Netherlands: Restoration of Eelgrass in the Western Wadden Sea

Project Stage:
Completed

Start Date:
2002-06-07

End Date:
2005-06-07

Primary Causes of Degradation

Degradation Description

In the beginning of the 1930s, the robust type of eelgrass vanished completely from the Wadden Sea. during the “˜wasting disease’ epidemic that affected eelgrass populations worldwide. The natural recovery of eelgrass was poor, probably due to intensive engineering activities (e.g. Nienhuis and de Bree, 1977; Nienhuis, 1983; Reise, 1985), poor visibility in the water column, fishery activities (Giesen, 1990ab; de Jonge et al., 2000) and increased nutrient loads in the 1970s and 1980s (van Katwijk et al., 1997, 1999, 2000). By that time, the abundance of eelgrass in the Wadden Sea had been reduced by more than 99% since the 1930s (de Jonge et al., 2000).

Reference Ecosystem Description

Seagrasses were commonly found in Dutch coastal waters well into the 20th century. At that time, the fields of Eelgrass (Zostera marina) in the Wadden Sea extended over a total area of between 65 and 150 square kilometres. There were two types of eelgrass–a robust type occurring below mean sea level (MSL) and a flexible type occurring in the intertidal zone.

Project Goals

The main objective of the experiment is to re-establish a self-sustaining eelgrass population in the western Wadden Sea, which will form the source for further expansion to other parts of the Dutch Wadden Sea.

Monitoring

The project does not have a monitoring plan.

Stakeholders

The stakeholders of the Wadden Sea area include politicians, civil servants, managers, interested citizens, NGO’s and the press.

Description of Project Activities:
The first step in the implementation of the project was to select planting locations in the western Wadden Sea. Sites were selected using the following criteria: (1) eelgrass used to grow naturally in the selected area in the past; (2) the area should have natural protection against prevailing winds; (3) the area should have some freshwater input; (4) no fishing activities or bait digging should be allowed in or in the proximity of the area. Balgzand, a tidal area of approximately 6,000 hectares in the western Wadden Sea, was ultimately chosen to be the main area for the project, as it fits all the selection criteria. Specific transplantation areas within Balgzand were chosen on the basis of a few additional criteria (van Katwijk and Wijgergangs, 2004): (1) sediment should be stable and not too coarse; (2) depth of the location should be between +15 and -20cm MSL; (3) wave exposure should be slight; (4) drainage should be moderate, with a film of water remaining on the tidal flats during low tide. The salinity at the selected transplantation site was about 24 PSU. The seedlings that were to be used for transplantation were collected at the tidal area Hond/Paap in the Ems Estuary of the eastern Dutch Wadden Sea. Seedlings were dug up by hand (only one seedling per 9m2 to guarantee genetic diversity), and were carefully rinsed and transported to the new locations at the temperature of collection. Transplantation to the experimental sites took place on 7 and 8 June 2002, and about 1,500 eelgrass seedlings were initially planted. A planting unit (PU) consisted of 37 (or 61) seedlings transplanted in a hexagon bed. The mutual distance between two neighboring shoots was constant (in six directions) by the nature of this hexagon shape. Shoot density was set at a mutual distance of 30cm (high density = 14 seedlings - m-2) or 50cm (low density = 5 seedlings - m-2). High density (HD) and low density (LD) PUs were always transplanted in pairs. The imaginary line between the two center points of all pairs of PUs was fixed at 50°N to assure similar exposure to the tidal currents. The transplanted plots were monitored on several occasions, and it was soon evident that survival of the shoots was low. It was learned that incorrect depth data for the experimental sites had been provided, and in July, new seedlings were transplanted to three new locations in the Balgzand area. This new transplantation was also unsuccessful, probably due to a prolonged period of unfavorable wind direction right after planting. In an effort to compensate for the lack of settled plants, seeding shoots were collected in August and were transplanted in September after a maturation period in the laboratory. Also in September, stabilization techniques were installed on the experimental location. They consisted of mussel seed, which was deposited on a bed of cockleshells, and wicker screens (as a spare technique). These stabilization techniques were intended to provide shelter for the eelgrass transplants that followed in June 2003, with 1,800 seedlings planted at depths of c. - 30 cm MSL. Finally, in June 2004, 1,400 additional seedlings were planted at locations where survival rates had been the highest in 2003.

Ecological Outcomes Achieved

Eliminate existing threats to the ecosystem:
After the initial failure due to poor depth data, about 50% of the newly transplanted shoots had grown into eelgrass plants by the end of the growing season at one location. The plants developed reproductive shoots, but no seeds were observed. Moreover, no seedlings were found to develop in spring 2004, probably due to the low seed production. As a result of the June 2004 planting, about 40% of the new seedlings survived by the end of the growing season and produced reproductive shoots. An intensive study into seed production proved that seeds were present, which supported expectations that developing seedlings were to be found by the spring 2005. During the course of the present study, survival ranged from 28 to 68% seven weeks after transplantation. Although these surviving plants produced reproductive shoots, winter survival and seedling growth were low. Nevertheless, an intensive study on seed maturation in 2004 showed the existence of relatively high numbers of ripe seeds (Bos et al., 2005). Hopefully this will result in the development of seedlings in 2005.

Factors limiting recovery of the ecosystem:
The initial failure of the transplantation was a result of poor depth data. The elevation of the tidal flats had been measured erroneously, resulting in a transplant at a depth of 22 cm below MSL, instead of at 5 cm below MSL. It is known from literature that conditions at 20 cm below MSL are too dynamic for unprotected eelgrass shoots. The second transplant--done in July--also failed, probably due to a prolonged period of unfavorable wind direction right after planting. A summer transplant is more risky anyway, as the shoots have smaller remaining energy reserves and are thus inhibited from taking root.

Socio-Economic & Community Outcomes Achieved

Key Lessons Learned

The success of transplantation could be affected negatively by local disturbances and therefore it was considered important to simultaneously work in different microhabitats during the eelgrass transplantations. Apart from transplanting at different locations, transplant density, transplant depth and number of shoots per planting unit were varied. High density PUs supported survival of transplants at those locations exposed to waves and currents, whereas planting density had no effect on survival of transplants in sheltered habitats (Bos et al., submitted). Shoots were transplanted at different depths in the safe range. However, none to locally small differences in survival of transplants were observed between depths of +2 and +8cm MSL (Bos et al., 2004; Bos et al., 2005). As mutual protection was expected to affect survival of transplants, the size of PUs was varied (37 or 61 plants). No significant differences were observed though.

Co-occurrence of blue mussels (Mytilus edulis) and eelgrass has often been described with special emphasis to the protective function of the blue mussel (e.g. van Katwijk and Hermus, 2000). Experiments done by Bouma et al. (subm.) showed that the blue mussel facilitates eelgrass by reducing the drag force on eelgrass shoots when exposed to currents. We tested this relationship at a relatively exposed location and found that transplanted eelgrass shoots had a significantly higher survival in mussel beds than without mussel bed protection (Fig. 3B; Bos et al., submitted). However, all plants disappeared towards the end of the growing season. This suggested that a protective mechanism of the mussel bed was present, but not strong enough to support long-term survival.

Sources and Amounts of Funding

This project was funded by the Dutch Directorate-General for Public Works and Water Management

Other Resources

Project Website: www.zeegras.nl

Contact:
Information Desk of the Department of Public Works, Directorate North Holland:
Email: info@dnh.rws.minvenw.nl

Primary Contact

Organizational Contact