Does benthic current velocity establish a critical threshold for algal removal by stream grazers?

Background/Question/Methods Water diverted from streams to meet human demands for freshwater alters current velocity near the streambed. Near-bed current provides an essential context for benthic organisms and can influence the ability of invertebrate grazers to remove or facilitate the growth of benthic algae. We hypothesized that lotic grazers adapted to fast current will lose their ability to control algae when near-bed current drops below a threshold velocity. To identify this putative threshold, and quantify the extent to which current mediates removal of benthic algal by grazers, we conducted a mesocosm experiment on the bank of Copper Creek, a high gradient mountain stream at the Rocky Mountain Biological Laboratory in Gothic, CO. Stream cobbles having small (6.45 cm2), attached ceramic tiles were colonized in the stream for 30 d prior to placement into 24, flow-through stream mesocosms. Each mesocosm maintained a gradient of near-bed current ranging 0-40 cm/s. Three grazers, Drunella coloradensis, Cinygmula spp., and Epeorus deceptivus, were applied separately or in combination to create 4 treatments replicated 5 times, plus 4 grazer controls. After 5 days, tiles were sampled for chlorophyll-a to estimate algal abundance. Grazer treatment chlorophyll values were subtracted from grazer controls to estimate algae removed.

Results/Conclusions The data indicated a threshold shift from algal accumulation to removal as near-bed current increased past ca. 10 cm/s. There was a 3-fold increase in chlorophyll-a at velocities ranging 0-5 cm/s, an equilibrium between accumulation and removal near 10 cm/s, and a steep decline in chlorophyll-a in the 10-20 cm/s range before values became asymptotic. This pattern was similar among the three grazers examined; however, the slope of the relationship differed (ANCOVA; p < 0.001). For example, the combined species treatment had a steeper slope (0.29) than did the single species treatments by a factor of 2. The combined grazer treatment also had the greatest stimulatory effect on algae; by contrast, Epeorus removed more than did any other treatment as near-bed current velocity increased. These results suggest that, not only do grazers have different effects on algae across the gradient of benthic current, their respective contributions to algal dynamics may be altered as near-bed current becomes constrained by decreased stream flow resulting from water abstraction.