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The Present Status of the River Rhine with Special Emphasis on Fisheries Development - T. Brenner1 A.D. Buijse2 M. Lauff3 J.F. Luquet4 E. Staub5


1 Ministry of Environment and Forestry Rheinland-Pfalz, P.O. Box 3160, D-55021 Mainz, Germany

2 Institute for Inland Water Management and Waste Water Treatment RIZA, P.O. Box 17, NL8200 AALelystad, The Netherlands

3 Administrations des Eaux et Forets, Boite Postale 2513, L1025 Luxembourg

4 Conseil Supérieur de la Peche, 23, Rue des Garennes, F 57155 Marly, France

5 Swiss Agency for the Environment, Forests and Landscape, CH 3003 Bern, Switzerland

Key words: Rhine, restoration, aquatic biodiversity, fish migration

ABSTRACT

The Rhine basin (1 320 km, 225 000 km2) is shared by nine countries (Switzerland, Italy, Liechtenstein, Austria, Germany, France, Luxemburg, Belgium and the Netherlands) with a population of about 54 million people and provides drinking water to 20 million of them. The Rhine is navigable from the North Sea up to Basel in Switzerland and is one of the most important international waterways in the world.

Floodplains were reclaimed as early as the Middle Ages and in the eighteenth and nineteenth century the channel of the Rhine had been subjected to drastic changes to improve navigation as well as the discharge of water, ice and sediment. From 1945 until the early 1970s water pollution due to domestic and industrial wastewater increased dramatically. Since then many measures have been taken by the riparian states, communities and by industry to reduce nutrients and pollutants. The total phosphorus inputs were reduced by 65 percent compared to 1985, the nitrogen inputs only declined by 26 percent.

Due to the improvement in water quality the number and abundance of the majority of fish species have increased and the Atlantic salmon (Salmo salar L.), which was formerly extinct, not only occurs in some tributaries but also reproduces naturally. In total over 60 species are present in the river basin. From its indigenous ichthyofauna of 44 species only Atlantic sturgeon (Acipenser sturio L.) has not been seen with certainty during the last decade. Most species other than the migratory species are self-sustainable, but the overall species composition is skewed towards a few ubiquitous ones, such as roach (Rutilus rutilus (L.) and bream (Abramis brama (L.). Twenty exotic species are present but nowhere dominate the fish community. New species (e.g. Abramis sapa Pallas, Proterorhinus marmoratus (Pallas) now appear more frequently as they reach the Rhine through the Rhine-Main-Danube canal. The commercial fishery is based mainly on eel (Anguilla anguilla (L.) and pikeperch (Stizostedion lucioperca (L.). Exploitation of migratory fish species is not remunerative and in the case of salmonids their fishing is banned. However, there is a flourishing recreational fishery. The "Salmon 2000 Programme" started by the Rhine Ministers of Environment under the coordination of the International Commission for the Protection of the Rhine (ICPR) has now been integrated in the programme on the sustainable development of the River Rhine "Rhine 2020" whose main objectives are ecology restoration, flood prevention and groundwater protection. Possibilities for the restoration of the River Rhine are limited by the multipurpose use of the river for shipping, hydropower, drinking water and agriculture. Further recovery is hampered by the numerous hydropower stations that interfere with downstream fish migration, the poor habitat diversity, the lack of lateral connectivity between main channel and floodplains and the cumulative unknown effects of thousands of synthesised components in water.

This paper describes the different national and international programmes for the restoration of the River Rhine, its tributaries and measures for the reintroduction of the Atlantic salmon such as stocking, habitat enhancement and construction of fish passages. The salmon has fulfilled a flagship role for a general improvement of the Rhine. The most significant positive recent development is the EU Water Framework Directive: EU member states are required to compile river basin management plans and rivers should have a good ecological status by the year 2015.

