Effects of the novel 'Flock & Lock' lake restoration technique on Daphnia in Lake Rauwbraken (The Netherlands)

We combined a low-dose flocculent ( polyaluminiumchloride) with a lanthanumenriched benthonite clay (Phoslock w ) into a novel restoration technique "Flock & Lock". This treatment was first applied in April 2008 to Lake Rauwbraken, where it stripped all phosphorus (P) from the water column and strongly reduced P-release from the sediment. The treatment had a strong positive effect on water quality, i.e. filamentous cyanobacteria were removed, chlorophyll-a concentration dropped to very low levels of 2 mg L 21 , however aluminium, lanthanum and suspended clay concentrations were at least temporally strongly elevated. Following the treatment, Daphnia galeata disappeared for 3 months from the water column within 1 week after application; this phenomenon was not observed in similar periods during 2 years before (2006, 2007) and 1 year after the application (2009). From our field observation and additional grazing (comparing the effects of flocculent, clay and combination of both) and survival experiments (food deprivation), we conclude that the disappearance of D. galeata from Lake Rauwbraken may have been caused by the combination of the physical effects due to flocks, grazing inhibition by flocks and clay, very low food concentrations and absence of predation refuge. However, effects were temporary and Daphnia recovered from the treatment.