Comparison of aquatic food chains using nitrogen isotopes

Area of Science:

• Ecology
• Environmental Science
• Biogeochemistry

Background:

• Nitrogen isotopes (delta(15)N) are valuable for modeling aquatic food webs and contaminant transfer.
• Cross-system comparisons of delta(15)N are challenging due to variations in the food chain base.
• Plankton delta(15)N is highly variable, complicating baseline assessments.

Purpose of the Study:

• To investigate baseline delta(15)N variation in aquatic ecosystems.
• To assess the impact of human population density and sewage on food web nitrogen isotopes.
• To determine the trophic structure variability among lakes after correcting for baseline shifts.

Main Methods:

• Utilized delta(15)N signatures in large primary consumers with stable tissue isotopic signatures.
• Correlated baseline delta(15)N with human population density in lake watersheds.
• Analyzed trophic levels in 40 lakes, adjusting for anthropogenic nutrient inputs.

Main Results:

• Baseline delta(15)N significantly increases with human population density, likely due to sewage input.
• Despite baseline shifts, the trophic levels leading to fish varied by only approximately one level across lakes.
• Confirmed the utility of delta(15)N in assessing anthropogenic nutrient impacts.

Conclusions:

• Large primary consumers provide a reliable indicator of baseline delta(15)N in aquatic food webs.
• Anthropogenic nutrient inputs, particularly sewage, significantly influence baseline nitrogen isotope values.
• Aquatic food chain lengths are more consistent across diverse lake systems than previously assumed when accounting for baseline variations.