INTRODUCTION

The River Rhine is 1 320 km long and flows from the Swiss Alps through Switzerland, France, Germany and the Netherlands to the North Sea. The 225 000 km2 catchment area of the Rhine extends over parts of Switzerland, Italy, Austria, Liechtenstein, Germany, France, Belgium, Luxembourg and the Netherlands and is populated by about 54 million people (Table 1 and Figure 1). A number of industrial centres such as Basel, the Ruhr region and Rotterdam are situated along the Rhine, formerly a wild stream, meandering through a wide floodplain, today a vital shipping route. Each day approximately 450 ships pass the Rhine at Lobith - Bimmen. In the year 2000 the transport on the river at the Dutch - German border was about 162 million tonnes and is expected to rise up to approximately 199 million tonnes in 2015 (Wetzel 2002). The river is also of importance for the water supply for agriculture and the drinking water provision for about 20 million people. Twenty-one hydropower plants on the Rhine mainstream have a total installed capacity of 2 186 MW. River Rhine has suffered severely from stream regulation and pollution.

The first documented human influence on the river with regard to canal construction to regulate the discharge took place in the Roman era. The construction of dykes on floodplains began in the early Middle Ages with the development of agriculture. However, until the eighteenth century the main channels were meandering and many river islands, floodplain forests and snag habitats were still present. Later on, the need for timber resulted in the disappearance of floodplain forests, whereas the wood was removed to facilitate shipping. River regulation began in the nineteenth century and was completed in the twentieth century with a series of weirs, locks and dams to control flooding, to produce hydropower and for shipping (Figure 2). The decline in water quality due to uncontrolled industrial and domestic discharges culminated in serious problems with drinking water and an overall degradation of the Rhine ecosystem from 1950 to 1970, when dissolved oxygen concentration became extremely low (Lelek 1989). As a result of the treatment of waste-water discharges during the 1970s - 1980s, dissolved oxygen levels returned to normal but nutrients, mainly nitrogen, still reach the river from diffuse agricultural sources. While concentrations of several heavy metals have been reduced over the last few decades, the sediments of the river forelands are still strongly contaminated and micropollutants are presumably the cause for the reduction in the bottom fauna (van den Brink, van der Velde, Buijse et al. 1996).

Table 1: Hydrological characteristics of the River Rhine

Total drainage area (km2)

185,000

225.000


(including "Alpine Rhine")

Total length (km)

1,250

1.320


(including "Alpine Rhine")

Mean discharge (m2/s)

2,280

**

Minimal discharge (m2/s)

590

**

Maximum discharge (m2/s)

11,800

**

** at Rees (Dutch border)

Figure. 1. The catchment area of the River Rhine

One can distinguish six stretches of the River Rhine: The Alpine Rhine from its source in the Alps to Lake Constance.

Figure 2. The Upper Rhine at Breisach (From ICPR 1991)

The High Rhine from the outflow of Lake Constance to approximately 170 km downstream at the city of Basel (This is the beginning of what is generally referred to be the Rhine). This stretch still has a riverine character despite having 11 dams with hydropower stations, as well as four dams upstream of the river Aare (Table 2).

Table 2: The most important tributaries of the River Rhine

Tributary

Length

Mosel (France and Germany)

550 km

Main (Germany)

524 km

Neckar (Germany)

370 km

Aare (Switzerland)

295 km

Lippe (Germany)

437 km

Ruhr (Germany)

235 km

Ill (France)

