seagrass ecology

The Wadden Sea and Zostera marina restoration

· News and info (in Dutch) about the project 2002-2006 (frequently updated)

· Background info: (Last updated 21 January 2004)

Reintroduction of seagrass (Zostera marina L.) in the western Wadden Sea.

The Wadden Sea is one of the world's largest international marine wetland reserves. Before the 1930s it contained large beds of subtidal and low-intertidal Zostera marina L, whereas many mid-intertidal flats were covered with a flexible form of Zostera marina L., and Zostera noltii Horneman. After the wasting disease in the 1930s, seagrasses survived only in the mid-intertidal zone (a narrow zone around 0 m mean sea level). Here, new losses occurred from the 1970s onwards. Increased turbidity, increased shellfish fisheries, increased construction activities and increased nutrient loads are the main factors that have caused the losses and lack of recovery, although the causes of the wasting disease losses during the 1930s are still open for dispute (den Hartog 1996, Philippart 1994, van Katwijk 2000). Currently, Dutch seagrass beds cover 2 km², German seagrass beds170 km² and Danish seagrass beds cover circa 30 km².

Since 1987, the University of Nijmegen, assigned by and in cooperation with the Dutch Government, investigates the possibilities for restoration of Zostera marina in the western Wadden Sea. Water clarity of the Dutch Wadden Sea has improved and shellfish fisheries have been locally prohibited. Experiments in the field, in outdoor mesocosms and in the laboratory, as well as literature, long-term environmental data and GIS studies, all provided knowledge of suitable donor populations, habitat requirements and potential habitats. (Giesen et al. 1990, de Jonge et al. 2000, van Katwijk 2000, 2003).

In 2002 transplanting has been started in the western Wadden Sea. Risks will be spread in space and time, the transplantations will be protected in the field during the first years (by the prevention of seed bearing shoots to drift to open sea) and the protective qualities of mussel ridges to provide refugia to the transplants will be investiged. The project will end in 2006, and is carried out in cooperation with Alterra Texel and RIKZ Haren.

Contact: M.M. van Katwijk

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(A vision of Ecological Coastal Protection)

This text is partly published in "Seas at the Millenium" by C. Sheppard (2000)

The Wadden Sea is one of the world's largest international marine wetland reserves (appr. 6000 km²), bordering the coasts of The Netherlands, Germany and Denmark. Before Dutch Wadden Sea contained large beds of subtidal and the 1930's, The low-intertidal marina L covering an area between 65 and 150 km² (figure 1; Zostera Oudemans et al. 1975). These seagrass beds were 1870; den Hartog and Polderman of great economic importance. The seagrass was used material, as roofing and isolation and to fill mattresses and cushions. Before 1857 it was used to build dikes (Martinet 1782; Sloet tot Oldhuis 1855; Oudemans et al. 1870). Considering the importance of dikes to The Netherlands, it is no wonder that in the past a proverb was used to describe the harvest ('good hay grass, good sea grass'), a special prayer day was held to invoke a bumper crop, and lyrical descriptions and poems about seagrass were written during the 18th and 19th century (Sloet tot Oldhuis 1855). Less is known about the past German and Danish beds. They had small or no economic value (van den Hoek et al. 1979).

During the 1930's, the seagrass cover was largely lost and the beds never recovered (e.g. den Hartog 1987; Reise et al. 1989). Presently, Z. marina occurs only in the mid-littoral; approximately 2 km² of Z. marina in The Netherlands (D.J. de Jong unpubl. results); in the German Wadden Sea, Z. noltii and Z. marina together cover approximately 170 km², and in the Danish part ca. 30 km² (Reise and Buhs 1991). The large-scale decline of Z. marina coincided with (1) the outbreak of wasting disease caused by the slime-mold Labyrinthula zosterae (2) increased diking and damming activities and (3) two subsequent years with a considerable deficit of sunlight. There is no consensus about which of these events (or combination of events) caused this decline (reviews in den Hartog 1996; de Jonge et al. 1996). Main causes for the lack of recovery of eelgrass stands in the Dutch Wadden Sea were thought to be high turbidity, and later shellfish fishery (van den Hoek et al. 1979; Giesen et al. 1990a;b de Jonge and de Jong 1992). The Dutch government is currently attempting to return seagrass to the Wadden Sea, in order to 'restore natural values' (Anonymous 1989). Seagrass beds are important as a nursery, shelter and feeding area for many fish and crustacean species (e.g. van Goor 1919; Heck et al. 1995; Horinouchi and Sano1999; Mattila et al. 1999; Valentine and Heck 1999).

