Juvenile fish recruitment in the large lowland river Oder: assessing the role of physical factors and habitat availability (dedikált példány) [antikvár]

1 INTRODUCTION Floodplain rivers in temperate zones have undergone profound modifications through direct and indirect humán influence (Fittkau & Reiss, 1983; Sparks et al, 1990; Swales, 1994; Ward, 1995). During the last 100 years, river channels have been straightened for ship traffic, dams have been constructed for hydroelectric power generation and flood control (Petts, 1984; Maddock, 1999) and considerable changes in water quality have occurred, particularly in lowland areas where the impacts of urbanisation and agriculture are pronounced (Sweeting, 1994). In many rivers, water pollution has been reduced markedly since the nineteen seventies and therefore, the water's chemical quality is no longer regarded as an impediment to the recovery of the aquatic biocoenosis (cf. Tittizer et al, 1990; Lelek & Buhse, 1992; Lelek, 1996; Van den Brink et al, 1996; Bakker, 1998). However, the modification of natural flow (Ward & Standford, 1983) and the resulting degradation of habitat (Bain et al, 1988; Boon, 1992) remains one of the most severe anthropogenic effects on river systems. At present, most flood plains are separated from the rivers by dikes, and flooding is inhibited (Reimer, 1991; Junk, 1999). In Germany, only about 10 % of the natural floodplain area are still in a nearly natural condition (Junk, 1999). In the United States, riparian forests have been destroyed on about 70 % of the rivers (Swift, 1984) and merely 2 % of the water courses run unimpeded (Vitousek et al, 1997). Accordingly, substantial losses of riverine species have been documented (Naiman & Bilby, 1998) and engineering works had in many cases devastating consequences for the fish fauna (cf. Welcomme, 1985; Lelek, 1987; Schiemer & Waidbacher, 1992). The ecological situation of the River Oder, one of the largest rivers in Central Europe, is no exception: between 1892 and 1931, the canalisation of the Lower Oder and the building of dikes progressed rapidly (Hermann, 1930; Kieseritzky, 1938). Dredging and the construction of groynes have deepened the river's main channel and left few natural river banks intact, resulting in a considerable demographic decline of sensitive fish species over the past decades (Trzebiatowski, 1999; Wolter et al, 1999). Despite these limitations, somé of the dynamic characters of a floodplain ecosystem have been maintained in the Lower Oder Valley, which make it suitable for thorough ecological investigations (Dohle, 1999). From an ecosystem perspective, large rivers have generally not been as well studied as small streams (Décamps, 1984; Sedell et al, 1989), partly because of methodological problems. As a result, the diversity and complexity of large river systems have been