Summary effect sizes for biomanipulation subgroups defined by different aspects of data quality. The diamond-shaped symbols show means based on meta-analysable Secchi depth and chlorophyll a data (with 95% confidence intervals indicated by the widths of the symbols). The number of individual effect sizes (n) is indicated for each subgroup. ‘Before’ and ‘During’ periods have been defined as in Figures 5 and 6. Forest plots showing all individual effect sizes are presented in Additional file 10.