Resource-Consumer Relationships and Baseline Stable Isotopic Signatures of Food Webs in Isolated Wetlands

Abstract

We examined isolated wetland food webs using stable C and N isotopes to understand resource-consumer relationships and controls on baseline isotopic signatures. Marshes were usually more 13C-enriched than cypress savannas and cypress gum swamps. Analysis of coarse particulate organic matter (CPOM) fractions indicated that C3 plants contributed the majority of organic matter to isolated wetlands. Individual wetlands of the same type were sometimes significantly different in δ13C and/or δ15N, suggesting that differences in stable isotopic signatures were related to baseline isotopic values rather than feeding preferences. Bi-plots were normalized using baseline values and resulting resource-consumer relationships were similar among the three wetland types. Food webs appeared to be supported by basal resources including periphyton and conditioned particulate organic matter, although indirect evidence suggested phytoplankton as an important basal resource. Differences in baseline isotopic signatures may have been partly a function of vascular plant decomposition based on significant positive relationships between the median δ13C of individual wetland food webs, the δ13C of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and CPOM pools, and a strong positive relationship between δ13C-DIC and δ13C-DOC. Mixing model calculations showed that other mechanisms were also important for establishing baseline isotopic signatures.