Integrated management of aquatic ecosystems is a common concern for water resources managers as well as for researchers. The 1992 French Water Act aims to preserve the biotic and ecological integrity of aquatic ecosystems as part of water management schemes, creating needs for new methodologies for studying these systems. This paper describes the partitioning of the Loire basin (105000 km 2 , France) into hydro-ecoregions tested at the mesohabitat scale. The following null hypotheses were examined in the four largest hydro-ecoregions of the basin: (1) differences in mesohabitat types distribution do not exist between regions; (2) the longitudinal structure of mesohabitat types distribution is not different between regions; and (3) the factors governing distribution and longitudinal evolution of mesohabitats distribution are not different between regions.
It was found that the four regions behaved in different ways in terms of distribution and longitudinal evolution of mesohabitats. Valley slope and stream order, the two tested control variables, do not play the same role in each region. If the region contains mainly alluvial rivers, slope and/or order do explain or predict mesohabitat distributions. If the region contains cohesive rivers, these factors do not, or poorly explain, mesohabitat distributions.
Since predictive models cannot be developed in most regions for mesohabitat distributions, it is necessary to build descriptive models at the regional scale. In addition to one regional predictive model, this paper provides such results for three regions of the Loire basin.