| Summary: | Background and aims – The host-specificity of epiphytic diatom species has long been debated. Scuba divers sampled epiphytic diatoms in the shallow Alpine Lake Valagola (average depth c. 2 m) along seven transects (length: 30–144 m) in West-East direction. The bottom of the tarn was covered by macrophytes dominated by Chara aspera and Potamogeton gramineus. Factors affecting epiphytic-diatom spatial distribution at a fine scale were tested. Methods – Dataset was tested using Redundancy Analysis (CANOCO package) and one-way ANOVA (NCSS package). Key results – The analysis separated sampling sites into two groups: the tarn shore dominated by Potamogeton gramineus , and the central area dominated by Chara aspera . Diatom species richness, diversity, and composition differed significantly between the two main host plants. Potamogeton gramineus assemblages were characterized by higher species richness and diversity, and by the large-celled, adnate diatom species Epithemia adnata, Rhopalodia gibba, Eunotia arcus and E. arcubus. Chara aspera was preferred by the small-celled, motile diatom species Brachysira neoexilis and Encyonopsis cesatii . Conclusions – The spatial distribution of epiphytic diatoms in the shallow, oligo-mesotrophic Lake Valagola is influenced by host plant composition and distribution. Epiphyton size structure suggests that Chara represents a less appropriate substrate for long diatoms.
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