Impact of climate on the vertical water column structure of Lajes Reservoir (Brazil): A tropical reservoir case

Ribeirão das Lajes Reservoir was constructed in 1905 for energy production, and is now used as a domestic water supply for about 1 million people. The reservoir was considered as being general monomictic, exhibiting water column mixing in the winter. Water column mixing, however, did not occur in the deepest parts of the reservoir in the warm year of 2005. Nevertheless, the long stratification period leads to an anoxic, nutrient-rich hypolimnion that exhibits poor water quality in the deepest part. The meteorological events of cold front passages in the reservoir region were not able to disrupt the water column stability, or affect its water quality for domestic supply. Maintenance of thermal stratification over most of the year was likely because of the low influence of wind, long water retention time, and the input of cold water from the tributary and from rain directly draining into the metalimnion.     

Fish zonation and indicator species for the evaluation of the ecological status of rivers: example of the Loire basin (France)

In the context of river alteration, ecologists are asked to develop tools for the assessment of river integrity. Fish are known to be good bioindicators of the ecological condition of rivers. The Loire basin (France) is often considered as relatively little impacted compared to most other large European systems. But curiously, no study clearly addressed the question of fish assemblages patterns in this system in order to assess this status. Thus, we studied fish assemblages along the river network in the Loire basin using self‐organizing maps (SOMs) and we built a fish typology. Four basic assemblages were described and indicator species were identified. These assemblages varied in terms of individual species patterns as well as in terms of flow preference guilds and species richness. A discriminant analysis carried out on environmental variables revealed that they could be mainly determined by the slope, temperature and depth. Finally, fish assemblages were arrayed along a longitudinal gradient and roughly fitted the theoretical zonation expected in European rivers with the succession of brown trout (Salmo trutta fario), grayling (Thymallus thymallus), barbel (Barbus barbus) and bream (Abramis brama) zones in a downstream direction. Such patterns are still rarely observed in large European systems. However, the fish assemblage characteristic of the bream zone occurred more frequently than predicted on the basis of environmental variables. Such deviations between field data and theory suggest lotic‐to‐lentic shifts probably due to anthropogenic disturbances, especially in the grayling and barbel zones. In these river sectors, eurytopic and limnophilic species tend to replace rheophilic ones. Finally, the method used in this study to investigate fish patterns may be helpful to detect disturbances and may serve as a tool for the establishment of management plans. Copyright © 2007 John Wiley & Sons, Ltd.