Stream Hydrology Controls the Longitudinal Bioreactive Footprint of Urban-Sourced Fine Particles

Area of Science:

• Environmental Science
• Ecology
• Biogeochemistry

Background:

• Urbanization increases the relevance of wastewater treatment plant (WWTP) effluents in river systems.
• Fine particles from WWTPs accumulate in streambed sediments, potentially increasing respiration and CO2 emissions.
• Intermittent streams are particularly vulnerable to the impacts of WWTP effluents due to reduced dilution capacity.

Purpose of the Study:

• To quantitatively assess the influence of WWTP-sourced fine particles on stream metabolic activity over one year.
• To investigate the longitudinal patterns of fine particle accumulation and their metabolic activity in an intermittent Mediterranean stream.
• To estimate the potential for carbon dioxide emissions from WWTP-derived fine particles in stream sediments.

Main Methods:

• Monitoring of fine particle standing stocks and their metabolic activity over an 820 m study reach.
• Assessment of longitudinal patterns, particularly during dry periods when stream dilution capacity is low (<40%).
• Estimation of in-stream bioreactive capacity and potential CO2 emissions based on observed data.

Main Results:

• Nutrient-rich and metabolically active fine particle stocks were found downstream of the WWTP.
• These effects propagated to the end of the study reach, especially during dry conditions.
• Estimated potential for substantial carbon dioxide emissions (3.1 g C/m2/d) from the bioreactive capacity of these fine particles.

Conclusions:

• WWTP-sourced fine particles significantly influence stream metabolic activity and CO2 emissions.
• Intermittent streams are particularly sensitive to these inputs, especially during low flow conditions.
• Carbon budgets for streams must account for fine particle accumulation downstream of point sources like WWTPs.