Abstract
Nitrate (NO3−) dual isotope analysis was performed in Zhanjiang Bay, which is a closed bay with intensive human activities in South China, to investigate seasonal changes in the main NO3− sources and their biogeochemical processes in the monsoon-controlled climate. The relatively low N/P ratios in Zhanjiang Bay suggests that nitrogen (N) is a limiting nutrient, which indicates that the increase of N is favorable for phytoplankton proliferation. However, a sufficient amount of ammonium was found in our study area owing to intensive human activities, which can support biological processes. Thus, less NO3− biological processes were found, indicating that NO3− isotopic characteristics may reveal details of the mixing from various sources. The Bayesian mixing model showed that NO3− in the upper bay originated from manure (43%), soil N (30%), N fertilizer (17%), and N precipitation (10%) during winter, which reflects the local human activities; while NO3- sources during summer were mainly N fertilizer (36%), soil N (32%), and manure (31%), indicating the source as the runoff from the upper river basin. Our results suggest that nitrate dual-isotope was very useful for tracing the main NO3− sources in the condition of the sufficient ammonium, and runoff exerted an important impact on the shift in NO3− sources between both the local source and the source from the upper river basin during the two seasons in this monsoon-controlled bay.
Highlights
Eutrophication in coastal aquatic ecosystems has prompted wide public concern owing to significantly increased nutrient inputs to coastal waters [1,2,3]
NO3 − sources and their biogeochemical processes, seasonal values of δ15 N-NO3 − and δ18 O-NO3 − were measured in Zhanjiang Bay in 2017, as were concentrations of nutrients (PO4 3- ), nitrite (NO2 − ), NO3 −, and ammonium (NH4 + ) and physicochemical parameters
The heavy rainfall and river discharge could dilute the NO3 − from manure, and the NO3 − from manure in winter was found to be significantly higher than that found in summer. These results suggested that runoff exerted an important impact on the shift in NO3 − sources between the local source and the source from the upper river basin during the two seasons
Summary
Eutrophication in coastal aquatic ecosystems has prompted wide public concern owing to significantly increased nutrient inputs to coastal waters [1,2,3]. N loads to the coastal environment have been increasing as a result of intensive industrial activity, agricultural activity, and rapid urbanization in coastal areas [4,5,6,7]. Heavy nitrate (NO3 − ) pollution has risen above acceptable levels in many areas, which greatly alters the N cycle in aquatic ecosystems, thereby endangering coastal ecosystems and biogeochemical cycles [8,9,10]. It is necessary to trace N sources and determine its recycling processes in coastal areas. The traditional, and simplest method to determine the nitrate pollution source is to identify the pollution source by investigating the land use type of the pollution area and combining it with the analysis of hydrochemical characteristics. Due to the diversity of nitrate sources, the mixing of point and nonpoint sources and the complex physical, chemical, and biological transformation
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