217 km

The Upper Rhine from Basle downstream to the city of Bingen. This was the most diversified part of the Rhine in the past and it is also known as the "furcation zone". Swift stretches extend over a length of about 190 km. The canalisation of the Upper Rhine, the so-called Tulla rectification, was carried out between 1817 and 1876 and had tremendous environmental consequences (Figure 3). As a result, the length of the Rhine in this stretch had been shortened to 81 km (23 percent of total length). Furthermore, 2218 islands that existed until 1825 disappeared. The once braided river system with islands, sand and gravel flats - a highly diverse system of various habitats in a dynamic environment - was transformed into a petrified canal with high current velocities. To counterbalance erosion and sedimentation 10 dams were built, with hydropower plants and with locks for navigation. For safety reasons and easier navigation, the main river channel has a bypass, which is an artificial canal on the French territory (between km 173 and km 227). The hydropower plant in Kembs can take a maximum discharge of 1 400 m3 s-1. The remaining water, with a mean discharge of 91 m3 s-1, flows through the former river channel, the so-called "Rest-Rhine". The minimum discharge in the summer period is 20 m3 s-1 and the maximum is about 256 m3 s-1. The first dam is about 700 km from the North Sea at Iffezheim and has been equipped in 2000/2002 with one of the largest fish passage structures in Europe. It is a modified vertical slot pass optimised by French and German fishery and hydraulic engineering experts. A fish pass for the next dam upstream at Gambsheim will be constructed in 2003/2004. The main tributary in this stretch on the right side is the dammed and navigable Main River, which is connected to the Danube system by the Rhine-Main-Danube canal. The River Ill flowing into the Rhine near Strasbourg is the most important French river in the Alsace region.

Figure 3. The Upper Rhine at Breisach (From ICPR 1991)

The Middle Rhine with its main tributary the Mosel River, also regulated, is located approximately midway between the 510 and 640 km mark. This stretch was declared in 2002 a world heritage site by UNESCO because of its beautiful landscape.

The Lower Rhine passes through the most populated and industrialized part of Germany before it flows into the Netherlands (Figure 3). In the Netherlands the Rhine enters a lowland area where it forms a river delta before it flows into the North Sea.

The Delta of the river has three branches and covers 25 000 km2, which corresponds to 67 percent of the total surface area of the country. The River Waal/Merwede is the main branch discharging 65 percent of the water; the Lower Rhine-Lek discharges about 21 percent and the River Ijssel discharges only 14 percent. Three weirs regulate the Lower Rhine-Lek. A fish pass could be installed at the first weir in Driel. The two other weirs in Amerongen and Hagestein will be equipped with fish passes in 2003/2004. The major part of the discharge from the Waal, the Lower Rhine-Lek and the Meuse converges in the Rhine-Meuse delta and flows from there into the North Sea from various points. The most important are the Haringvliet sluices and the Nieuwe Waterweg. The barrages in the Haringvliet sluices have a discharge programme that ensures that the river discharge of the Nieuwe Waterweg is maintained at about 1 500 m3 s-1 to prevent saltwater penetration. Since most Dutch arms are strongly regulated by huge dams, free entrance for migrating fish species from the North Sea to the Lower Rhine estuary is only possible via the Nieuwe Waterweg near Rotterdam, a highly industrialised area with many harbours. All Rhine branches in the Netherlands are canalized and there is no natural river delta left. Since the closure of the Afsluitdam in 1932 and the Haringvliet in 1970 the tidal influence in the river estuary is very much reduced and the Haringsvliet-Holland Diep and Lake Ijssel are managed as freshwater lakes. This was part of a large plan to protect the densely populated delta of the Rhine, Meuse and Scheldt against flooding, to control the watersystem and improve the supply of fresh water. But this has also caused severe ecological damage. In the near future the Dutch government wants to partially open the Haringvliet sluices, to be followed up by a gradual opening of the gap. It is expected that these measures will improve the estuarine ecosystem. Initial experiments indicate that the fish migration is improved by partial opening of the barrier. Besides the barriers separating the river from the sea, there are no obstacles for fish migration in the Waal and Ijssel.

INTERNATIONAL REHABILITATION PROGRAMMES

INTERNATIONAL COMMISSION FOR THE PROTECTION OF THE RHINE

The International Commission for the Protection of the Rhine against Pollution (ICPR; IKSR in German) was initiated by the Netherlands in the 1950s because of the concern over pollution of the Rhine and its implications for the drinking water supply. The ICPR started as a common forum of the member countries bordering the Rhine: Switzerland, France, Germany, Luxembourg and the Netherlands for periodical meetings and the formulation of pollution control agreements. On 1 November 1986, 10 to 30 tons of plant-protecting agents were discharged in fire-fighting water into the Rhine at the Sandoz plant in Schweizerhalle near Basel (Lelek 1989). This resulted in a massive fish kill, mainly of eel, of which an estimated 200 tonnes died. With this accident the extent to which the Rhine ecosystem was endangered became apparent and this stimulated the ICPR to promote an international river restoration plan called the Rhine Action Programme "Salmon 2000" (IKSR 1987; Brenner 1993). It has four goals:

