River ecosystems are subject to multiple stressors that affect their structure and functioning. Ecosystem structure refers to characteristics such as channel form, water quality or the composition of biological communities, whereas ecosystem functioning refers to processes such as metabolism, organic matter decomposition or secondary production. Structure and functioning respond in contrasting and complementary ways to environmental stressors. Moreover, assessing the response of ecosystem functioning to stressors is critical to understand the effects on the ecosystem services that produce direct benefits to humans. Yet, there is more information on structural than on functional parameters, and despite the many approaches available to measure river ecosystem processes, structural approaches are more widely used, especially in management. One reason for this discrepancy is the lack of synthetic studies analyzing river ecosystem functioning in a way that is useful for both scientists and managers. Here, we present a synthesis of key river ecosystem processes, which provides a description of the main characteristics of each process, including criteria guiding their measurement as well as their respective sensitivity to stressors. We also discuss the current limitations, potential improvements and future steps that the use of functional measures in rivers needs to face.

12 páginas ; 3 Figuras ; 1 TablaWe examined the effects of riparian vegetation removal on algal dynamics and stream\ud nutrient retention efficiency by comparing NH4-N and PO4-P uptake lengths from a logged and an\ud unlogged reach in Riera Major, a forested Mediterranean stream in northeastern Spain. From June to\ud September 1995, we executed 6 short-term additions of N (as NH4Cl) and P (as Na2HPO4) in a 200-\ud m section to measure nutrient uptake lengths. The study site included 2 clearly differentiated reaches\ud in terms of canopy cover by riparian trees: the first 100 m were completely logged (i.e., the logged\ud reach) and the remaining 100 m were left intact (i.e., the shaded reach). Trees were removed from\ud the banks of the logged reach in the winter previous to our sampling. In the shaded reach, riparian\ud vegetation was dominated by alders (Alnus glutinosa). The study was conducted during summer and\ud fall months when differences in light availability between the 2 reaches were greatest because of\ud forest canopy conditions. Algal biomass and % of stream surface covered by algae were higher in\ud the logged than in the shaded reach, indicating that logging had a stimulatory effect on algae in the\ud stream. Overall, nutrient retention efficiency was higher (i.e., shorter uptake lengths) in the logged\ud than in the shaded reach, especially for PO4-P. Despite a greater increase in PO4-P retention efficiency\ud relative to that of NH4-N following logging, retention efficiency for NH4-N was higher than for PO4-\ud P in both study reaches. The PO4-P mass-transfer coefficient was correlated with primary production\ud in both study reaches, indicating that algal activity plays an important role in controlling PO4-P\ud dynamics in this stream. In contrast, the NH4-N mass-transfer coefficient showed a positive relationship\ud only with % of algal coverage in the logged reach, and was not correlated with any algal-related\ud parameter in the shaded reach. The lack of correlation with algal production suggests that mechanisms\ud other than algal activity (i.e., microbial heterotrophic processes or abiotic mechanisms) may\ud also influence NH4-N retention in this stream. Overall, this study shows that logging disturbances\ud in small shaded streams may alter in-stream ecological features that lead to changes in stream nutrient\ud retention efficiency. Moreover, it emphasizes that alteration of the tight linkage between the\ud stream channel and the adjacent riparian zone may directly and indirectly impact biogeochemical\ud processes with implications for stream ecosystem functioning.Spanish Comisio´n Interministerial de Ciencia y\ud Tecnologı´a (CICYT), grant number AMB99-\ud 0499, and by the European program: Nitrogen\ud Control by Landscape Structures in Agricultural\ud Environments (NICOLAS, n8 ENV4-CT-97-\ud 0395).Peer reviewe