Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen, China.
Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen, China; Key Laboratory of the Coastal and Wetland Ecosystems, Xiamen University, Xiamen, China.
J Environ Manage. 2018 May 1;213:382-391. doi: 10.1016/j.jenvman.2018.02.051.
Increased anthropogenic nutrient input and losses has caused eutrophication problems in freshwater and coastal ecosystems worldwide. High-frequency observations and modeling of river fluxes in subtropical regions are required to understand nutrient cycling and predict water quality and ecological responses. In 2014, a normal hydrologic year, we carried out daily sampling of the North Jiulong River in southeast China, which drains an agricultural watershed and experiences the Asian monsoon climate. We focused on the distinct characteristics of two important inorganic nitrogen forms (ammonium and nitrate). Our results show contrasting hydrological controls on the seasonal timing and magnitude of ammonium and nitrate concentrations and loads, likely due to differing sources and transport pathways (surface runoff versus baseflow) to the river. Both nitrogen concentrations were enriched in the dry season and diluted in the wet season. Arrival of rains in the pre-wet period in March caused a "first flush" peak event with the highest concentrations of the year, during which ammonium peaked two weeks earlier than nitrate. In contrast, the majority of nitrogen transport occurred during the lower concentrations of the wet season, with seven storms inducing flood events that lasted 24% of the time, contributed 52% of the runoff, and exported 47% of the ammonium and 42% of the nitrate. We found that seasonally piecewise LOADEST models (for pre-wet, wet and post-wet periods) performed better (5-8% error) than a year-round model (12-24% error) in estimating monthly nitrogen loads. However, not all nitrogen dynamics are easily synthesized by this approach, and extreme floods might produce a greater deviation in estimating nitrogen loads. These findings represent important implications for coastal ecology and provide opportunity on improving observation and modeling.
人为营养物质的输入增加和流失导致了世界范围内淡水和沿海生态系统的富营养化问题。为了了解营养物质循环，预测水质和生态响应，需要对亚热带地区的河流通量进行高频观测和建模。2014 年，在一个正常的水文年，我们对中国东南部的九龙北河进行了每日采样，该河流经一个农业流域，经历了亚洲季风气候。我们重点研究了两种重要无机氮形式（铵和硝酸盐）的明显特征。我们的结果表明，铵和硝酸盐浓度和负荷的季节性时间和幅度受到不同的水文控制，这可能是由于不同的来源和运输途径（地表径流与基流）到河流。氮浓度在旱季富集，在雨季稀释。3 月前湿期的降雨导致了“第一次冲洗”高峰事件，年浓度最高，其中铵的峰值比硝酸盐早两周。相比之下，大部分氮在雨季的低浓度期间输送，7 次风暴导致了持续时间为 24%的洪水事件，贡献了 52%的径流量，输出了 47%的铵和 42%的硝酸盐。我们发现，季节性分段 LOADEST 模型（前湿期、湿期和后湿期）在估计月氮负荷方面的表现优于全年模型（误差为 12-24%）（误差为 5-8%）。然而，并非所有氮动态都很容易通过这种方法综合，极端洪水可能会导致氮负荷估计产生更大的偏差。这些发现对沿海生态系统具有重要意义，并为改进观测和建模提供了机会。
