1. Macroinvertebrate assemblages were studied in the glacial river West‐Jökulsá, originating from the Hofsjökull Ice Cap in central Iceland at an altitude of 860 m. Sampling sites were distributed from the source to 45 km downstream at 160 m a.s.l. Comparative studies were carried out on non‐glacial rivers and tributaries in the area, at similar altitudes and distances from the glacial source. 2. Detrended correspondence analysis (DCA) demonstrated that species composition of benthic macroinvertebrates was related to the distance from the glacier. Assemblages at sampling sites furthest from the glacier were similar in species composition to sites in non‐glacial rivers. Temporal variation was small compared with longitudinal zonation. 3. Based on canonical correspondence analysis (CCA) of data from the main glacial river, distance from the glacier, altitude, bryophyte biomass and the Pfankuch Index of channel stability were the measured explanatory variables having a significant effect on the structure of macroinvertebrate assemblages, accounting for 31% of the total variation in the data set. When data from all the rivers were analysed, altitude, bryophyte biomass, channel slope, suspended sediment concentration and maximum water temperature explained 21% of the variance. 4. Macroinvertebrate communities were in general agreement with the predictions of the conceptual model of Milner & Petts (1994) for the upstream reaches. The assemblages consisted mainly of Orthocladiinae and Diamesinae (Chironomidae), although other taxa such as Simuliidae, Plecoptera and Trichoptera were also found in low numbers. Shredders were lacking from the benthic communities, apparently because of continued glacial influence in the river even 45 km downstream from the glacier and lack of allochthononus inputs from riparian vegetation.

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The genus Fragilaria was originally described by Lyngbye in 1819, although it was only in 1927 that a typus generis was designated by Boyer (1927): Fragilaria pectinalis (O.F.Müller) Lyngbye. Williams & Round (1987), later refined and completed by Tuji & Williams (2006a), provided a detailed description for the genus. Almost all species, nowadays transferred to the genus Fragilaria, were originally described in the genus Synedra Ehrenberg in the nineteenth century. Only a handful of scientists worked on this group then, such as