The presence of potential seagrass habitats is the first condition for successful restoration. In the Wadden Sea, a distinction can be made in a higher and a lower zone of potential habitats along the tidal gradient, each suitable for differing morphotypes of Z. marina (figure 2; chapter 6). The higher zone is inhabited by mostly annual plants. When emerged, the plants lay flat on the moist sediment, in this way protected from desiccation. The lower zone (that disappeared during the thirties) was inhabited by perennial plants, with their stiff sheaths being vulnerable to desiccation during low tide, but more resistant to high water dynamics than the former morphotype (Harmsen 1936). Between the two seagrass zones, a bare zone exists, where the habitat is too dynamic for the high morphotype, and the periods of emergence last too long for the low morphotype. The upper limit for Z. marina growth in the high zone is delineated by the degree of desiccation, whereas the low zone is limited by light availability and/or strong currents due to the presence of channels (figure 2; chapter 6).

Important factors influencing the occurrence of Z. marina are: turbidity (chapter 2), disturbance (e.g. de Jonge and de Jong 1992), water dynamics (chapter 5), sediment dynamics (e.g. Boley 1988; Fonseca 1996), degree of desiccation (e.g. Harmsen 1936; Hermus 1995), nutrient level (e.g. chapter 3 and 4) and salinity (chapter 4). To show how these factors determine the suitability of potential habitats, two main factors are distinguished:

1. 1. Dynamics, involving water dynamics, sediment dynamics and derived effects, grain size of the sediment (positively correlated with water dynamics), turbidity (idem) and degree of desiccation (idem, via coarsening of the sediment, and see below). Increased water and sediment dynamics will remove plants or will lower their productivity, making them more vulnerable to other stresses. Increased turbidity negatively influences Z. marina when light is near to limiting. Increased degree of desiccation has greatest effects on the high edge of both zones (figure 2). Dynamics may increase as a consequence of construction activities, disappearance of subtidal seagrass and increased fishery activities, including shell fisheries. The latter may additionally increase the degree of desiccation when the disappearance of oyster and mussel beds results in an increased superficial drainage.

2. 2. The interactive effect of nutrients and salinity (figure 3). High nutrient loads negatively effect Z. marina. High salinity stresses the plants, which will aggravate the negative effects of high nutrient loads. Also, nutrients stimulate algal growth which subsequently causes increased light limitation. Finally, increased nutrient loads in the water causes an increased shoot:root ratio which makes the plants more vulnerable to high water dynamics.

Figure 4 shows how these main factors act on the edges of the two Z. marina zones. Disturbance, in the Wadden Sea mainly caused by shellfish exploitation, acts locally and indiscriminately in both Z. marina zones.

In the Wadden Sea, dynamics, disturbance and nutrient loads have increased during the 20th century, whereas the overall salinity has remained equal. As a result, the area suitable for reestablishment of Z. marina has decreased (figure 3). However, since the end of the eighties, turbidity levels in the Wadden Sea have decreased, nutrients levels have decreased or stabilised, and shell fisheries are prohibited in some areas. Both active and passive restoration of Z. marina seems now more feasible, although further decreases of these factors are desirable so as to increase the area of potential Z. marina habitats. Until then, potential Z. marina habitats will be confined to undisturbed sheltered locations and locations with freshwater influence.

Captions to the figures

Figure 1. The Wadden Sea and the decline of Zostera marina in the western part during the thirties. Presently, one mid-intertidal Z. marina bed of 1.6 ha remains in this area (D.J. de Jong unpubl. results). Black arrows indicate major freshwater influences.

Figure 2. Zonation of Zostera marina in the Wadden Sea along the tidal gradient in relation to wave energy (orbital velocity ant the sediment surface, based on model calculations) and emergence time during low tide. Delimiting factors are indicated for the upper and lower limit of both zones. MSL Mean Sea Level, LT Low Tide.

Figure 3. Interactive effect of nutrients and salinity on potential Zostera marina habitats, deduced from a laboratory experiment and field observations (van Katwijk et al. in press). Darker shades indicate greatest Z. marina vitality.

Figure 4. Conceptual model of the potential Zostera marina habitats (shaded) along a tidal gradient in the Wadden Sea, in relation to dynamics (sediment and water dynamics, and related factors, turbidity, grain size and degree of desiccation), and the interactive effect of nutrients (direct and indirect effects of the enrichment of the total system and salinity). MSL Mean Sea Level, LT Low Tide. The arrow indicates the development in the Wadden Sea during the last century.