mn¥ ELSEV I ER Aquatic Botany48 ( ! 994) 99-108 Variations in the stable isotope composition of aquatic plants and their implications for food web analysis Paul I. Boon *'a, Stuart E. Bunn b aMurray-DarlingFreshwaterResearch Centre, P.O. Box 921, Albury, N.S.W. 2640, Australia bCentre for Catchment and In-stream Research, Faculty of Environmental Sciences, Griffith University,Nathan, Qld. 4111, Australia (Accepted2 March 1994) Abstract The use of stable isotopes to identify the structure of aquatic food webs is predicated upon there being significant and consistent differences in the isotopic composition of the various classes of primary producers. The structure of food webs will be interpreted incor- rectly if variations in the isotopic composition of primary producers are not taken into account. We detected significant temporal and spatial variations in the Jm3C and J~SN values of aquatic plants collected from three small, lentic water bodies in south-eastern Australia. Carbon- and nitrogen-isotope values of individual taxa could each vary by up to 10 delta units, according to site and season. The magnitude of these variations is suffi- ciently great for them to have important consequences for the interpretation of aquatic food webs. Greater attention must be given to fluctuations in the isotopic signatures oftbe primary producers if the structure of aquatic food webs is to be properly understood. 1. Introduction Stable isotopes, particularly J 13 C and d !5N, are commonly used to analyse food webs in aquatic systems (Peterson and Fry, 1987; Ehleringer and Osmond, 1989). As large spatial or temporal variations in the isotopic signatures of primary pro- *Corresponding author. 0304-3770/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSD10304-3770(94 )00395-3