PRIMARY RESEARCH PAPER Effects of stream velocity and phosphorus concentrations on alkaline phosphatase activity and carbon:phosphorus ratios in periphyton Daniel L. Hiatt . Jeffrey A. Back . Ryan S. King Received: 21 May 2018 / Revised: 23 July 2018 / Accepted: 27 July 2018 / Published online: 3 August 2018 Ó Springer Nature Switzerland AG 2018 Abstract We studied several streams spanning a steep dissolved phosphorus (PO 4 –P) gradient to test the hypothesis that faster stream velocity would reduce alkaline phosphatase activity (APA) and car- bon:phosphorus (C:P) of benthic periphyton because higher velocities should increase the supply rate of dissolved phosphorus at the community–water inter- face. We tested the hypothesis that the differences in APA and C:P between fast and slow velocity locations within a stream reach would decline as stream PO 4 –P concentrations increased, and, therefore, velocity effects should be the greatest at low levels of PO 4 –P. APA declined in response to both the increased water velocity and PO 4 –P, but the effect of velocity on APA was negligible at the highest levels of PO 4 –P. Further, we found a strong, negative relationship between periphyton C:P and PO 4 –P levels as hypothesized, but did not detect significant relationship between C:P and velocity after accounting for the effects of PO 4 –P. The lack of an effect of velocity on C:P is probably due to the higher levels of APA in low-velocity, low PO 4 –P reaches, as the higher APA rates reflect an alternative pathway for acquiring sufficient PO 4 –P to sustain periphytic growth and metabolism. These results have important implications for stream ecosystem function because of the increasing frequency of extreme weather events associated with the climate change, particularly droughts that reduce or eliminate peren- nial stream flow, and further illustrate the important effects of stream flow on biogeochemical processes. Keywords Flow regime Á Flow intermittence Á Water velocity Á Stoichiometry Á Exoenzymes Á Eutrophication Á Streams Á Ecoenzymes Á Benthic periphyton Introduction The concentration of nutrients such as nitrogen (N) and phosphorus (P) are directly related to overall productivity of an aquatic system (Chro ´st & Overbeck, 1987; Stevenson et al., 1991; Chessman et al., 1992; Rier et al., 2011; Lang et al., 2012; Proia et al., 2012). In many cases of freshwater nutrient limitation, nitrogen and phosphorus are in limited availability to Handling editor: Eric Larson D. L. Hiatt Á R. S. King Department of Biology, Baylor University, One Bear Place 97388, Waco, TX 76798-7388, USA J. A. Back Á R. S. King Center for Reservoir and Aquatic Systems Research, Baylor University, One Bear Place 97388, Waco, TX 76798-7388, USA D. L. Hiatt (&) Department of Crop Sciences, University of Illinois, AW- 101 Turner Hall, MC – 046, 1102 S. Goodwin Avenue, Urbana, IL 61801-4730, USA e-mail: DHiatt@Illinois.edu 123 Hydrobiologia (2019) 826:173–182 https://doi.org/10.1007/s10750-018-3727-4