Research on the method system for formulating nutrient standards in cold region rivers and its application: taking the mainstream of the Heilongjiang River as an example

Cold-region rivers possess unique ecological characteristics and environmental conditions. Scientifically defining their nutrient-carrying capacity thresholds holds significant practical value for safeguarding river ecological health and supporting scientific water environment management. This study focused on the mainstream of the cold-region river, the Heilongjiang River. It screened nine indicators, including pH, EC, DO, COD_Mn, COD_Cr, BOD₅, NH₃-N, TN and TP, based on the water quality monitoring data from 2016 to 2024 and the investigation results of biological communities. The international nutrient benchmark formulation methods were integrated to construct the benchmark system of cold-region rivers. The results indicated: 1) The Heilongjiang River is characterized by a long freezing period (November to the next March), low temperatures suppressing microbial activity, and permafrost affecting mineral cycling. Epiphytic algae and plankton should be given priority as response indicators. 2) Natural background levels were determined using the group distribution method, ternary method, and reference point distribution method. These were integrated with pressure-response analysis to validate ecological threshold coupling, yielding the following reference values for the non-frozen period: TP 0.047 mg/L, TN 0.82 mg/L, NH₃-N 0.28 mg/L, COD_Mn 6.0 mg/L, COD_Cr 15 mg/L, BOD₅ 1.89 mg/L, pH 7.35, EC 15.9 mS/m, and DO 8.4 mg/L. 3) To address specific challenges during the freezing period, including ice cover obstruction of reoxygenation, low-temperature degradation inhibition, and nutrient salt concentration, corrections were applied using degradation inhibition coefficients, ion concentration effects, and biological response offset calibration. The adjusted indicators were as follows: TP 0.061 mg/L (correction factor 1.3), TN 0.98 mg/L (correction factor 1.2), NH₃-N 0.29 mg/L (flux calibration), COD_Mn 7.8 mg/L (degradation inhibition+inflection point offset), COD_Cr 18.75 mg/L (degradation inhibition+correction factor 1.25), BOD₅ 1.51 mg/L (activity inhibition+correction factor 0.8), pH 7.35 (maintained stable), EC 19.5 mS/m (background flux calibration), DO 6.8 mg/L (correction factor 0.8). This study establishes a benchmarking method system of "statistical analysis + pressure response + freezing period correction" suitable for rivers in the cold region, which can provide theoretical references for water environment management and the formulation of river nutrient benchmarks in the Heilongjiang River.