Establishment of stream nutrient criteria by comparing reference conditions with ecological thresholds in a typical...

The establishment of numeric nutrient criteria is essential to aid the control of nutrient pollution and for protecting and restoring healthy ecological conditions. However, it's necessary to determine whether regional nutrient criteria can be defined in stream ecosystems with a poor ecological status. A database of periphytic diatom samples was collected in July and August 2011 and 2012. In total 172 samples were included in the database with matching environmental variables. Here, percentile estimates, nonparametric change-point analysis (nCPA) and Threshold Indicator Taxa ANalysis (TITAN) were conducted to detect the reference conditions and ecological thresholds along a total nitrogen (TN) and total phosphorus (TP) gradient and ammonia nitrogen (NH₃-N) for the development of nutrient criteria in the streams of the Lake Dianchi basin. The results highlighted the possibility of establishing regional criteria for nutrient concentrations, which we recommended to be no more than 1.39 mg L^-1 for TN, 0.04 mg L^-1 for TP and 0.17 mg L^-1 for NH₃-N to prevent nuisance growths of tolerant taxa, and 0.38 mg L^-1 for TN, 0.02 mg L^-1 for TP and 0.02 mg L^-1 for NH₃-N to maintain high quality waters in streams. Additionally, the influence of excessive background nutrient enrichment on the threshold response, and the ecological interaction with other stressors (HQI, etc.) in the nutrient dynamic process need to be considered to establish the eventual nutrient criteria, regardless of which technique is applied.