Over the past two decades a gradual improvement of the quality of Rhine water has been achieved through close international cooperation. However, improving the water quality is not enough for creating a viable river ecosystem. Geomorphological aspects are also considered essential for ecological rehabilitation of the regulated Rhine and the International Commission for the Protection of the Rhine has developed the Ecological Master Plan to improve the ecosystem of the river (ICPR 1991).

Habitat diversity along the Rhine shows considerable deficits. Some stretches of the originally freely flowing Rhine and its numerous tributaries, such as the Mosel, Main and Neckar, have been turned into a series of impoundments. Numerous engineering measures along the main channel of the Rhine and of almost all its tributaries have fundamentally changed the hydrological and morphological conditions. More than 85 percent of the floodplains have been cut off from the Upper and Lower Rhine leading to a considerable loss of habitat and of animal and plant species typical of the river. The implementation of the Ecological Master Plan aims at counterbalancing the impacts.

The most important targets defined in the Ecological Master Plan for the Rhine are the restoration of the main stream as the backbone of the Rhine ecosystem and its main tributaries, their functioning as a habitat for migratory fish and preservation and protection, the improvement and extension of areas of ecological importance along the Rhine and in the Rhine valley to provide suitable habitats for autochtho-nous plant and animal species. These measures should allow the return of migratory fish species such as Atlantic salmon and the restoration of the connection between the river channel and the bordering riparian zones and floodplains. Furthermore, the aim of preservation and restoration is to increase the diversity of indigenous animals and plants, to increase spawning and nursery grounds, to create self-sustaining food chains and to create areas of refuge in case of large-scale contamination (Schulte, Wülwer-Leidig 1994). Since the adoption of this plan, numerous projects and studies have been carried out to improve fish migration (e.g. on the rivers Lek, Sieg, Ahr, Saynbach, Rhine at Iffezheim, Ill, Aare) and to restore spawning and feeding grounds in the Rhine and its tributaries. The results of the Rhine Action Programme in water quality, hydrology and ecology have been published (Weidmann and Meder 1994; Ministerium für Umwelt und Forsten 1996; ICPR 1994; IKSR 1999 a; IKSR b).

The most relevant present activities of the ICPR comprise the action plan for flood defence. Until 1993 floods, including flood-warning systems, were considered to be a local problem. The flooding of 1993 and 1995 on the rivers Rhine, Mosel and Meuse brought this topic to international interest. Dykes were at risk of bursting in the Netherlands and several hundred thousand people were evacuated. The damage was estimated to be several billion US dollars.

In January 1998 the Rhine Ministers adopted the "Action Plan for Flood Defence for the River Rhine", that was aimed at the improvement of precautionary flood protection. This plan defines four action targets: (1) Reduce damage risks; (2) Reduce flood levels; (3) Increase awareness of flooding and (4) Improve the system of flood forecasting (Rother 2002, Conversion of Forecasts into Warnings; unpublished manuscript). The Action Plan will be implemented within the next 20 years. The reduction of damages by up to 10 percent is expected to be achieved by the year 2005 and up to 25 percent by 2020 (ICPR 1998).

The International Commission for the Protection of the Rhine (ICPR) produced an inventory of measures undertaken and underway for restoration and conservation of the Rhine (IKSR 2001). Measures for the improvement of the ecosystem include, flood prevention, water quality and ground water conservation. The measures with the regard to the improvement of the ecosystem are specified for the top of the catchment and for the Upper, Middle and Lower Rhine. Among the measures is the lowering of summer dykes (> 20 km2 per section), reactivation of dammed old river branches and connection of floodplains with the main stream (>25 km2 river branches, additional dredging is included), construction of fish passes at the existing power stations and dams (in the main stream and on the tributaries which are part of the diadro-mous-fish programme), nature improvement of more then 3 500 km of tributaries.

