Algal Colonization under Four Experimentally-Controlled Current Regimes in High Mountain Stream

Abstract

Algal colonization and assemblage development were examined on unglazed ceramic tiles in flow-through troughs over a 42-d period (summer 1987) in the regulated upper Colorado River (∼2400 m a.s.l.). The unusually constant discharge of the stream, coupled with the trough design, allowed the establishment and maintenance of four controlled current regimes that differed with respect to range of free-stream velocity and associated near-substratum hydraulic conditions: 1) depositional (free-stream current velocity <1 cm/s); 2) slow and hydraulically-smooth (range 14-20 cm/s); 3) fast and hydraulically-smooth (range 36-47 cm/s); and, 4) fast and hydraulically-rough or turbulently mixed (range 15-46 cm/s). Biomass was on average 30-40× higher in the two slow velocity treatments than in the two fast velocity treatments. Chlorophytes, bacillariophytes, and cyanophytes exhibited distinctly different successional trajectories and end-points in all treatments. Ulothrix zonata, established by day 4, was the dominant alga at all velocities for the first two weeks, after which it was replaced by a more diverse assemblage of chlorophytes, except in the Fast-rough regime. Diatoms were common in all treatments, but were only numerically dominant during the latter part of the study. Cyanophytes dominated only in the two lowest current regimes and only at the end of the study. Scanning electron microscopy revealed that physiognomy of algal assemblages also varied across current regimes. The two low velocity treatments were characterized by dense, upright filaments within one week, while three-dimensional assemblage structure in the two high velocity treatments did not develop over the 42-d period. Differences in species composition, successional trajectories and physiognomy across treatments demonstrated that current regime (free-stream velocity and pattern of flow) was an important determinant of algal assemblage structure in the absence of hydrologic and grazing disturbances.