Benthic macroinvertebrate assemblages in relation to high ammonia loading: A 5-year fertilization experiment in 5 subtropical ponds

Nitrogen pollution, especially ammonia, and its impacts on aquatic ecosystems are always hot topics worldwide. Evaluating the toxicity effect of ammonia on aquatic organisms is the essential basis for nitrogen management. Benthic macroinvertebrates are widely used to evaluate ammonia toxicity based on acute and chronic lab tests. In comparison, responses of macroinvertebrates under field and controlled conditions were rarely studied. To explore the effect of ammonia on macroinvertebrate assemblages and the underlying mechanisms under field conditions, a 5-year fertilization experiment was conducted in 5 quasi-natural ponds located in the Yangtze River floodplain. One control (TN0, no artificial ammonia loading) and four treatments (TN2, TN10, TN20, TN100; ordered by artificial ammonia loading from low to high) were set. The results showed that (1) species number of macroinvertebrates differed little among the ponds, while total density and biomass were positively correlated with unionized ammonia concentration (NH3), indicating that increased ammonia loading had no adverse impact on macroinvertebrate abundance; (2) all ponds were dominated by gathering collectors and the biomass was higher in the ponds with higher ammonia loading resulting from the more phytoplankton promoted by ammonia loading and improved internal phosphorus release; (3) the biomass of predators also increased with the increasing NH3 which may be due to the bottom-up effect through their prey; (4) some species, such as Limnodrilus hoffmeisteri, survived and were dominant species in the ponds with higher NH3 compared with 96 h median lethal concentration from acute lab test. The results suggested that higher ammonia loading increased macroinvertebrate abundance, mainly contributed by gathering collectors and predators. Unlike previous acute and chronic lab tests, macroinvertebrates showed extremely high tolerance to NH3 in field conditions. This study supported that ammonia toxicity to aquatic organisms was scale-dependent and should be evaluated at multiple scales.