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Beck, Whitney S. ¹ ; Poff, N. LeRoy ²

¹ Colorado State University
² Colorado State University

Streamflow is a master variable that regulates biological communities in lotic ecosystems. Laboratory studies have shown that current velocity influences how algal communities respond to stream water nutrients. Faster velocities can enhance delivery of limiting nutrients or decrease the boundary layer between the water column and algal cells to promote nutrient uptake (Borchardt et al. 1994). Furthermore, faster velocities may inhibit insect mobility and thus grazing rates (Opshal et al. 2003). Although current velocity may be independently physiologically stressful, in low-nutrient streams with high grazing rates, current velocity could be expected to increase algal biomass.

We tested this hypothesis under field conditions using nutrient diffusing substrates deployed at riffle and pool sites in five Poudre watershed streams. We included control, N, P, and N+P treatments to test the interactive effects of current velocity and nutrient additions. After a three-week incubation period, we analyzed growth discs for algal biomass (chlorophyll a), organic matter (ash-free dry mass), and relative autotrophy (ash-free dry mass/ chlorophyll a). We measured NO₃^-, total N, and orthophosphate concentrations in fast and slow velocities. We ran ANOVAs for each site and velocity to determine whether nutrient treatments differed from controls. We also ran ANCOVAs using site and nutrient treatments as factors and velocity as a covariate to quantify potential interactions.

We found differences in nutrient limitation between experiments located in fast versus slow velocities at three of the five streams. For instance, at one stream N was the limiting nutrient for algal biomass in fast velocities, but P was the limiting nutrient in slow velocities. Overall, we found that N was the primary limiting nutrient for algal biomass across all sites and velocities. Algal biomass significantly increased with velocity, and we found a positive interaction between current velocity and the treatments containing the limiting nutrient (N and N+P). Organic matter was not influenced by the velocity or nutrient treatments, but relative autotrophy was enhanced by current velocity and N treatments. Stream water nutrient concentrations did not change with current velocity.

It is likely that decreased insect grazing led to the general positive relationship between algae and current velocity across all nutrient treatments, but the current velocity and nutrient interaction was probably driven by increased uptake on the limiting nutrient treatments. These experiments have demonstrated that current velocity influences algal responses to nutrients in a field setting. The generalizability of this relationship should be explored across a broader stream water nutrient gradient.

Borchardt, M. A., Hoffmann, J. P., & Cook, P. W., 1994, Phosphorus Uptake Kinetics of Spirogyra Fluviatilis (charophyceae) in Flowing Water: Journal of Phycology, v. 30(3), p. 403–417.

Opsahl, R. W., Wellnitz, T., & Poff, N. L., 2003, Current velocity and invertebrate grazing regulate stream algae: results of an in situ electrical exclusion: Hydrobiologia, v. 499(1–3), p. 135–145.