Comparison of guts contents analysis and stable isotopes analysis in determining food web structure in three tropical streams

Kirk O. Winemiller, Steve C. Zeug and Clint R. Robertson

Recently, the use of stable isotopes to evaluate food web structure has become increasingly commonplace.  Isotopic analysis provides information on longer-term feeding trends based only on assimilated dietary items.  Stomach contents analysis, although only providing a record of very recent feeding activity, provides a more detailed picture of feeding relationships than is generally possible using isotopes in diverse systems.  Winemiller (1990) evaluated food web structure in three streams in Tortuguero and Corcovado National Parks, Costa Rica, using guts contents analysis.  The present study analyzes food web structure in the same streams using stable isotope analysis, and compares the structure resulting from both analyses.

Aquatic Food-web Structure along the Longitudinal Gradient of the Bladen Monkey River, Belize

Kirk Winemiller, Rodney Honeycutt, Peter Esselman, Will Heyman, Allison Pease, Elizabeth Carrera, Donmale Gbaanador, Josiah Payne, John Putegnat, Gabriela Tamez

This project examines food-web structure and primary production sources supporting the aquatic fauna of the Bladen River and upland tributaries, the Monkey River estuary, and adjacent coastal waters using stable isotope methodology.  Bladen-Monkey River watershed lies within one of the most important conservation regions of Central America.  The watershed contains a variety of terrestrial and aquatic ecosystems that support rich biodiversity, and also influences important and sensitive coastal and coral reef ecosystems.  Ratios of the stable isotopes of carbon are being used to estimate assimilation of carbon from various plant production sources by animals.  To identify the predominant energy source for aquatic consumers, a mixing model is employed with the following basal elements:  periphyton, macroalgae and vascular plants (aquatic macrophytes, grass, tree leaves, seeds, fruits).  Nitrogen isotopic ratios are used as indicators of trophic position (vertical position in the food web) in aquatic systems, where ¹⁵N enrichment increases predictably with trophic level of consumers.  The heavier ¹⁵N accumulates in consumers as nitrogen moves up the food web, and as a result top consumers tend to have higher values for δ¹⁵N than consumers near the base of the web.  In early 2006, we collected tissue samples from several of the most common plants and animals in these aquatic ecosystems for laboratory analysis (mass spectrometry) of ratios of heavy and light stable isotopes of carbon and nitrogen.  Knowledge of food web structure and the energy sources that support faunal elements of the aquatic community will greatly increase understanding of tropical river ecology.  It is anticipated that the proportion of allochthonous basal inputs (plant material of terrestrial origin) to the aquatic food web will decline from upstream sites in the Bladen River to the meso-haline waters near the Monkey River mouth.  We hope that this food-web project will yield scientific information and insights that will assist local efforts in "ridge-to-reef” ecological watershed conservation.

Updated Wednesday March 01, 2006