River phosphorus cycling: separating biotic and abiotic uptake during short-term changes in sewage effluent loading

Area of Science:

• Environmental Science
• River Ecology
• Biogeochemistry

Background:

• Point sources of phosphorus (P) loading pose a continuous threat to riverine ecosystems.
• Understanding in-river P processing and retention is crucial for protecting downstream environments from elevated soluble reactive phosphorus (SRP) concentrations.

Purpose of the Study:

• To quantify the biogeochemical response of a small river catchment to a pulse of treated sewage effluent.
• To differentiate the contributions of algae, bacteria, and abiotic sorption to in-river P cycling without using radioisotopes.
• To assess the temporal dynamics of P retention and release from the river ecosystem.

Main Methods:

• In-situ whole stream metabolism measurements.
• Batch and column simulation experiments.
• SRP mass balance analysis.
• Comparison of P uptake rates by different biological components (algae, bacteria) and abiotic sorption.

Main Results:

• The river ecosystem shifted from a P sink during effluent input to a P source after cessation.
• 65-70% of SRP was retained during the 14-day effluent exposure and remained sequestered for at least two weeks.
• Heterotrophic production increased by 67%, leading to a 50% rise in biological P uptake rate.
• Both biological uptake (algae, bacteria, sediment) and abiotic sorption contributed to P retention.

Conclusions:

• River ecosystems exhibit a significant capacity for P retention from treated sewage effluent through combined biological and abiotic mechanisms.
• The river ecosystem demonstrated a 'long memory' of P input, storing sequestered P for extended periods post-loading.
• Increased heterotrophic activity plays a key role in enhancing biological P uptake during effluent discharge events.