Long-term Changes in the Food Web of the Wabash River, Indiana

Methods

Summary

We will collect fish consumers and fish predators in the Wabash River to construct food webs for the current ecosystem. Historical food webs will rely on preserved specimens from natural history collections (Illinois Natural History Survey). Tissue samples from organisms will be analyzed using amino-acid stable isotope methods. The amino acid stable isotope method is advantageous because one need not collect autotrophs or basal herbivores (e.g., molluscs) to determine basal values in food web calculations, as the ¹⁵N isotopic signatures of some amino acids (e.g., glutamic acid) change substantially between trophic levels, while others (e.g., phenylalanine) essentially remain the same. By analyzing the δ¹⁵N (¹⁵N/¹⁴N ratio) value of both amino acids, one can determine the original algal signature (from phenylalanine) and the number of trophic transfers (from glutamic acid). These analyses will allow us to identify links from producers to consumer and predator fish from potential organisms (green algae, blue-green algae, aquatic plants, terrestrial C3 plants, and terrestrial C4 plants).

Challenges

There are not any difficult challenges for this project. My lab has done similar projects in the past using different isotope ratio approaches (bulk tissue). The analytical approach is simple from our perspective because we send tissue samples to a professional laboratory. The results are interpreted using software called a mixing model, that produces estimates of carbon sources and food web height (number of links from the bottom of the food web).

Pre Analysis Plan

Our pilot data using a few fish samples showed that carbon sources in the Wabash River food web are mostly from green algae and terrestrial trees. We do not yet have results for nitrogen to show us the food web height, but we expect that predators will be at higher levels than other consumer fishes. The interesting results will be if any of the fishes turn out to be omnivores, eating plants and animals. Results will be used to construct a detailed food web with strong and weaker links. We will collaborate with other scientists to model the resulting food web.