The programme of sustainable development of the Rhine River "Rhine 2020-programme for Sustainable Development of the Rhine" succeeds the successful "Rhine Action Programme" and is coordinated by the International Commission for the Protection of the Rhine (ICPR). The focal points of the future Rhine protection policy are: further improvement of the Rhine ecosystem, improvement of flood prevention and protection and groundwater protection. The execution of the EU Water Framework Directive will assist with the implementation of the essential parts of the programme "Rhine 2020". Continued monitoring of the state of the Rhine and further improvement of water quality remain essential. The activities include the Rhine ecosystem improvement, restoration of the former habitat connectivity and of the up- and downstream fish migration from Lake Constance to the North Sea, as well as ecological enhancement of the tributaries as mentioned in the Programme on Migratory Fish. The programme "Rhine 2020" was drafted in an open dialogue among the Rhine bordering countries. Participating in the discussions were groups representing nature protection, flood protection, industry, agriculture, navigation and drinking water supply. There is great support for acceptance of the ICPR programme. Within the framework of this holistic approach monitoring of the progress is an essential part of the programme.

More than 45 individual flood prevention control activities are currently underway or in the planning phase between Basel and the Netherlands. Important retention areas have been created along the Upper Rhine, e.g. polders at Moder, Altenheim, Daxlander Aue, Flotzgrün, Strasbourg, an agricultural weir at Kehl and specific management of the hydroelectric power plants. On behalf of the ICPR and with reference to the EU Water Framework Directive a feasibility study for the restoration of the ecological connectivity for the Upper Rhine between Iffezheim and Basel and its tributaries is in progress. A preliminary report (October 2002) includes considerations of a possibility to reactivate this natural stretch of the Rhine. The first phase of this study is a review of the existing data and identification of goals which should be achieved regarding the ecological connectivity for individual fish species. The second phase should result in specific recommendations (Bericht 2002).

WATER FRAMEWORK DIRECTIVE OF THE EUROPEAN UNION

The EU Water Framework Directive was put into force on 22/12/2000 after more than 10 years of drafting and negotiations among the member states, Non-Governmental Organizations and numerous other stakeholders. The directive sets a common framework within which Member States must work to protect and enhance all natural surface, ground, coastal and estuarine waters and aims to achieve good water status in 15 years. Regulated water bodies have to be developed to their ecological potential. Furthermore, under this Directive, member states have to identify all the river basins situated within their national territory and assign them to the individual river basin districts. River basins covering the territory of more than one member state will be assigned to an international river basin district. For surface waters, the definition of 'good' is based on a new concept of 'ecological quality', taking into account biology, chemistry and their physical features. In this context, monitoring of water quality and its ecological condition is a key requirement. According to the timetable to the end of 2006 monitoring programmes have to be operational as a basis for the water management. Benthic invertebrates, fish and aquatic macrophytes are most frequently used as ecological indicators and were selected as indicators of the ecological status of rivers in the EU Water Framework Directive (European Union 2000). These measures coincide with the pre-existing monitoring programmes of the ICPR.

IRMA: INTERREG-RHINE-MEUSE-ACTIVITIES

The EU funded project IRMA, "Interreg-Rhine-Meuse-Activities" for the improvement of flood prevention along the rivers Rhine and Meuse. The project was set up to finance a joint flood control programme within the catchment areas of the rivers Rhine and Meuse with approximately 191 000 km2 and with 60 million inhabitants. Besides the EU member states, Switzerland is also participating in this programme on a project basis. As defined by the EU the main objective of the IRMA programme is:

"To prevent damage caused by floods to all living creatures and to important functions of the catchment area of the rivers and therefore to create a balance between the activities of the population in the areas, the socio-economic development and sustainable management of the natural water resources".

This main objective combines three important elements:

Taking these elements into consideration, water should be retained in the catchments as much as possible. The rivers should have space to discharge and high water should have the opportunity to flow into retention areas and floodplains. The awareness of high water must be raised, knowledge must be improved, legislation drafted and favourable conditions created. The spatial planning, water management and risk management must be integrated into one policy concept in order to prevent flood damage.

