Denitrification and Nitrogen Burial in Swiss Lakes

Area of Science:

• Environmental Science
• Limnology
• Biogeochemistry

Background:

• Anthropogenic activities have imbalanced Earth's nitrogen (N) cycle due to excessive N inputs.
• Freshwater lakes play a crucial role in removing N from surface waters through denitrification (DN) and net sedimentation (NS).
• Limited data exists on the environmental conditions controlling these N removal processes and their long-term rates in individual lakes.

Purpose of the Study:

• To quantify N elimination rates in Swiss lakes using long-term monitoring data.
• To identify key environmental factors controlling denitrification and net sedimentation of N.
• To assess the impact of hydraulic loading rate on N removal efficiency.

Main Methods:

• Whole-lake nitrogen budgets were estimated for 21 Swiss lakes over approximately 20 years.
• Statistical analyses were performed to identify predictors of denitrification and net sedimentation rates.
• Nitrogen removal efficiency (NRE) was calculated and its relationship with environmental variables, including hydraulic loading rate (Qs), was examined.

Main Results:

• Bottom water nitrate concentration was the primary predictor of denitrification (DN).
• DN rates correlated positively with external N load and the area-specific hydraulic loading rate (Qs).
• Net sedimentation (NS) of N was strongly related to total phosphorus (P) concentration.
• Nitrogen removal efficiency (NRE) was negatively related to Qs, improving predictability without N or P loading data.

Conclusions:

• The hydraulic loading rate (Qs) is a critical, previously underappreciated factor influencing N removal efficiency in lakes.
• Phosphorus management alone may not significantly reduce N export unless coupled with N management strategies.
• Understanding these N removal mechanisms is vital for managing freshwater ecosystems impacted by N pollution.