Effects of Benthic Fauna on Nitrogen and Phosphate Removal in Constructed Wetlands: a Meta-Analysis

Benthic fauna can directly or indirectly remove pollutants in wetlands, and the effect of introducing benthic fauna on effluent nitrogen (N) and phosphate (P) concentrations in constructed wetlands (CWs) has been extensively researched. However, systematic understanding of the variability in pollutant removal efficiency across studies remains limited, particularly regarding key influencing factors. We conducted a meta-analysis of 799 pairwise comparisons from 14 publications to assess the effects of benthic fauna introduction on effluent N and P concentrations in CWs. Results showed that (1) the impact of benthic fauna on effluent quality was species-dependent, adding T. tubifex or C. riparius significantly reduced effluent total phosphate (TP) concentration, adding B. aeruginosa also reduced effluent N concentrations; (2) high density benthic fauna addition reduced effluent TP and nitrate-nitrogen (NO3--N) concentrations, whereas low-density additions did not; (3) the introduction of benthic fauna reduced effluent TP concentration in free water surface constructed wetlands (FWSCWs) and TN concentration in subsurface flow constructed wetlands (SSFCWs); (4) the introduction of benthic fauna negatively affected effluent N concentration in CWs with a single plant species, while the effect diminished in CWs with mixed plant species; (5) introducing benthic fauna significantly reduced effluent TN and NO3--N concentrations in the long experiment durations (>60 days) and reduced effluent TP concentrations in the short experimental durations (<60 days). These results indicate that the effects of benthic fauna introduction on nutrient removal are influenced by species identity, density, CW type, plant species richness, and experiment durations. Specifically, introducing high-density benthic fauna, particularly T. tubifex or C. riparius into CWs with single plant species can enhance nutrient removal capacity.