Labile DOC increases P removal efficiencies in the benthic zone and shifts P thresholds towards higher P concentrations

Biofilms in river sediments play a key role in P retention in aquatic ecosystems. Most studies in freshwater ecosystems focus mainly on the autotrophic component of biofilms but little is known about the role of heterotrophic components on P removal. It is known that DOC in some streams is of low bioavailability, hence, resulting in severe DOC and P co-limitation of heterotrophic biofilm growth which could then constrain P removal efficiency. How DOC limitation affects P removal efficiency in the benthic zone and how it modifies P thresholds (i.e. concentration from which the removal efficiency decreases) are still open questions.

We performed an experiment in the MOBICOS (Streamside Mobile Mesocosms) in the Holtemme River (Germany) to study the role of labile DOC on P thresholds in P retention in the bed-sediment biofilm community. Our flume experiment followed a BACI design (before: no DOC addition; after: labile DOC addition at C:P molar ratios >100; control: basal P and DOC concentrations; impact: P concentrations ranging from 25 µg P/L to 420 µg P/L).

Our results show that labile DOC increases the P removal efficiency of the system (i.e. P water mass balances in the flumes) and shifts P thresholds for P removal towards higher P concentrations meaning that at a given P concentration higher P removal efficiency is achieved if the system is supplied with labile DOC. Labile DOC activated the heterotrophic component in the flumes and benthic biofilms receiving labile DOC show higher bacterial density and higher P accumulation compared to the ones not receiving labile DOC.

Our results demonstrate that the heterotrophic biofilm community plays a key role in in-stream phosphorus retention. As it relies on availability of labile DOC, the interaction of DOC and P dynamics need consideration in models for stream nutrient processing and retention.