River Systems 

Abstract: The ecology of large rivers is a long neglected but now rapidly expanding field . Several theories have been proposed during the last decade on hierarchically structured determinants of ecosystem processes, community organization and biod1VerSlty (FRIS$ELL et al . 1986 ; WARD 1989; WARD oL STANFORD 1995 b) . The applicability of VANNOTE et al .'s (1980) river continuum concept of carbon flux to high order river reaches with extensive floodplain areas was questioned (SEDELL et al . 1989). The floodpulse concept (JuNx et al . 1989) refers specifically to these lateral exchange processes between the river and the semi-terrestrial adjoining area, that are inundated by regular or irregular spates . The river productivity model emphasizes, in contrast, that autochthonous production in the channel and connected backwaters may be significant in the carbon dynamics of large artificially constrained rivers (THORP 1994). In a recent article, REYNOLDS & DESCY (1996) discussed the significance of hydraulic storage as a function of channel morphology for river phytoplankton recruitment and production . They argue that reach retentivity, in relation to growth performances of algal species, can explain the nature of river phytoplankton associations and their productivity. For streams retentiveness means the physical ability of channels to retain particulate organic matter as food. It also defines the refuge capacity allowing higher community persistence (HILDREW 1991; HILDREW et al . 1996 ; LANCASTER et al . 1996). We propose that a mechanistic model relating hydraulic retention to biological functions can be applied for different scales of hydrological storage to form a general framework for the understanding of the ecology of large rivers . Inshore retention may be specifically critical in channelized and regulated rivers .