Benthic cyanobacterial mats serve as a refuge and food for the amphipod Gammarus fasciatus

We investigated the ecological role of Lyngbya wollei (Farlow ex Gomont, comb. nov.) cyanobacterial mats in aquatic ecosystems, especially as a food source and shelter for the amphipod Gammarus fasciatus (Say). For this, we offered amphipods a choice between mats of L. wollei and either chlorophytes (Spirogyra, Rhizoclonium) or an artificial mat made of acrylic wool in laboratory experiment. Moreover, we reconstructed in situ amphipod diet using a dual isotope mixing model (δ¹³C and δ¹⁵N). G. fasciatus consistently selected the substratum offering the best light refuge (Acrylic > Lyngbya = Rhizoclonium > Spirogyra). Neither the presence/absence of saxitoxins, nor the tissue elemental composition and epiphyte abundance exerted any significant effect on substratum choice. L. wollei and its epiphytes constituted 36 and 24%, respectively, of the in situ diet of G. fasciatus, whereas chlorophytes, macrophytes, and associated epiphytes represented a less important fraction of its diet. Benthic cyanobacterial mats should be considered a good shelter and food source for small omnivorous invertebrates such as amphipods.