SUMMARY1. In an enclosure experiment carried out in a ditch receiving sulphate-enriched seepage water, iron(II) chloride was added to the sedim ent In the sediment pore water of the iron-treated enclosures sulphide levels decreased to very low values (< 1 jumol I"1) immediately after the iron addition while in the control enclosures sulphide reached values up to 500 \imol I-1. 2. The sulphide levels in the sediment pore water were also strongly correlated w ith temperature. In summer, phosphate m obilization was observed in the non-treated enclosures while in the iron-treated enclosures phosphate levels remained low. 3. Total phosphate levels increased greatly in the water layer of the noivtreated enclosures, coincident with an algal bloom and increased turbidity. It is suggested that phosphate mobilization in summer is caused by the reduction of iron(III) phosphate complexes and in this high sulphate water body probably also by the reduction of iron(III) by sulphide and the consequential precipitation of iron(II) sulphide. 4. Iron addition appeared to prevent sulphide toxicity in Potamogeton acutifolius Link which was planted in the enclosures immediately after iron(II) addition. In the non-irontreated enclosures P. acutifolius plants decayed w ithin a few weeks probably as a result of sulphide toxicity
Macrophytes are one of the biological quality elements to be included in an integrated description of the ecological status of flowing waters under the European Union Water Framework Directive (WFD). However existing knowledge on macrophyte assemblage patterns in un-impacted European streams and rivers is limited and several problems may therefore arise regarding their assignment to ecological quality classes. In the present paper we investigated community structure i.e. composition, richness and diversity measures in 60 un-impacted stream and river sites situated throughout Europe. We found that the macrophyte community structure varied considerable among the main stream types. Moving from the small streams in upland areas (core stream type 1) to middle-sized lowland streams (core stream type 2) there was a clear transition in species richness, diversity and community structure. Especially there was a shift from a predominance of species poor mosses and liverwort dominated communities in core 1 sites to more species rich communities dominated by vascular plants in core 2 sites. We found that the macrophyte communities responded to most of the features underlying the typological framework defined in WFD. However our results raise the question whether the previous translation of the WFD typology is adequate for an evaluation of stream quality based on macrophytes. First, by using this typology we may overlook an important community type, that is characteristic of small-sized relatively steep-gradient streams being an intermediate type between core 1 and core 2 streams. Second, the natural variability in most of the calculated metrics is higher when using the pre-defined typology. This implies that we may improve our ability to detect impacts in streams and rivers by refining the typology to more adequately describe the macrophyte communities.
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