Biodiversity of Macroinvertebrates in Oxbow-Lakes of Early Glacial River Basins in Northern Poland

River basins are very complex systems which include both abiotic and biotic elements. Such a high number of components results in a situation that normal functioning of river with adjacent areas depends on a set of hydrological and geological processes. They influence biological diversity observed in river basins (Arscott et al., 2005; Marshall et al., 2006). Proper assessment of a lotic system should include not only the main watercourse but also wetlands flooded in spring and autumn. The diversity of habitats in natural river valleys increases biological diversity of aquatic ecosystems and thus the quality of environment (e.g., Boulton et al., 1992; Clausen & Biggs, 1997; Tockner et al., 1999; Gibbins et al., 2001; Sheldon et al., 2002; Arscott et al., 2003; Robinson et al., 2003, 2004; Arscott et al., 2005; Whiles & Goldowitz, 2005; Gallardo et al., 2008; Reese & Batzer, 2007; Obolewski, 2011a; Obolewski & Glińska-Lewczuk, 2011). Each meandering, lowland River is forming its riverbed constantly. Often after rapid floods it turns out that a river flows in new riverbed and the cut-off fragments transform into oxbow-lakes (Amoros & Roux, 1988). They can be filled up during river rises and undergo succession (Junk et al., 1989; Tockner et al., 2000). Due to the diversity of river rise intensity, the connectivity between an oxbow-lake and a river can vary. Therefore, we distinguish lentic, semi-lotic and lotic oxbow-lakes. The first type is supplied by river waters under high water table level or by ground waters while the two remaining types are partly or totally connected to a river. Limited exchange of water in a river valley as well as its agricultural use causes that oxbow-lake drainage area often undergoes anthropopression which leads to quick eutrophication and massive phytoplankton blooms. As a result, water contains considerable amounts of biogenes, mineral salts but low oxygen content. Additional unfavourable factors are hydromorphological features of oxbows, i.e. small area (between a few hundreds squared meters and a few hectares) and depth which usually does not exceed 3 meters. As a result oxbows quickly react to changes in temperature and thanks to that they are perfect objects for the research on climate changes even in the global depiction (Klimaszyk, 2004). The structure and functioning of wetland ecosystems, including oxbow-lakes, are directly and indirectly connected with the fluctuations in water table level of rivers during floods or flow pulsations (Junk et al., 1989; Tockner et al., 2000). According to Amoros & Roux (1988),