Beating the blues by floc & lock

Blooms and scums of cyanobacteria led to prolonged swimming bans in Lake Rauwbraken (The Netherlands). We mitigated this nuisance by imposing phosphorus limitation on the lake. Our lake system analysis revealed that, over 40 years history, small dispersed sources built up a legacy phosphorus pool in the sediment and that the internal sediment release of phosphorus was the main phosphorus-source to the cyanobacteria (Chapter 2). The practically and economically most feasible approach was to reduce the internal phosphorus load by applying a solid phase phosphorus sorbent (SPS). We combined the flocculent poly aluminium chloride (PAC; Floc; Chapter 3) with the SPS lanthanum modified bentonite (Lock; Phoslock®; Chapter 3) as sinking weight to sink a present bloom, additional lanthanum modified bentonite was applied to the reduce internal phosphorus load. The treatment first stripped the water column of phosphorus using a flocculant (Floc) (Chapters 3, 4), shifting the lake from a (hyper) eutrophic state to a mesotrophic state. It also provided an instantaneous cure to the nuisance and lifted the imposed swimming ban (Chapter 4). Longer term, water quality remained good for more than a decade, but due to ongoing and increased external phosphorus loads, the lake is returning to a eutrophic state (Chapter 5). Directly after its application, the treatment caused a temporarily decrease in the Daphnia population in the lake (Chapter 4). Four years after the treatment Microcystis cells emerged from the lake sediment and produced a minor low-toxic scum. The event did not lead to a proliferation into a nuisance bloom (Chapter 5). Other than this mystery scum, the lake remained free of cyanobacterial nuisances. Directly after the application the Dutch maximum permissible filterable lanthanum concentration was exceeded for after 75 days (Chapter 6). Three years after the application, part of the LMB is relocated to the deeper part in the lake (Chapter 6). The lanthanum from the LMB is taken-up by Crayfish (Procambarus fallax f. virginalis) and macrophytes (Chapter 6), no ill effects were observed in the Crayfish nor in the lake itself. Successful mitigation of high densities of toxic cyanobacteria depends on strict definition of the problem, lake system analysis and knowledge of the human factor: socio-limnology (Chapter 7).