Primary consumer stable nitrogen isotopes as indicators of nutrient source.

Area of Science:

• Environmental Science
• Ecology
• Biogeochemistry

Background:

• Non-point source pollution of nitrogen and phosphorus drives eutrophication in inland waters.
• Identifying nutrient sources is challenging due to the diffuse and variable nature of inputs.
• Stable nitrogen isotopes (delta15N) in aquatic organisms show potential for tracing anthropogenic nutrient sources.

Purpose of the Study:

• To investigate the relationship between land use, nitrogen loading, and stable nitrogen isotope values in aquatic consumers.
• To determine if delta15N can serve as an indicator of nutrient sources in Danish lakes.
• To model primary consumer delta15N (PCdelta15N) using watershed characteristics.

Main Methods:

• Measured delta15N in aquatic consumers across 27 Danish lakes with varying trophic states and land uses.
• Utilized an information-theoretic approach (AIC) to model PCdelta15N.
• Analyzed relationships between PCdelta15N, limnological data, nitrogen loading, and nitrogen sources.

Main Results:

• PCdelta15N varied significantly across lakes (>14%).
• Models incorporating land use/land cover performed best in explaining PCdelta15N variation.
• Elevated delta15N was quantitatively linked to urban and agricultural land use, particularly in riparian buffers.

Conclusions:

• Stable nitrogen isotopes in aquatic consumers are effective indicators of nutrient sources.
• Land use, not just nitrogen loading, is crucial for understanding elevated delta15N.
• Urban and agricultural watershed land use, including riparian zones, significantly influences aquatic nitrogen cycling and isotopic signatures.