Coupling High-Frequency Stream Metabolism and Nutrient Monitoring to Explore Biogeochemical Controls on Downstream...

Area of Science:

• Environmental Science
• Biogeochemistry
• Ecology

Background:

• Instream biogeochemical process measurements are typically short-term and localized.
• Understanding seasonal nitrate dynamics at the river-reach scale is crucial for water quality management.

Purpose of the Study:

• To quantify the net effects of biogeochemical processes on seasonal baseflow nitrate retention.
• To investigate the role of stream metabolism and biota in nitrogen dynamics using in situ sensors.

Main Methods:

• Utilized dual-station, high-frequency in situ nitrate sensors.
• Coupled nitrate data with high-frequency measurements of stream metabolism and dissolved inorganic carbon.
• Calculated nitrate assimilation from net primary production and estimated nitrification/denitrification using mass-balance.

Main Results:

• Instream processes removed >30-90% of baseflow nitrate load over a 6.5 km reach.
• Observed a seasonal switch from nitrate assimilation (spring/early summer) to denitrification (after midsummer).
• Synchronicity noted between metabolic shifts (autotrophy to heterotrophy) and changes in nitrate processing.

Conclusions:

• Seasonal shifts in metabolic pathways significantly drive instream nitrate retention and downstream delivery.
• Continuous in situ stream sensor networks provide novel opportunities for quantifying nitrogen dynamics in fluvial systems.
• River biota play a critical role in the fate and transport of nitrogen in river networks.