What is the purpose of stable isotope analysis in freshwater food webs, and what inferences do δ13C and δ15N provide?

• A. Stable isotopes measure water chemistry; δ13C indicates water source, δ15N tracks temperature.
• B. Stable isotopes help identify trophic relationships; δ13C indicates carbon sources (allochthonous vs autochthonous) and δ15N increases with trophic level.
• C. Stable isotopes are used to date sediment layers.
• D. Stable isotopes measure nutrient uptake in plants only.

Answer: (B)

Stable isotope analysis in freshwater food webs helps map feeding relationships by tracing energy flow through the ecosystem. Carbon isotopes (δ13C) change only a small amount as nutrients move from prey to predator, so they reveal the carbon source in an organism’s diet—whether it comes from autochthonous, in-water production like algae, or from allochthonous inputs such as terrestrial leaves. Nitrogen isotopes (δ15N) become progressively enriched with each step up the food chain, so higher δ15N values indicate a higher trophic level. Together, δ13C distinguishes where the carbon in an organism’s diet originates, while δ15N provides the consumer’s relative position in the food web, enabling researchers to reconstruct who eats whom in freshwater systems.