STABLE ISOTOPIC DETERMINATION OF AQUATIC FOOD WEBS IN KARSTIC WATERSHEDS

Food webs from three geographically isolated, hydrologically similar karstic watersheds are being constructed. The hypothesis is that the food webs will be similar. If so, a conceptual model of karstic ecosystems can be generated from these data. Samples of guano, detritus, and terrestrial vegetation were collected and represent possible carbon sources. Samples of animals (snails, isopods, and amphipods) and water were also collected. All samples were analyzed for 13/12 C and 15/14 N isotopic ratios. A cross plot of 15/14 N and 13/12 C of all samples will be used to determine the trophic level of the animals as the 15/14 N ratio that is transferred to the organism that consumes it; however it is consistently and uniformly shifted to a value three parts per thousand higher. The cross plot also allows determination of the carbon source as 13/12 C isotopic ratios of the various “pools” of organic material are unique and transferred directly to any organism that consumes it. Preliminary data from Hunter's Cave show that the majority of the carbon isotopic ratios plot in a narrow range from -24‰ to -31‰ with four clusters at approximately -3‰, 0‰, 2‰, and 5‰ on the nitrogen axis. From this spatial division it is suggested that there is clear division between food sources and consumers in this karstic system.