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Chironomid larvae and pupae were studied in selected Mediterranean rivers with the aim of identifying pool and riffle taxa assemblages and of analysing their response to ecological quality gradients. Macroinvertebrate samples were collected in six Italian rivers along a pool-riffle sequence in three seasons following a multihabitat sampling technique. Chironomids were identified as genus/species, other macroinvertebrates as family/genus. The main physico-chemical, hydromorphological and geographical data were collected. Samples were ascribed to five quality classes according to the STAR_ICM index. Based on Chironomid taxa, principal component analysis (PCA) axis 1 represented an organic pollution gradient, axis 2 represented seasonality. Pool and riffle samples were significantly different according to taxa assemblages. Similar results were obtained with PCA based on the whole macrobenthic community. Indicator value (IndVal) analysis facilitated the detection of the Chironomid indicators of high/good quality pools (e.g. Conchapelopia pallidula, Rheopelopia ornata, Epoicocladius ephemerae) and riffles (Tvetenia calvescens, Eukiefferiella gracei). The Berger-Parker dominance index based on Chironomid assemblages in pools was correlated to PCA axis 1 and performed well in discriminating between quality classes. In riffles, no correlations to PCA axes were detected and a wide overlap between quality classes was present. Thus, assessment in the analysed river type may focus on pool mesohabitat as this seems to represent best the ecological gradient of sites.

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Class boundaries of three European assessment systems based on macroinvertebrates were compared and harmonized. Three different approaches to comparison, one based on regression analysis and the other two on statistical testing, were described and used, however only one was considered useful for the harmonization of boundaries. In all cases, the calculations were based on a set of six Intercalibration Common Metrics, combined into a simple multimetric index (ICMi). The ICMi was calculated for three test datasets from Italy, Poland and the UK, all belonging to the same stream type (small lowland siliceous sand rivers). For comparison, a regression model was employed to convert national assessment boundary values into ICMi values. The ICMi was also calculated on samples included in a strictly WFD-compliant benchmark dataset. The values of the ICMi obtained for the quality classes Good and High for the test and benchmark datasets were statistically compared. When significant differences were observed in the harmonization phase, the boundaries of the national method were refined until no further differences were observed. For the test datasets and assessment systems of Italy (IBE index) and Poland (Polish BMWP index) small refinements of the boundaries between High/Good and Good/Moderate classes were sufficient to remove the differences from the benchmark dataset. After harmonization, in the studied stream type, the percentage of samples requiring restoration to Good quality increased by 22 and 6% for Italy and Poland, respectively. For the UK dataset (EQI ASPT) the comparison to benchmark dataset showed no significant differences, thus no harmonization was proposed. A general discussion of the options used to compare boundaries based on the ICMi and their potential for harmonization is provided. Lastly, the option of harmonizing class boundaries through comparison to an external, benchmarking dataset and then re-setting them until no differences are found is supported.

313In accordance with the aims of the E.U. funded AQEM Project, an assessment system module based on aquatic macroinvertebrates was developed for small sized rivers in the southern Apennines (south Italy). Eleven stream sites, impacted to a greater or a lesser extent by organic pollution and/or habitat impairment and chosen to cover the whole degradation gradient present in the geographical area were sampled in three seasons. The samples were collected following a proportional, multihabitat procedure, afterwards considering separately the replicates collected in the depositional (pool) and transport (riffle) areas for the analysis. A PCA multivariate analysis was performed to extract the main axes of variation of the biological community, which resulted in the first axis being strongly correlated to ecological quality. The final assessment module is based on a multimetric system, structured by selecting the best metrics in simulating the first axis gradient. The system considers a total of 15 different metrics, mainly providing information concerning tolerance to pollution, taxa richness, habitat features and trophic structure of the community. In accordance with the WFD requirements, some of these metrics are based on abundance classes of taxa. Depositional and transport units, due to the observed dissimilarity in the structure of their benthic communities, were kept separate during the development of the assessment system to retain this potentially useful information and to clear interpretation of the results. Both 'riffle' and 'pool' invertebrate data showed clear differences in ecological quality between sites. Nevertheless, the final assessment module is based on the macroinvertebrates inhabiting depositional areas of rivers only, because the metrics for these river units showed a better performance than those examined for the transport river units. The application of the assessment module requires 10 replicates to be quantitatively collected, for a total area of 0.5 m 2 . In terms of sampling and identification effort, the assessment module shows a good comparability with the standard Italian method presently in use and might thus be easily applied for river sites classification according to the Water Framework Directive in southern Italy. The site classification obtained with the proposed multimetric index shows a very good correspondence with the post-classification based on multivariate analysis.