The EU Water Framework Directive (WFD) requires river assessment systems based on benthic invertebrates. The AQEM project is developing, at a European scale, such a methodology, based on a comparison of communities of reference streams and degraded streams. The project is focussing on three main impact types: morphological degradation, organic pollution and acidification. This paper presents the outline of the AQEM project with special emphasis on: -an overview of assessment systems with benthic invertebrates presently applied in Europe -an overview of stream typology approaches in Europe -the selection process of the stream types investigated in AQEM -the criteria used to select reference sites -the design of the sampling programme -the microhabitat-based sampling method applied in AQEM -the links and possibilities of integration between AQEM and existing assessment systems.

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We analysed the spatial patterns in macroinvertebrate taxon richness and abundance at two scales: sampling unit and basin. We sampled 12 stream sites in three zones of Portugal, differing in climate geomorphology and water chemistry. At a sampling unit scale, substratum organic matter content, depth and the dominant size of substratum particles were correlated with numbers of taxa and individuals. We propose that the number of taxa at a small scale depends on the number of individuals, which in turn is the result of organic matter accumulation, hydrologic and substratum characteristics. The environmental parameters better explaining the large-scale biological data were temperature, minimum size of substratum particles and pH. Regardless of the relative importance of variable types and mechanisms regulating stream invertebrates along the climatic gradient, rivers from the North and Centre appeared to be richer in taxa than the typically Mediterranean streams in the South. IntroductionOne important attribute of communities is their species richness and diversity. Various mechanisms have been indicated as controlling species diversity, including productivity, habitat heterogeneity and biotic interactions (TOWNSEND, 1989;TOWNSEND et al., 2002). Rivers and their adjacent riparian zones are considered to be dynamic, complex and diverse systems (RISSER, 1990;NAIMAN et al., 1993). Benthic macroinvertebrates are an important component of the river biota. At a large scale, diversity of invertebrates along and among rivers is affected by factors such as longitudinal zonation of river abiotic conditions (VANNOTE et al., 1980;CLENAGHAN et al., 1998), channel width and catchment size (MALMQVIST and HOFFSTEN, 2000), discharge (CORTES et al., 2002), temperature and pH (TOWNSEND et al., 1983;CLARKE and SCRUTON, 1997).At a stream reach scale, invertebrates have, in general, a clumped distribution, which is assumed to be related to the mosaic of interchanging conditions in substratum, flow conditions, depth and many others (TOWNSEND, 1989;CORTES et al., 2002). These conditions are likely to change at a scale of only a few metres or centimetres. At this small scale, factors indicated as regulating macroinvertebrate distribution and richness are current velocity, substratum particle size (MARCHANT et al., 1985;WILLIAMS and MOORE, 1986;ARUNACHALAM et al., 1991), substratum stability (MALMQVIST and OTTO, 1987), organic matter in the Internat. Rev. Hydrobiol.

The composition of macroinvertebrate assemblages was studied at 31 sites in the Sado basin, a lowland area with intermittent rivers located in southern Portugal. Riparian features of the sites, including bankfull width, composition, cover, shade, and relative amount of tree debris, were also measured, and local and basin abiotic variables were determined. Woody vegetation in the Sado basin was species poor and was dominated by the ash Fraxinus angustifolia, the alder Alnus glutinosa, and the willows Salix alba and S. salvifolia. The study sites included a broad range of cover and riparian longitudinal integrity. Macroinvertebrate assemblages revealed low levels of richness (4-29 taxa, average 17.6) and Shannon's diversity (0.65-2.87 bits, average 1.89), and were dominated by species or groups (especially chironomids) that tolerate organic pollution and habitat disturbance. Assemblages were composed of collector-gatherers (5.4-100%, average 58.1), and to a lesser extent collector-filterers (0-81.3%, average 14.1) and shredders (0-25.5%, average 10.6). Partial canonical correspondence analysis was used to relate the macroinvertebrate assemblages to the riparian characteristics of a site and to local and basin abiotic variables. Out of 8 riparian variables, overhanging tree shade, total tree cover, and abundance of F. angustifolia and A. glutinosa were significant predictors of assemblage variation. Of the 21 abiotic variables considered, only conductivity, distance from source, and the average number of flowless summer months were significant predictors of assemblage variation. Total macroinvertebrate variation was divided into portions that were: 1) explained solely by abiotic variables (ϳ14%); 2) explained solely by riparian variables (ϳ18%); 3) explained by both riparian and abiotic variables (ϳ2%); and 4) unexplained (ϳ66%). Riparian features had greater influence than other environmental characteristics on the composition of macroinvertebrate assemblages in the Sado basin, perhaps because riparian features are closely related to food types.

