| Zusammenfassung: | ABSTRACT Little is known about the structural composition of macrophytic communities in Irish small streams and headwaters, despite their important influence on the biological integrity of the river catchment as a whole. A subset of 103 small stream and headwater vegetation plots were extracted from the River Macrophyte Database (RMD) with the aim of identifying the vegetation groups that occur there, to describe their composition and determine whether these vegetation groups have affinities to British and European vegetation communities or are unique to Ireland. The RMD has been recently collated as part of a wider project that aims to construct a vegetation classification system for Irish rivers. Hierarchical Cluster Analysis and Non-metric Multidimensional Scaling (NMDS) were used to identify these species assemblages. Nine cluster groups, representing different vegetation assemblages were identified; five of these groups were of biological significance and these are described in detail. Of these, two groups were typical of slow-flowing calcareous waters of lowland regions, one group characteristic of fast-flowing calcareous streams and two were typical of acidic fast-flowing waters found more commonly at a higher altitude. A Canonical Correspondence Analysis (CCA) biplot showed the relationship between the species distribution and three chosen environmental factors—flow rate, conductivity and substrate type. These factors were found to have significant influence on macrophyte distribution, despite the small percentage of the variance explained during analysis. Additional factors that were not available for analysis, e.g. nutrient levels, that may explain a greater percentage of the variation in macrophyte distribution are discussed.
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