De Levende Natuur nummer 5 van 2004 (English summary)




DLN 2004-5

Dynamic gradients back in the Delta region in Southwest Netherlands

Blauw, T.S., H.A. Haas & L.A. Adriaanse

The Dutch Delta region in Southwest Netherlands originally consisted of interconnected estuaries, interfacing the rivers Rhine, Meuse and Scheldt with the North Sea. The execution of the Delta Project during the period 1960-1986 has changed this area dramatically. This huge project, which was initiated after the storm surge of 1953, resulted in a system of dams and sluices that guarantees protection of the region against flooding from the sea. The new infrastructure has fundamentally changed the hydraulic, morphological and ecological characteristics of the region. The open interconnected estuaries with dynamic gradients between riverine and marine systems were transformed into isolated salt, brackish and freshwater lakes, disconnected from the rivers and the sea. This resulted in a loss of estuarine gradients and buffering capacity within these water systems. Also all kinds of ecological problems became visible in the different lakes. The negative effects of the Delta Works and the future threats hanging over the region make it essential to take measures now, so the three provinces that cover the Delta region have jointly developed an integrated future vision plan for the Delta: ‘View on The Delta’. In this vision the restoration of the Delta’s natural processes (estuarine dynamics) is one of the major goals.

Recovery of the estuary landscape in the Zuiderdiep

Broek, T. van den & W.H.M. Cornelissen

In the area of the brackish Haringvliet tidal wetlands used to occur at large scale. During last centuries these wetlands were turned into polders and used for agricultural purposes. To manage flood risks the Haringvliet sluices were build in 1970 as a part of the Deltaworks and the Zuiderdiep (a former part of the Haringvliet) became an important part of the fresh water supply for the polders. Again large areas of tidal wetlands disappeared in the Haringvliet because of the absence of water table fluctuations on regular daily basis. Recovery of tidal wetlands is an important topic in Dutch nature management nowadays and a study is conducted to focus on the possibilities of the recreation of tidal wetlands in the Zuiderdiep area. Partial reopening of the Haringvliet sluices will create opportunities for brackish tidal wetlands (model Brakke getijdenkreek) and using the pumping engine at the North Sea site for the inlet of salt water in the Zuiderdiep will create opportunities for saline tidal wetands (model Zoute slufter). The habitats and vegetation types in the models are described.To ensure fresh water supply for the polders, a new fresh water channel will be designed.

Restoration of estuarian values in the North of The Netherlands

Meijer, M.L., K. Borrius, M. van Wieringen & Z. Jager

In the North of The Netherlands there is a lot of experience with smaller restoration methods such as fish-migration constructions and salt marsh re-establishment. First priority of restoration is the improvement of estuarine values of the only remaining estuarium, the Ems. Especially the salt marshes and seegrass meadows have to be expanded. On the long term restoration of Lauwersmeer as an estuarium, in connection with streams coming form the sandy grounds more upstream is considered the most promising improvement of the estuaria in the North.

Measures to improve brackish waters

Fortuin, A.W.

Three examples of improving brackish watersystem quality in the south-west part of The Netherlands by dredging and complementary measures are given. In Katse Gat the dredging was combined with increasing the waterlevel and pumping in water out of the adjacent marine Oosterschelde to the new established level. In Flaauwers- and Weversinlaag the dredging was combined with placing seepage tubes in order to increase seepage of oligotrophic marine water from the Oosterschelde into the watersystems. Rammekenskreek is a permanently stratified watersystem due to the high salt content by seepage of the lower layer. Due to the growing formation of reefs of Electra crustulenta (Bryozoa) that block the drainage channels the upper layer has become thinner. The upper layer now mixes with the hypertrophic lower layer due to wind effects, resulting in a decreased water quality. Apart from dredging the shallow parts of the watersystem the blocking reefs were removed and a siphon-weir construction was built in order to drain the lower layer separately. The upper layer will increase to an extent, that mixing between the layers will become impossible.

Results of a reduced tidal regime in Polder Breebaart

Peletier, H., H. Wanningen, B. Speelman & P. Esselink

Polder Breebaart is located in the NE of The Netherlands, on the edge of the Eems-Dollard estuary. The polder was embanked in 1979, and became a nature reserve in 1991. A plan to restore tidal influence was developed by the Foundation "Het Groninger Landschap" together with Water Board "Hunze en Aa’s". Since 2001, seawater flows in from the estuary through a culvert in the seawall. The effects of this drastic change in the ecological situation in the former polder are monitored by following the populations of the microflora, vegetation, macrofauna, fish and birds. After three years on the sediment-rich outer edges dense algal mats have developed with species characteristic of brackish shallow tidal flats. The original vegetation has disappeared completely to make place for vegetation adapted to saltwater such as Glasswort, Annual seablite, Lesser sea spurrey and Sea aster. The bottom fauna did not come in yet, its density is still very low. Fish in the former polder are diadromous and estuarine species. Birds such as Avocet and Black-headed gull are dominant in the area. In the former Breebaart polder sediment accumulates at a rate of 30 cm per year. The area will consequently change rapidly in a morphological sense. A clear fresh-to-salt gradient has not been achieved yet. The monitoring programme has a high educative value; many volunteers participate. The development of a new natural situation in this former polder can be regarded as a model for other areas that could become brackish again, influenced by tides - the situation of past times. Moreover, the development in Breebaart shows that natural conservancy, participation of the public and artificial nature development can go together quite well.

