Hydrodynamic impacts on tidal-scale dissolved inorganic nitrogen cycling and export across the estuarine turbidity maxim
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Hydrodynamic impacts on tidal-scale dissolved inorganic nitrogen cycling and export across the estuarine turbidity maxima to coast Dan Yu . Nengwang Chen
. Peng Cheng . Fengling Yu . Huasheng Hong
Received: 26 July 2020 / Accepted: 21 October 2020 / Published online: 27 October 2020 Ó Springer Nature Switzerland AG 2020
Abstract Estuarine turbidity maxima (ETM) is a transition zone subject to the influence of river flow and tides. Here we showed the distinct impacts of fluvial and tidal hydrodynamics on dissolved inorganic nitrogen (DIN) cycling and export across the ETM to coast. We conducted tidal-scale hourly measurements at the ETM zone of the Jiulong River Estuary in Southeast China in May and December 2015. Generally, ammonium-N (NH4-N) and nitrateN (NO3-N) increased in ebb tides, primarily controlled by freshwater input. In contrast, nitrite-N (NO2-N) increased in flood tides, largely due to the horizontal advection of NO2-N rich water from the middle estuary (5–10 PSU). During the fresh–saline water mixing period with high suspended particulate matters (SPM), the stronger tides and smaller river discharge in December increased SPM and NO2-N in the ETM, indicating stronger ammonium oxidation in the water column. During the low tide period when freshwater
dominated and particles were deposited, the increase of NH4-N in the water column was related to the external source supply (e.g., wetland effluent), while the decline of NO3-N and NO2-N was likely associated with denitrification occurring in anoxic fluid muds and sediments. The larger DIN flux was found in May with larger river discharge, weaker tides and longer duration of the freshwater dominated period than December. This study highlights the combined effects of river and tides on hydrodynamics, which largely determine the major N sources, processes (e.g., nitrification and denitrification) and DIN fluxes across the ETM to coast. Keywords Estuarine turbidity maxima Dissolved inorganic nitrogen Tidal variation Nitrogen cycling Land–sea interface
Introduction Responsible Editor: Marguerite A. Xenopoulos D. Yu N. Chen (&) H. Hong State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361102, China e-mail: [email protected] D. Yu N. Chen P. Cheng F. Yu H. Hong Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen 361102, Fujian, China
Estuaries, as a transitional zone connecting rivers and the open sea, are among the most productive ecosystems in the world, due to their diverse environment with abundant nutrients (mainly nitrogen and phosphorus) derived from the watershed (Bianchi 2007). Terrestrial nutrients are utilized or transformed in estuaries before being transported to the sea (Brion et al. 2008; Erler et al. 2014; Schlarbaum et al. 2010).
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Therefore, estuaries play an important role in controlling the export of nutrients from the watershed to the sea, and
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