen together with master students from University of Southern Denmark will monitor the development of eelgrass plants closely by using under water videos along with physical monitoring. Based on these results, it will be possible to describe the loss of plants over time.

Written by Camilla Vestergaard, SEGES
Updated: 18.12.2014