COS 14-2 - Trophic assessment of Mammoth Cave NP Food Webs Using Stable Isotope Analysis

Abstract: Mammoth Cave National Park contains a karst system of interconnected underground rivers connected through springs to the mainstem of the Green River that runs through the park. Though connected, these systems—including the mainstem river, springs, and cave streams—contain vastly different fauna and environmental conditions. Given the connectivity of this system, we were motivated to study the food webs of these aquatic systems, including the energy pathways, food web size, and trophic positions of key groups of organisms. Stable isotope ratios of carbon and nitrogen can provide valuable information to identify and quantify trophic pathways and processes occurring in these connected systems. Coupling information from isotopic ratios with modern tools for analyzing isotopic data, including Bayesian mixing models, provides robust estimates of the sources of carbon in these food webs, and allows for the calculation of food web metrics, like trophic position and niche breadth. We utilized modern analytical tools to examine the aquatic food webs in Mammoth Cave National Park, including the R, packages SIBER, tRophic Position, and MixSIAR. MixSIAR uses Bayesian mixing models to estimate of the contribution of different sources to a consumer’s diet, tRophic Position incorporates a Bayesian model to calculate the trophic position using stable isotopes with one or two baselines, and SIBER quantifies the niche breadth of food webs while standardizing for sample size. Wielding these tools, we addressed two a priori hypotheses. H1: Fish and invertebrates derive progressively more energy from allochthonous than autochthonous sources— moving from the mainstem Green River to the springs to cave streams—due to decreasing sunlight availability and more outside carbon coming from coarse and fine particulate organic matter flushed into cave streams through springs. H2: Food webs will become progressively compressed—moving from the mainstem, to the springs, to cave streams—due to progressively less resource availability in these systems. Results from this study will provide basic information about how food webs are structured in unique, understudied, underground aquatic ecosystems, as well as how these systems relate to interconnect surface food webs.