Mechanisms influencing community composition and succession in mountain stream periphyton: interactions between scouring history, grazing, and irradiance

Abstract

A field experiment with a factorial design was used to examine changes in benthic periphyton exposed to different regimes of scouring (scoured/non-scoured), light (open/shaded), and grazing (grazed/non-grazed) to investigate the role of environmental heterogeneity in shaping assemblages of periphyton in Rocky Mountain streams. Clay tile substrata were positioned at the start of the experiment either in the main channel (to receive bedload scouring) or a protected side channel (where scouring did not occur) of a 3rd-order, subalpine stream (elevation 3000 m asl). After 45 d, tiles were transferred to in situ stream channels where light intensity and densities of a mayfly grazer (Rhithrogena robusta) were manipulated. Scoured tiles carried ∼2× greater algal biovolume than non-scoured tiles 1 wk after high flows receded. The chrysophyte Hydrurus foetidus and the cyanobacterium Chamaesiphon incrustans predominated on scoured tiles, whereas diatoms, which were rare on scoured tiles, were the most abundant on non-scoured tiles. Non-scoured tiles also had 3× greater algal species richness than scoured tiles. Thus, seasonal high flows resulted in higher algal biomass but lower algal diversity than base flows at the spatial scale of individual cobbles. Scouring history, light, and grazer manipulations influenced the final periphyton assemblage structure in post-scour experiments. Scouring effects persisted throughout the experiment, in that scoured tiles had greater periphytic ash-free dry mass (AFDM) and lower algal biovolume than non-scoured tiles at the end of the experiment. Light had little direct influence on periphyton. Tiles in open channels receiving full sunlight (irradiance = 1696 μmol m−2 s−1) supported less periphytic AFDM than shaded tiles (506 μmol m−2 s−1), but light had no effect on total algal biovolume and influenced the abundance of only 2 of 5 algal taxa. Grazers depressed periphytic AFDM, algal biovolume, and the abundance of all major taxa except H. foetidus. Differences in community composition resulting from scoured and non-scoured treatments influenced periphytic and algal response to grazers and light. No single factor drove periphytic heterogeneity. Post-scouring community structure and interactions between grazers and light were important processes producing a mosaic of patches.