De Levende Natuur nummer 6 van 2004 (English summary)


DLN 2004-6

Wetland restoration and water management in the Hunze-area

Schollema, P.P., U. Vegter, H. Wanningen & B. Zoer

In the river valley of the Hunze (north-eastern part of The Netherlands) wetland restoration takes place on a relatively large scale. Restoring the river Hunze with its characteristic river processes like flooding and sedimentation are important elements. In some parts of the river valley restoration of mesotrofic fens fed by calcareous groundwater is in preparation. Important in this process is the different view on water management in the area. Together with water conservation, creating areas for water storage and improvement of water quality the recovery of eutrofic fens and the lake Zuidlaardermeer takes place. Furthermore groundwater abstraction and nature development are going along in the area to a certain extent. The first ideas of wetland restoration fifteen years ago have already led to some new nature reserves in the area, for example the Annermoeras. One of the reasons of this success is the cooperation between the different organisations responsible for nature and water management. Some characteristic species have already returned in the area. It is expected that within ten years a large part of the river Hunze will be restored.

New government policies impose risks for nature and landscape

Oostenbrugge, R. & J.L. Tersteeg

This years Nature Balance shows that the condition of the natural environment in The Netherlands is rather mixed. The quality of river systems and natural streams gradually improves, but heathlands and coastal dunes suffer from acidification and extensive eutrophication. Some groups of species are doing rather well, like bats, but for others the situation can be very serious. Two thirds of the butterfly species, for instance, have shown a decline in numbers over the past ten years. Also birds of the rural areas are diminishing.

The Netherlands has made good progress in designating a considerable number of conservation areas under the European ‘Natura 2000 Network’. These provide for protected habitats of which the environmental conditions should be optimized by the year 2015. However, many of these conservation areas are in close proximity to areas of intensive human activity, like urban and agricultural areas. As a result, the exposure to dessication, acidification and eutrophication can be severe.

Urbanization also affects landscape quality. The new physical planning policy of the national government is now concentrating all efforts into twenty large-scale natural heritage sites. The location of these national protected areas is well chosen. However, implementation of the new policy is handed over to regional authorities while objectives, checks and balances are largely left undefined.

Toxic cyanobacteria are no problem for Zebra mussels

Dionisio Pires, L.M.

Most lakes in The Netherlands are, despite several phosporus-reducing measures, still highly turbid. This turbidity is mainly caused by high algal biomasses, especially of toxic cyanobacteria. One way of reducing the nuisance of cyanobacteria is by using grazers, which are capable of removing cyanobacteria from the water column. In The Netherlands it has been suggested to use Zebra mussels (Dreissena polymorpha) in the restoration of eutrophic systems by biomanipulation (Reeders, 1990), because of their high filtering capacity. In order to see if Zebra mussels can be used in lake restoration programmes, the feeding behaviour of the Zebra mussel was studied using laboratory reared phytoplankton and lake seston. Also, mussels were exposed to different phytoplankton species to study survival. Furthermore, the fate of the toxic microcystins in mussel tissue was followed. Zebra mussels cleared all seston components at equal rates. Analysis of pseudofaeces, however, showed that especially detritus was expelled and cyanobacteria and other phytoplankton were ingested. No differences in survival of the mussels were observed between different food treatments. Mussels had no problems with the toxic cyanobacteria as compared to non-toxic cyanobacteria and green algae. Microcystins were shown to be transformed by the mussels into a non-toxic conjugate. It is concluded that the presence of toxic cyanobacteria will not hinder the mussels in their filtration. The mussels could be useful aids in clearing lakes in The Netherlands from toxic cyanobacteria. Providing hard substrate in the form of shells and stones for the mussels to settle may help as a restoration measure.

Flora developments in the ‘Gelderse Poort’; results of 15 years of nature rehabilitation

Peters, B.W.E., G.H.S. Kurstjens & T. Teunissen

In 1990 large scale nature rehabilitation was started in the floodplain area ‘De Gelderse Poort’ (Geldern Gateway), in the eastern part of The Netherlands. This is the area where the river Rhine enters the country and splits up in three branches. Agricultural floodplains are transformed into new nature areas through governmental funding and clay mining. The clay mining and brick industry is an important economic partner in the process. The management is handed over to nature conservation organisations. Main purpose of the nature management plan is to reactivate natural riverine processes, such as river inundation, dynamic sand and gravel depositions, erosion, natural grazing, spontaneous forest development and groundwater seepage. In 2003 and 2004 a large scale floristic survey was carried out, to census the results of this nature restoration policy in terms of floristic developments. The results demonstrate clearly that many rare and characteristic riverine plant species are now recolonising the ‘Gelderse Poort’. Especially the sandy levees and other morphologically active parts of the floodplain attract many species that abandoned the area many decades ago due to agricultural intensification. Although these positive developments are still in their first stages, it is obvious that nature management based upon restoration of ecological processes shows fantastic results. The project also indicates that nature management based upon just a few target species or rigid management goals can deprive us of many unexpected successes and should be avoided in areas where the natural processes are still alive and kicking.

Distribution trends of marsh and peat-bog moths in The Netherlands

Meulen, J. van der, M. Coenen & D. Groenendijk

Distribution trends of fifty-five moth species characteristic of marshes and peat-bogs in the western part of The Netherlands was assessed by comparing the numbers of observations in the course of the 20th century for each species and of their rate of abundance in the period between 1970 and 2000. The data used for the calculation came from the national database for Lepidoptera at Dutch Butterfly Conservation, which contains over 500,000 records of moths. The relative abundance of all fifty-five species, i.e. the percentage of the total area where a particular species was recorded, showed a decline of nearly all species since 1970. A similar rate of decline was found when we used another method for calculating trends to correct for differences in observation intensity. We compared these values with those from four other European countries in the Atlantic biogeographical zone, Great Britain, Ireland, Germany and (a small part of) Portugal. The relative abundance of the majority of marshland moths in The Netherlands was found to be higher than those in other European countries. We therefore conclude that The Netherlands have a special responsibility for the conservation of the moths of its marshes and peat-bogs.