By the end of 1999, when the committing period ended, 153 projects were approved with a total EU contribution of 141 million Euro as IRMA grants. National counterparts contribute approximately 278 million Euro. The total cost of the programme amounts to about 419 million Euro. The programme has improved cooperation among the member states in spatial planning, water management and damage prevention. All measures focus on the creation; restoration and preservation of the former overflow areas/retention basins and infiltration. Besides cooperation and knowledge transfer, the IRMA programme has produced a number of quantifiable indicators.

The projects will reduce the peak discharges by 0.5 percent to 20 percent and the maximum flood water level by lowering the maximum water levels to approximately 140 mm on average (2 mm to 1 200 mm). The serious floods in 1993 and 1995 resulted in a policy decision named "Room for the Rivers". Following this concept the floodplain area will increase by approximately 215 km2 and including this 368 km2 of potential retention areas in the entire catchment of the rivers Rhine and Meuse are being restored. A total of 103 kilometres of previously straightened stretches of rivers and tributaries are subject of a re-meandering and re-development projects. Approximately 215 million m3 of additional retention capacity is expected to be created, the bulk of this being along the Rhine, e.g. more than 60 million m3 will become available along the Upper Rhine.

In the Netherlands the "Room for the Rivers" focuses predominantly on excavation of floodplains and retention areas in the hinterlands. The floodplains, however, contain diffuse contaminated sediments, a heritage of the twentieth century. The total amount of contaminated soils is estimated at 50-80 million m3, with high levels of HCB, PCBs, DDT, PACs, Hg, Cu, Zn and As. Since the mid-1990s the focus of water management was directed towards water quantity while before that it was mostly water quality that was addressed.

Floodplain lakes

Floodplains contain a large number of water bodies, which have originated from spontaneous diversion of streams (former meanders, lakes with a permanent open connection with the main channel, oxbow lakes), from dyke bursts in the past (break-through lakes) and more recently from sand, gravel and clay extraction (pits). Depending on geomorphological and hydrological situation and on seasons these floodplain water bodies are subject to a variety of hydrological regimes. When they are connected to the river by flooding river channel fish may freely enter floodplains.

Secondary channels

Large temperate rivers have experienced dramatic environmental changes that have resulted in a loss of some fish species. But due to their dynamic nature, rivers and streams have shown a remarkable capacity for recovery. Following the water quality improvement, a gradual increase in species richness in the Rhine was noted during the 1980s. Presently, however, ecological improvement is stagnating, indicating that restoration measures must not only address water quality improvements. This stagnation is considered to be due to the poor habitat diversity that presently exists in these regulated rivers. In order to enhance the heterogeneity of aquatic biotopes and biodiversity of the aquatic fauna and flora the reconnection of abandoned channels to the main channels has been proposed.

One possibility for restoration of riverine biotopes is to create permanently flowing channels on the floodplain. Such channels can make a valuable contribution to the ecological rehabilitation of rivers. However, they have to be fitted into the landscape without affecting existing interests such as shipping and protection against flooding. Such channels have been established for example along the Upper Rhine in Leimersheim and along the Waal River, the main branch of the Lower Rhine, where two secondary channels were created in 1994. The results of a 5 year monitoring programme show that excavated secondary channels function as a biotope for riverine fish, including the more demanding rheophilic species (Simons et al. 2001).

Floodplain development

The floodplains along the main channel of the Rhine are normally enclosed by a low summer or minor dyke and a high winter or major dyke. The summer dykes enable agriculture on the floodplains and the winter dykes prevent the hinterland from flooding. Due to the presence of these dykes, the floodplains are at present separated from the main channel and the typical and most important aquatic-terrestrial transition zone has been lost.

However, the flood plains of the Rhine in the Netherlands have several hundred relatively large water bodies (1 - 200 ha) such as gravel pits. Several water bodies are connected with the river. Some water bodies are inundated only during winter and spring and are isolated from the main channel whereas others are inundated only at high water floods.

