Assessing the mode of action of Phoslock® in the control of phosphorus release from the bed sediments in a shallow lake (Loch Flemington, UK)

Phoslock® is increasingly used worldwide to control sediment phosphorus (P) release and cyanobacterial blooms despite the fact that little is known about its mode of action in lake bed sediments. This study quantified the effects of Phoslock® on sediment elemental composition and P fractionation (one year pre- and post-application of 170 g Phoslock® m−2) in an attempt to address these knowledge gaps. Post-application, sediment La content was significantly higher in the top 10 cm of the sediment compared to pre-application conditions. Mass balance calculations indicated that the applied mass of La had the potential to bind 25% of potentially release-sensitive P (Pmobile; sum ‘labile P’, ‘reductant-soluble P’ and ‘organic P’ fraction) present in the top 4 cm or 10% of Pmobile present in the top 10 cm of the sediment. Assessing variation in sediment P partitioning indicated that the application caused a significant increase in the mass of P present in the more refractory ‘apatite bound P’ fraction between post-application month 4 and 7 compared to Pmobile. This suggests that Phoslock® controls sediment P release by increasing the mass of P permanently bound in the sediment. To address uncertainty in estimating product dose required to control sediment P release we conducted laboratory assays using intact sediment cores to which we added serial additions of Phoslock® under either aerobic or anaerobic conditions. The laboratory experiment indicated that the original dose to Loch Flemington was sufficient to control sediment P release under aerobic conditions but that significant P release will occur should prolonged anaerobic conditions persist. However, Phoslock® may be a viable option to control sediment P-release under anaerobic conditions which would require an estimated additional application of up to 510 g Phoslock® m−2. A conceptual model is proposed for the use of P-capping agents in lake remediation projects which is likely to increase cost-effectiveness and reduce non-target effects by applying multiple smaller doses compared to a single high dose