How scour disturbance affects Didymosphenia geminata abundance and the associated epiphytic diatom community

Background/Question/Methods Blooms of the diatom Didymosphenia geminata have become increasingly common, reaching nuisance proportions in montane streams throughout North America, Europe, and most recently, New Zealand. The diatom's extracellular polysaccharide stalk comprises >90% of the algae's biomass. This matrix may persist long after its cells senesce and die. Historically a rare diatom, little is known about the factors promoting D. geminata blooms. To better understand the role of scour disturbance in regulating D. geminata abundance, we undertook a manipulative field experiment to examine how scour frequency affected 1) substrate cover by D. geminata, and 2) the epiphytic diatom community associated with D. geminata. The 19d experiment was conducted in the East River of Gothic, Colorado, consisting of 4 scour disturbance treatments within 10 replicate blocks. For each block, four stream cobbles of similar size and D. geminata cover were randomly assigned to be scoured 1, 3 or 6 times, or serve as a control. Scouring was accomplished by scrubbing the upper surface of the cobble with a scrub brush in a standard manner. Every third day, D. geminata percent cover was estimated, and on the last day, cobble surfaces were sampled to determine Chlorophyll-a, periphytic AFDM, and algal species composition.

Results/Conclusions Data from the study showed that scour treatments effectively eliminated D. geminata cover, but that recovery was rapid. Cobbles scoured 3 and 6 times were not significantly different from controls, whereas cobble scoured only once showed less mat cover (ANOVA, P > 0.05). Epiphytic diatom abundance was positively correlated to D. geminata biomass, and was dominated by Achnanthidium and Rossithithidium spp., which comprised 55% of cell counts across treatments. Likewise, diatom species richness increased significantly with decreased scour frequency (r2 = 0.32, P =0.04). These data suggest D. geminata blooms are resilient to scour and that less frequently disturbed mats show greater community complexity.