The Rivers Rhine and Meuse have served as vital European transport arteries for centuries, as well as sites for urban and industrial development and water resources. Thus, the two rivers are of considerable economic importance but have been subject to substantial anthropogenically derived changes over time. The River Rhine, a combined glacier-rainfall river, originates in Switzerland (2,200 m above sea level) and flows over 1,250 km through France, Germany, and the Netherlands with a drainage area of 185,000 km2. In the Netherlands, it divides into three branches: Waal (65 percent of discharge), Lek (21 percent discharge), and Ijssel (14 percent discharge). The River Meuse is fed by rainwater, originates in France (410 m above sea level), and flows over 890 km through Belgium and the Netherlands, with a drainage area of 33,000 km2. Both rivers flow into a lowland area where they form a river delta before entering the North Sea (van den Brink 1994). The earliest documented human influence on these rivers occurred in the Roman era and involved the construction of canals to regulate discharge into the Dutch Rhine tributaries (van Urk & Smit 1989) and embankments started in the Middle Ages. In the 18th century, the Rhine River floodplain was still tens of kilometers wide, and the river meandered and supported an extensive riparian forest. Large-scale river regulation began in the 19th century, with construction of dams and sluices for sea flood protection, dams for river regulation, and groins (breakwater boulder piles extending laterally into the river), weirs, and dykes to facilitate shipping. These changes impeded natural meandering and formation of side channels, cutoff channels, and oxbow lakes; consequently, the floodplain shrank dramatically (van den Brink 1994). Since then, nearly all the floodplain forests have been cut, the riparian forests have been largely removed, and the existing riparian areas have been degraded. The significant loss to the aesthetic and recreational services is self-evident.
The water quality of the main channels of both rivers has changed considerably since measurements began in the early 1900s, with increased levels of nutrients (nitrate and phosphate), salts (chloride, sodium, and sulphate), and heavy metals (cadmium, mercury, lead, and zinc) (van de Weijden & Middleburg 1989). In addition, increasing levels of organic micropollutants such as polychlorinated biphenyls (PCBs), para-aminohippuric acid (PAHs), insecticides, and herbicides have contaminated the sediments. The lower sections of the two rivers accumulate inputs from several countries upstream and are the most polluted. In the 1960s—1970s, oxygen levels were extremely low, which affected the abstraction and provision of quality drinking water. More recently, construction of sewage treatment plants has improved the Rhine, although the Meuse still suffers from low oxygen, particularly in summer (van den Brink 1994). As a result of thermal pollution from power plants and industries, water temperature in the lower Rhine and Meuse has risen by 2 to 4° C since 1900.
Not surprisingly, there have been dramatic changes in the biotic communities of the rivers. Plankton biomass in the river channels has increased, and is now dominated by a few ubiquitous centric diatoms and green algae (Admiraal et al. 1993). These add a considerable economic cost on filtration of the abstracted water. At present, the waters of the lower Rhine are dominated by sodium chloride instead of calcium bicarbonate (chloride levels increasing from < 20 mg/l in 1874 to > 200 mg/l in 1985; van den Brink et al 1990), which, together with the increased temperature, has created an environment that permitted the invasion of several exotic brackish-water and eurythermic macro-invertebrate species. These include exotic species introduced from North America and Eastern Asia and others that have immigrated from the Mediterranean and Ponte Caspian areas. One species is the benthic filter-feeding amphipod crustacean Corophium curvispinum, an invader originally from the southern Ponte Caspian area, which has expanded its range since 1900 from the rivers entering the Caspian and Black Seas via canals and rivers to western Europe, probably aided by shipping. It was first documented in the middle then lower Rhine in 1987, and within a couple of years increased explosively to become the most abundant species in the Rhine system. This species also reached the Belgian part of the River Meuse in 1981 and the Dutch part by the end of the 1980s. This invader has had a significant impact on the Rhine ecosystem (Neumann 2002). Its high fecundity, short generation time, and small size have led to massive densities (rising from 2/m2 in 1987 to 200,000/m2 in 1991 on stones of groins in the lower Rhine, van den Brink et al. 1993a), increased filter-feeding activity, and competition for food and space with other species, including other exotic invaders such as the amphi-pod crustacean, Gammarus tigrinus, and the zebra mussel, Dreissenapolymorpha. Gam-marus tigrinus invaded the lower Rhine in 1983, reducing abundance of the native amphipod G. pulex. Dreissena spp. invaded Europe from the Black Sea and Caspian Sea over two centuries ago, before the Industrial Revolution, but disappeared from the lower Rhine in 1960s due to the poor water quality and high levels of cadmium. Reductions in cadmium levels lead to Dreissena polymorrpha's re-establishment in 1975 and its subsequent rapid population increase. However, the species has now dramatically declined since 1987 due to competition for space with the nonnative Corophium (van der Velde et al. 1994). Meanwhile, a number of native brackish-water crustaceans such as the benthic amphipod Gammarus zaddachi range more than 100 km upstream of their original distribution boundary, as a result of increased river salinity (van den Brink et al. 1990, 1993b).
The rivers' benthic community is now largely a pollution-tolerant one, with typical pollution-sensitive aquatic insects (ephemeropteran, trichopteran, and plecopteran species) having disappeared prior to 1940. Because the latter two families are involved in detrital breakdown, the decomposition process was likely affected. Species richness of macroinvertebrates declined from 83 around 1900 to 40 in 1987. The fish community was completely dominated by cyprinids (particularly roach) in the 1970s, and anadromous and rheophilous species declined or disappeared altogether (van der Velde et al. 1990). For example, the salmon (Salmo salar), was overexploited and became extinct despite large-scale restocking attempts, thus negatively impacting recreational services provided by the rivers. In addition, changes in the macroinvertebrate communities and the invasion of exotic species led to changes in the river food web structure and the diet of the major predatory fish (Kelleher et al. 1998).
Over the past two decades, various restoration management measures were implemented that began to reverse some of these impacts (Jungwirth et al. 2002). The Rhine Action Plan established by the Dutch government involved all the countries bordering the river and implemented various measures to restore water quality and habitat structure. Discharge of raw sewage and industrial wastes has decreased. The much-publicized Sandoz incident in 1986 (Lelek & Kohler 1990; Mason 1996), which involved huge inputs of insecticides following a major fire in a chemical plant in Switzerland, led to the closure of water diversion plants along the river and other controls and restrictions. Despite such setbacks, some evidence of success has been seen in the rediscovery of several benthic riverine species in the lower Rhine, including the net-spinning caddis fly Hydropsyche conturbernalis, the water bug Aphelocheirus aestivalis, the damselfly Calopteryx slendens, and the freshwater mussels Anadonta anatina and Unio pictorum (van den Brink et al. 1990). The number of fish species has increased, rising from a low of 12 in 1971 to 25 in 1987 (van der Velde et al. 1990). Further reduction of the pollution loads in the entire drainage basin has focused on nutrients, heavy metals, and organic micropollutants (such as PCBs and PAHs). Restoration of wetland vegetation, floodplain lake water quality, and, in particular, connections between the main channel, floodplain lakes, and side channels were suggested as being particularly important from a biodiversity perspective (van den Brink 1994). Indeed, the creation of permanently flowing secondary channels on the Rhine floodplain in 1994 showed that within five years these artificial secondary channels function well as an appropriate habitat for riverine species, including the more demanding rheophilic species (those that prefer to live in running water), and have thus contributed to the ecological value of the river (Simons et al. 2001). Jungwirth et al. (2002) give a number of other examples of similar river and floodplain restoration projects.
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