Restoration of brackish marshes in SW Netherlands - Redshank Program halfway

Beijersbergen, J. & P.J. van der Reest

The organizations for nature conservation and management in the province Zeeland have proposed a program for the restoration of brackish marshes in the early nineties: the so called Redshank Program. The program meant to relieve the decline of estuarine animal and plant life in and around the Oosterschelde, an estuary in SW Netherlands, after building a storm surge barrier. The program listed 44 different projects. 33 projects were actually planned. So far 13 projects have been executed. A small project (56 ha) was carried out in the Van Haaftenpolder. The morphology and hydrology of a former tidal creek was restored by means of dredging and some arable fields were turned into natural pastures by removing the artificial drainage system and nutritive soil layers. The project was quite successful because coastal breeding birds, like Avocet, Little plover and Common tern returned and rare plant species of wet dune slacks established. A larger project (214 ha) was carried out in the Prunje area. Agricultural fields were transformed into brackish marshland and natural pastures by natural engineering. Creeks which were filled in and leveled in the past were completely dug out. Saline seepage was enhanced by placing seepage conducts. The area was rapidly colonized by breeding waders and terns; salt marsh vegetation and brackish water plants were restored on a large scale. Both projects demonstrate that good results can be achieved by natural engineering on brackish marshes and focus the attention on the importance of the water circulation and the dimension of water bodies as key factors for a good brackish water system.

Developing a salt-marsh through de-embankment of a summerpolder in the Wadden Sea

Vries, H.J. de

To develop more mainland salt marsh, in 2001 the first summerpolder of 135 ha was de-embarked in Noard-Fryslân Bûtendyks (Wadden Sea, province of Fryslân). The main subject is to monitor and analyse surface-elevation and vegetation development in the new salt marsh. In this contribution the results after two years. Sedimentation on the surface is the highest near the (artificial) creeks in ungrazed parts. Halophytes have rapidly invaded the area at the expense of less salt-tolerant plant species. Based on the vegetation development the project appears to be successful within two years after de-embankment. The project area is (temporarily) less used by grazing Barnacle– and Brent geese. Due to the large areas of surrounding salt-marsh and grassland outside the Delta dike, the agricultural land nearby is not affected by higher geese densities. The suitability for breeding coastal- and meadowbirds dropped, but already shows a slight improvement for a few species.

Fish in the transition between fresh and marine water

Jager, Z., J. Kranenbarg & A.D. Vethaak

The Dutch estuarine fish fauna comprises about 100 species, which can be grouped according to so called ecological functional guilds that make different use of the estuary. One of these guilds is the group of diadromous species, migratory fish which spend part of their live in fresh water and part in marine waters. Another relevant guild of estuarine fish are the resident species that can spend their whole life in brackish water and are tolerant to the dynamic and harsh estuarine environment. The fishes that inhabit our (former) estuaries have suffered by the changes that occurred in the transition between fresh and marine water. These were brought along by a number of developments: ‘dry feet’, ‘safety’ and ‘human impact’. The Netherlands are a low-lying country in the delta of the rivers Rhine, Meuse, Scheldt and Ems. Starting in the Roman era the sea level rise forced the inhabitants to raise artificial mounts and to construct dikes. The water that could flow to sea unhindered before now was discharged through sluices or by pumps. As a consequence the outflowing water reached high current velocities which surpassed the swimming capacity of fish, and the diadromous fish coming from the sea could no longer enter the inland fresh water bodies. Their life cycle was disrupted. On the other side, fresh water fish were flushed out to sea and were not able to return. Flounders developed ulcers in the vicinity of sluices. During the 20th century safety became a big issue, and the coastal defence was enforced. The impact of these changes are explained by the developments that occurred in the populations of two species, the anadromous Smelt (Osmerus eperlanus) (which spends most of his life in estuarine waters) and the Twaite shad (Alosa fallax). A box illustrates the trends in the incidence of ulcers in Flounder (Platichthys flesus).

Seagrasses in The Netherlands

Jong, D.J. de, M.M. van Katwijk & Z. Jager

Despite their name seagrasses are not sea plants but estuarine plants. For good growth and survival they need brackish circumstances. As a consequence of recent measures to protect the hinterland from storm floods and salt intrusion the growing conditions for these species have declined severely in the last decades, bringing both species to a level of almost or complete extinction in the Dutch salt waters. Restoration of seagrass populations in The Netherlands requires restoration of more estuarine circumstances in the water systems.