Contrasting seasonal variations in riverine nitrogen and phosphorus concentrations in China: implications for N/P...

Rivers are critical conduits of nitrogen (N) and phosphorus (P), essential nutrients that regulate the productivity and resilience of freshwater ecosystems through seasonal fluctuations. However, comprehensive assessments of the seasonal dynamics of N, P, and their stoichiometric ratios in river ecosystems, as along with the underlying natural and anthropogenic drivers of the magnitude of these seasonal variations, remain scarce. In this study, we conduct an investigation of seasonal variations in total nitrogen (TN), total phosphorus (TP) concentrations, and their stoichiometric ratios across 150 medium-sized river basins in China. Our findings reveal starkly contrasting seasonal trends: TP concentration peaks in spring and summer, while TN concentration reaches maximum levels in winter, resulting pronounced intra-annual fluctuations in TN:TP ratios. These divergent dynamics are modulated by a complex interplay of hydrological variability, temporal autocorrelation in nutrient concentrations, and temperature-driven processes. Seasonal fluctuation in the TN:TP ratio leads to spatiotemporal variability in nutrient constraints. Seasonal differences in water temperature and the magnitude of TN variability explain 22 % and 14 % of this variation, respectively, while climatic factors and anthropogenic influences account for 15 % and 13 %. Phosphorus-limitation dominates most river systems year-round, but seasonal shifts toward N-P co-limitation occur in southern basins during warmer months. Water temperature emerged as a primary driver, influencing the magnitude of seasonal variability in nutrient concentrations, with hydrological discharge and human activities providing additional influence. The seasonal variability of TN and TP is substantially greater in northern basins than in southern basins, and this regional heterogeneity is closely associated with differences in climate, land use, and anthropogenic inputs. These findings underscore the need for regionally adaptive, season-specific nutrient management strategies to address growing N/P imbalances exacerbated by climate change and anthropogenic impacts.