Abstract

The dry and wet depositions of atmospheric ammonia (NH3) is one of the important pathways of nitrogen loads to aquatic ecosystems and should be managed to mitigate lake eutrophication. Crop and livestock agriculture, one of the largest emitters of NH3 in Asian countries, are known to cause high spatial and seasonal variation of NH3 and influence the surrounding lake basin areas via its dry and wet deposition. However, the spatial characteristics of the NH3 concentration in basin scale are not completely understood for regulation in NH3 emission because a paucity of monitoring network is currently available for NH3 in many Asian countries. Here we aim to clarify dominant factors of spatial and seasonal variations of the NH3 concentration in a shallow eutrophic lake basin (Lake Kasumigaura basin) surrounded by agricultural areas in eastern Japan. Passive sampling over various land use categories in the basin was conducted at 36 sites in total from October 10, 2018 to January 14, 2020. Mean NH3 concentration in the observational period was in the order of farmland (6.6 ppbv), lake (5.9 ppbv), rural (3.5 ppbv), and forest (2.6 ppbv). Interestingly, the observed NH3 concentration near the livestock houses were higher in winter than summer, which was inconsistent with knowledge of seasonal changes of current NH3 emission inventory based on temperature-driven volatilization process. Comparing monthly NH3 concentrations at farmland and lake sites with various meteorological factors, we suggested the importance of seasonal advection of NH3 from high emission sources to which has been rarely paid attention by the previous past studies. Consequently, a higher nitrogen input by dry deposition of NH3 was preliminarily estimated over the lake than that of wet deposition of total nitrogen. Our finding of a role of seasonal advection in NH3 spatial pattens should be considered for lake ecosystem management since its deposition is known to be closely related to the ecological processes such as phytoplankton blooming.

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