Shallow lake restoration and water quality management by the combined effects of polyaluminium chloride addition and benthivorous fish removal: a field mesocosm experiment

Polyaluminium chloride (PAC) has been used in lake restoration and water quality management as a flocculent to remove algal biomass and organic matter from the water column and increase water transparency. However, benthivorous fish may suspend the sedimented flocs and reduce the efficiency of PAC in improving water quality. Therefore, we hypothesized that PAC is more efficient in reducing algal biomass, suspended solids and total phosphorus concentrations in the water, and increasing water transparency, when benthivorous fish is removed. To test this hypothesis, we performed a field experiment combining the presence and absence of PAC (2 mg Al l(-1)) with the presence and absence of benthivorous fish (350 g m(-2)). Contrary to our hypothesis, no significant interactions were found between the effects of PAC addition and benthivorous fish removal on the measured variables. Both techniques significantly decreased chlorophyll a and phosphorus concentrations and increased water transparency, but their effects were independent of each other. The above results suggest that either of the treatments may improve water quality, but this goal would be better achieved by the combination of both techniques than by the application of one or other technique in isolation.