Spatial and temporal ecological risk assessment of unionized ammonia nitrogen in Tai Lake, China (2004-2015)

Ammonia toxicity varies largely due to its pH- and temperature-dependent speciation (unionized ammonia nitrogen, NH₃-N). The seasonal and long-term trend of ammonia risk in ecologically significant sections of Tai Lake, China was unknown. In this study, a two-level (deterministic and quantitative) method was developed to assess the special ecological risks posed by NH₃-N at 37 sites during two seasons (February and September) of 2014 in Tai Lake. The long-term temporal (2004-2015) risk posed by NH₃-N was also assessed by comparing annual quantitative risk values (probability of exceeding acute or chronic threshold values) in three key sections of Tai Lake. The results indicated the species living in the Tai Lake were at a 0.04% and 32.45% chance of risk due to acute exposure, and a 1.97% and 92.05% chance of risk due to chronic exposure in February and September of 2014, respectively. Alarmingly, the chronic ecological risks of NH₃-N in the Lanshanzui section of the Tai Lake remained >30% from 2004 to 2011. The chronic risk of NH₃-N in all three key sections of Tai Lake started to decrease in 2011. This was likely the consequence of the control practice of eutrophication implemented in the Tai Lake. A significant decline in diversity of the benthic invertebrate community of the Tai Lake could be associated with continuous exposure to ammonia over decades given different sensitivity of taxa to ammonia. The results laid a scientific foundation for risk assessment and management of ammonia in Tai Lake, China, and the developed two-level risk assessment approach can also be applied to other similar aquatic regions.