Microbial mats as shelter microhabitat for amphipods in an intermittent karstic spring

Microbial mats represent complex communities where cyanobacteria and diatoms as key organisms provide shelter for diverse assemblages of aquatic invertebrates, like the small stygophilous amphipod Synurella ambulans. Studies addressing such communities in the karst springs have rarely examined springheads, and have ignored intermittent springs. During high flow conditions the stygophilic crustaceans are flushed to the surface of a temporary stream Krcic where microbial mats prevent their drift and enables their successful retreat into underground in the periods of drought. The objective of this study was to characterize the microbial mat community of the Krcic Spring as a shelter for S. ambulans during strong current and high water level. Representative samples for diatom and cyanobacterial species identification and composition, as well as the fresh mat material for potential animal activity and cyanobacterial phylogenetic analysis were collected. The most dominant diatom was Achnanthidium minutissimum, whilst Fragilaria capucina, Meridion circulare, Navicula cryptocephala and Nitzschia palea had abundance greater than 0.5%. Morphological observations of cyanobacteria revealed that Phormidium favosum was the most dominant, with Hydrocoleum muscicola as a subdominant. Cyanobacterial phylogenetic relationship revealed two distinct clusters: ?(i) "Phormidium cluster", confirming morphological observations in both winter and spring samples, and ?(ii) "Wilmottia cluster", a first report for Croatia and found exclusively in the winter sample. Laboratory observations revealed a small stygophilic amphipod S. ambulans, hiding and feeding inside the pockets of fresh microbial mat. The intermittent Krcic Spring as a predator-free and competitor-free ecosystem provides a spatiotemporal conformity between microbial mat and stygophilous amphipod.