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The generalization of ecological results from temporary streams needs the study of those ecosystems across a broad scale of running waters, covering the geographical ecological constraints involved (e.g., precipitation, temperature and geology). For the particular situation of Mediterranean streams in Southern Europe, high changes in water level, with unpredicting flood events, represent an important factor determining the structure and function of Mediterranean streams. This variability, inducing accentuated changes in the macroinvertebrate community, tends to influence the assessment methodologies. A set of metrics commonly used in Europe to assess organic degradation (Shannon-Wiener index, evenness, richness, BBI, IBE, BMWP , ASPT , DSFI, EPT, number of Trichoptera families, percentage of Gasteropoda, Oligochaeta and Diptera) and a multimetric index (IM9) developed to assess organic degradation in southern siliceous Portuguese basins were tested to evaluate quality at two contrasting sites (one unimpaired and another impaired by an identified point source of pollution). The multimetric index (IM9) composed by: ASPT (average score per taxon); GOLD (one minus percentage of Gasteropoda, Oligochaeta and Diptera); and TRICF (number of Trichoptera families), was the most suitable assessment methodology. IM9 presented a quite stable temporal pattern from February in late winter until June in early summer, even under the effect of episodic floods. A stepwise regression showed that most of tested metrics were significantly related to environmental variables (soluble reactive phosphorous, dissolved inorganic nitrogen and dissolved oxygen). Only richness, IBE and BMWP were not significantly influenced by environmental variables. Future research must be done covering the complete gradient of organic degradation, including the extension of multimetric assessment methodologies to temporary streams located in other regions under different geological and climatic conditions.

a b s t r a c tLarge wood is a critical element in stream ecosystems, but only a subset of wood pieces actually provide hydraulic, geomorphic, and ecological functions. We test the current paradigm that larger pieces provide more function, and examine the role wildfires may play in affecting functionality of recruited wood. We conducted a cross-basin analysis in nine central Portugal watersheds, obtaining a variety of measurements on 1483 wood pieces (diameter ≥ 0.05 m; length ≥ 0.5 m) in 27 streams burned within six years prior. We examined nonlinear relationships and indirect effects on function using Generalized Additive Modeling and Structural Equation Modeling. Variables with direct effects on function were piece diameter, rootwads, anchoring, position (bridging, ramping, loose), longitudinal distance along the stream continuum, and the ratio of piece length to channel width. The effect of length ratio on function was nonlinear. Relatively long pieces were more likely to be functional until they were ∼3 times the channel width, at which point longer pieces became less likely to be functional. Post-fire wood likely lacked complexity and longer pieces were more likely to be bridging; both conditions may have prohibited them from interacting with the wetted area. Wildfires had indirect effects on function. Burned pieces were more likely to be large in diameter (thus more likely functional) but not anchored (thus less likely functional); these antagonistic effects may be the reason burned status had no direct effect on function. Our results challenge the wellestablished idea that the function of wood in streams is simply a matter of wood size, along with indicators of longevity (e.g. stability and decay status). Relatively long pieces may in fact provide less function to the stream, at least until they break or are transported further downstream. Practitioners installing wood to streams should consider pieces with wide diameter and rootwads, approximately 3 times the channel width, and anchored but not bridging the channel.

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