VARIATIONS IN THE STABLE-ISOTOPE COMPOSITION OF AQUATIC PLANTS AND THEIR IMPLICATIONS FOR FOOD-WEB ANALYSIS

The use of stable isotopes to identify the structure of aquatic food w ebs is predicated upon there being significant and consistent differen ces in the isotopic composition of the various classes of primary prod ucers. The structure of food webs will be interpreted incorrectly if v ariations in the isotopic composition of primary producers are not tak en into account. We detected significant temporal and spatial variatio ns in the deltaC-13 and deltaN-15 values of aquatic plants collected f rom three small, lentic water bodies in south-eastern Australia. Carbo n- and nitrogen-isotope values of individual taxa could each vary by u p to 10 delta units, according to site and season. The magnitude of th ese variations is sufficiently great for them to have important conseq uences for the interpretation of aquatic food webs. Greater attention must be given to fluctuations in the isotopic signatures of the primar y producers if the structure of aquatic food webs is to be properly un derstood.