Influence of Substrate Conditioning on Periphytic Growth in a Heterotrophic Woodland Stream

Abstract

The influence of in situ substrate conditioning on periphyton colonization and growth was investigated during fall and spring in Walker Branch, a shaded, woodland stream in eastern Tennessee. Ceramic cylinders were conditioned by placing them in the stream under black plastic for >75 d. After conditioning, pairs of conditioned and unconditioned cylinders were placed in a shaded riffle and periphyton development was monitored for 35 (spring) or 40 (fall) d. The only variable measured that was significantly greater on conditioned than on unconditioned cylinders in both seasons was ATP, but significant differences only lasted until day 9. Taxonomic structure of the algae differed among seasons. Achnanthes spp. dominated during the fall experiment and their relative abundances were significantly greater on conditioned than on unconditioned cylinders through day 8 of the experiment, but not after. Meridion circulare dominated early in spring, whereas filamentous cyanophytes were only observed on day 35. Neither these taxa nor the other dominant taxa demonstrated a consistent preference for one substrate type over another. Conditioning never resulted in enhanced densities of algal cells or units during spring, when discharge was high and variable; only rarely did conditioning lead to enhanced algal settlement or growth during fall, when discharge was low and stable. Results from this study suggest that the influence of substrate conditioning on periphyton growth and development is relatively short-term. Conditioning is predicted to have more important consequences on stream structure and function in systems with a high frequency of disturbance, where periphyton communities are continuously set back to an early seral stage, than in systems with a low frequency of disturbance.