In the Netherlands, in 1989 river restoration started by connecting permanently water bodies on the floodplains to the main channel. At several locations of the floodplains, secondary channels were dug and isolated backwaters were connected with a downstream opening to the main channel. Only a few years after their creation, secondary channels provided nursery areas for rheophilic cyprinids. Connectivity of a water body with the main channel and the presence of flowing water are important factors for the structure of the young-of the-year (YOY) fish community in floodplain water bodies (Grift et al. 2001).

Spawning conditions for especially the rheophilic cyprinids (e.g. Aspius aspius (L.), Leuciscus idus (L.), Barbus barbus (L.), Gobio gobio (L.) have been declining dramatically. At present the eurytopic species Abramis brama (L.), Blicca bjoerkna (L.), Rutilus rutilus (L.), Alburnus alburnus (L.) and Stizostedion lucioperca (L.) dominate the riverine fish community.

Therefore, the development and restoration of floodplain waters offers increased habitat diversity for fish and benefits recruitment of fish larvae (Freyhof et al. 2000; Grift 2001). In a pilot study of 14 water bodies on the floodplains of the Lower Rhine 20 species were caught (Buijse and Vriese 1996).

The most important floodplains in Germany are at the Upper Rhine (Dister 1991), such as Rastatt (1800 ha), Kühkopf (2800 ha) and the Lampertheimer Rhine (500 ha). On the German Lower Rhine is the Xantener Altrhein (600 ha). The fish fauna of important spawning sites in different types of water bodies in the inundation area of the Upper Rhine between Philippsburg (km 388) and Mannheim (km 412) was studied by Gebhardt (1990). Inundation areas provide habitats for a variety of fish species, many of which spawn only there.

Aquatic biodiversity

Van den Brink et al. (1996) discusses the diversity of aquatic biota in the Lower Rhine. The present species richness in the main channels is still relatively low, despite major water quality improvements. Although the present biodiversity has vastly improved when compared with the situation a few decades ago, it is evident that many species are eurytopic, including many exotics. Further biodiversity recovery is hindered because of river regulation and normalization, which have caused the deterioration and functional isolation of main channel and floodplain biotopes. The importance of connectivity differs among the aquatic taxa. The authors conclude that floodplain lakes contribute significantly to the total biodiversity of the entire riverine ecosystem. The redevelopment of active secondary channels is required to restore the most typical riverine habitats and biota.

BIOLOGICAL MONITORING PROGRAMMES

NATIONAL AND INTERNATIONAL MONITORING PROGRAMMES

National (e.g. Bakker et al. 1998) and international (IKSR 2002 a, 2002c) monitoring programmes show the trends in the physical, chemical and ecological state of water bodies.

Aquatic macrophytes

The present diversity of aquatic macrophytes in the main channels of the Rhine is rather poor as compared with the former situation or with the present diversity in floodplain lakes. At present, about 70 percent of the species recorded have been found only in the floodplain lakes. The other 30 percent can be found both in the main channel and floodplain lakes biotopes. Although the historical data are scarce, it can be concluded that many species have disappeared or became rare. Deterioration of aquatic macrophytes in this regulated river is probably caused by increased river dynamics, e.g. larger differences between summer and winter water levels, higher stream velocities and a higher frequency of summer spates. The reduced number of oligo- and mesotrophic species and an increase in the eutrophic ones may indicate an increase in eutrophication. The number of exotic aquatic macrophytes in the Rhine and its floodplains is low.

Aquatic macroinvertebrates

During the latest surveys for the ICPR "Rhine 2020" Programme more than 300 species or higher taxa were recorded. Most of the species occurred in the Higher Rhine and in the southern Upper Rhine. The number includes 28 species or higher taxa of neozoa (exotics). Ship canals connecting the Rhine with the Rhone, Ems and Danube rivers enable the migration of aquatic fauna between these rivers, e.g. the invasion of the amphipod Corophium curvispinum from the Caspian Sea and also influencing the changes in the density of zebra mussel (Dreissena polymorpha) on the stones. The invasions are related to increases in salinity, nutrient load and to a higher water temperature. The exotics are well adapted to the higher chlorine concentration and higher water temperature in the river. The biomass of neozoa exceeds that of native species.

