Temporal characteristics and trends of nitrogen loadings in lake Taihu, China and its influencing mechanism at multiple timescales

Abstract

Climate warming impact on excessive nitrogen (N) load in sediment favours cyanobacterial blooms in eutrophic waters. The nitrate (NO₃^--N) and ammonium (NH₄⁺-N) are two forms of N loads that contribute to algae blooms. However, little attention is paid to the impact of environmental factors on N loads variations at different time scales. This paper used a well-calibrated and validated EFDC model to investigate the temporal patterns and trends of ammonium and nitrate from June 2016 to June 2017. This paper presented the relationship and effects between these variations and environmental factors using data from satellite and reanalysis-based observations obtained for six meteorological parameters. The relationship and effects between these variations and environmental factors were also examined at different timescales (i.e., daily, monthly and seasonal scales). Model calibration results indicated that measured values reasonably matched simulated values. The validation results revealed that relative error (RE) values were within an acceptable range. The REs of ammonium at East Taihu (S12) and Xu Lake (S23) sampling sites were 55.83% and 57.61%, while that of nitrate was 24.37% (S12) and 41.08%, respectively. The daily analysis of NH₄⁺-N and NO₃^--N variations was 7.318 ± 3.876 (g/m²/day) and 0.0275 ± 0.222 (g/m²/day), respectively. The monthly analysis showed NH₄⁺-N and NO₃^-N range from 2.04 to 12.04 (g/m²/day) and 0.0008 to 0.064 (g/m²/day), respectively. The magnitude NH₄⁺-N and NO₃^--N varied and showed distinct inter-monthly variations. , The relationship between sediment fluxes and meteorological parameters showed the magnitude of correlation coefficient (r) and strength of correlation varied significantly. At daily scales, the relationship of NH₄⁺-N and NO₃^--N had a significant positive correlation with all meteorological parameters. At monthly, the correlation coefficient (r) of NH₄⁺-N and NO₃^-N were heterogenous. At daily and monthly scales, air temperature and wind speed are the main drivers affecting sediment N loads' dynamics; however, the influence of relative humidity, precipitation, and evaporation on N loads are smaller. The study demonstrates the contribution of meteorological conditions to the magnitude and timing of N loadings variability in water bodies. The findings provide more insight into lake ecosystem protection and environmental remediation.

Keywords: EDFC; Lake taihu; Meteorological factors; Nitrogen; Regression; Sediment digenesis; Temporal analysis; Trend analysis.