The effect of water velocity on stable carbon and nitrogen isotope signatures of periphyton

Water movement affects carbon and nitrogen isotopic signatures of algae, possibly through effects on boundarylayer thickness. We describe a laboratory experiment carried out in artificial streams, which supports this hypothesis. Periphyton d13C and d15N signatures were significantly depleted in 13C and 15N when grown at higher water velocity over the range of 5–62 cm s21, the normal range found in small streams. d13C signatures ranged between 216.7‰ and 228.1‰ from the lowest to the highest water velocity, respectively. Similarly, d15N signatures ranged between 7.2‰ and 2.3‰ from the lowest to the highest water velocity, respectively. This pattern was found for algal communities growing on glass (mainly diatoms) as well as for those growing on rock (mainly filamentous green algae). For both C and N, the slopes of the responses were not different between the periphyton communities growing on each substrate, although the effect was statistically weaker for the communities on rocks. The intercept for d13C was significantly higher for the communities on rocks, but not different for d15N. Thus, while both isotopes are fractionated to a greater extent as velocity increases, the diatom communities growing on glass appeared to fractionate C isotopes more than the filamentous green algae growing on rock. This relationship between the stable isotopic signatures of aquatic plants and water velocity will hopefully allow a better understanding of the differences in isotopic signatures of fish in different habitats. Stable carbon and nitrogen isotopes (d13C and d15N) are increasingly used in ecology to study food-web structure and functions. The influence of abiotic factors, such as water velocity, on the isotopic signatures of algae is of particular interest because algae are at the base of the food web, and such factors can indirectly influence the signatures of primary and secondary consumers. In order to properly interpret isotopic signatures of invertebrates and fish, it is therefore necessary to understand how physical factors affect the signatures of primary producers. Field studies on aquatic plants have generally reported d13C values enriched in 13C (lower degree of isotopic fractionation) in low-energy lentic systems than in high-energy lotic systems (Keely and Sandquist 1992; Hecky and Hesslein 1995). In low-energy environments, boundary layers of aquatic plants are thicker due to more stagnant water. This results in lower diffusion rates and reduced isotopic discrimination against the heavier 13C. Moreover, phytoplankton, which normally live in open-water turbulent environments, generally exhibit d13C values depleted in 13C (higher degree of isotopic fractionation) than littoral benthic algae (Hecky and Hesslein 1995). Laboratory experiments have shown that d13C values for individual algal species were decreased under conditions of high turbulence (Degens et al. 1968; France and Holmquist 1997). Osmond et al. (1981) also found that freshwater macrophytes from low-energy lake1 Present address: Biological Sciences, The University of Lethbridge, 4401 University Drive, Lethbridge, Alberta T1K 3M4.