The dependence of the macroinvertebrates on dissolved oxygen content is shown in Figure 4. During the periods of low oxygen content the number of insects decreases drastically (IKSR 2002c).

At present the Lower Rhine main channels have a low diversity of aquatic macroinvertebrates. Fifty-two percent of the aquatic insect taxa for which historical data exist occur only in the floodplain lake biotopes, 17 percent only in the main channels and 31 percent in both biotopes. Formerly, 46 percent of the insect species occurred in the floodplain lakes only, 37 percent in the channel biotopes and 17 percent in both biotopes. This means that the biodiversity of typical riverine taxa has been decreasing. On the other hand, unlike aquatic insects, the biodiversity of snails, mussels and especially macrocrustaceans is at present higher than that during the start of the last century (van den Brink et al. 1996).

Fish fauna and distribution

Lelek (1989) describes the historical changes in the fish fauna and a detailed information on the fish fauna in the Rhine is given in the publications of Lelek and Buhse (1992) and IKSR (1997; 2002 a). The latest information on the fish fauna in the Dutch part of the Rhine is summarized in Raat (2001).

The surveys are usually done by electro-fishing. The composition of the fish fauna in the Dutch Rhine is also monitored yearly by fyke and trawl fisheries. Information about the fish fauna also comes from surveys at the intakes to the water-cooling systems of power plants (Weibel 1991).

The biological inventory of 2000 (IKSR 2002 a) gave evidence of an impressive regeneration of the biocoenosis of the Rhine. Sixty-three fish species are present in the river basin (Tables 3 and 4; Figure 5). The indigenous ichthyofauna consists of 44 fish species and of these only the Atlantic sturgeon (Acipenser sturio L.) has not been recorded. Atlantic sturgeon disappeared because of excessive fishing, closure of the Rhine-Meuse delta and river channel degradation in the German part of the Rhine. The migratory allis shad (Alosa alosa (L.) and twaite shad (Alosa fallax Lacepede) which had disappeared from the river have again been recorded. Although blageon (Leuciscus souffia Risso) did not occur in the ICPR 2000 survey it is a common species in the Upper Rhine (Schwarz 1998). The stock of bullhead (Cottus gobio L.) seems to be more widespread, not only distributed in the Lower Rhine (Köhler, Lelek and Cazomier 1993). The presence of the young of rheophilous species such as barbel (Barbus barbus (L.) shows that the river provides enough dissolved oxygen for their existence.

Figure 4. Macroinvertebrates and oxygen content in the Rhine at Bimmen (From IKSR 2002 c)

Twenty exotic species are present but they do not dominate the fish community. Because of the connection of the Danube basin through the Rhine-Main-Danube canal new species, such as white-eye bream (Abramis sapa (Pallas) and tubenose goby (Proterorhinus marmoratus (Pallas), appear more frequently (Schadt 2000; Lelek 1996; Lelek and Brenner 2002). The white finned gudgeon (Gobio albipinnatus Lukasch) has appeared recently (Freyhof, Staas and Steinmann 1998; Freyhof et al. 2000), probably due to stocking. The present fish fauna is dominated by eury-topic cyprinids. Rheophilous species have declined in numbers and anadromous fish have become scarce or extinct. Because of their greater tolerance to environmental changes the dominant species are bream, white bream and roach. Phytophilous northern pike (Esox lucius L.) and rudd (Scardinius erythophthalmus (L.) decreased in density with the decline of riverine vegetation in the Rhine (Raat 2001). Sea lamprey (Petromyzon marinus L.) and river lamprey (Lampreta fluviatilis (L.) are common but have decreased in numbers due to the closure of the Zuiderzee in 1932 and the closure of the Haringvliet in 1970. However, since the 1980s captures of these species in the Netherlands have slightly increased.


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