Spatial and Temporal Variation of Nitrogen Concentration and Speciation in Runoff and Surface Water in the St. Lucie Estuary Watershed, South Florida.

Spatial and Temporal variation of Nitrogen Concentration and Speciation in Runoff and Surface Water in the St. Lucie Estuary Watershed,   South Florida

Liguang Li¹, Zhigang Li², Suli Li², Yongshan Wan³, Xiaoe Yang⁴, Peter J. Stoffella¹ and Zhenli He¹

¹ University of Florida, IFAS, Indian River Research and Education Center, Fort Pierce, FL 34945, USA

liguangli@ufl.edu; pjs@ufl.edu; zhe@ufl.edu;

²Agronomy Department, Guangxi University, Nanning, Guangxi, China lizhigangnn@163.com; lisuli88@163.com；

³South Florida Water Management District, 3301 Guns Club Road, West Palm Beach, FL 33416, USA ywan@sfwmd.gov;

⁴Ministry of Education Key Laboratory of Environmental Remediation and Ecological Health, Zhejiang University, Hangzhou, China.  xyang@zju.edu.cn

Increased inputs of nutrients have been considered to accelerate water eutrophication of St. Lucie Estuary and Indian River Lagoon in south Florida. In this study runoff and surface water samples were monthly collected from representative agricultural fields and waterways that carry stormwater from fields to the surface water bodies and analyzed for concentration of different nitrogen (N) forms (organic and inorganic N, dissolved and particulate N) and related physical-chemical properties. The objectives were to determine runoff N concentration and attenuation from agricultural fields to discharge point.  Total N concentrations in water samples were mostly above the critical levels of US EPA for surface waters (0.5-1.0 mg L^-1) and dissolved organic N was the dominant form. Concentration of most N forms generally declined from the fields to the discharge point. Of the measured N species, only NH₄-N showed no significant seasonal variations. The concentrations of all other N forms were higher in summer and autumn and lower in winter and spring.  Concentrations of organic N, dissolved N, or total N were significantly correlated with rainfall, water pH, electrical conductivity (EC) and turbidity. However, no significant relationships between N concentration in water and fertilization practices were noted.

Keyword: Runoff, Stormwater, Nitrogen, Agricultural, Rainfall