Coral carbonate-bound isotopes reveal monsoonal influence on nitrogen sources in Southeastern China's Greater Bay Area from the mid-Holocene until the Anthropocene.

Most anthropogenic nitrogen (N) reaches coastal waters via rivers carrying increasing loads of sewage, fertilizer, and sediments. To understand anthropogenic N impacts, we need to understand historical N-dynamics before human influence. Stable isotope ratios of N preserved in carbonates are one way to create temporal N records. However, records that span periods of human occupation are scarce, limiting our ability to contextualize modern N dynamics. Here, we produce a fossil-bound N-record using coral subfossils, spanning 6700 years in China's Greater Bay Area (GBA). We found that during the mid-to-late Holocene, the GBA's coastal N was dominated by fluvial sources. The weakening of the Asia monsoon throughout the late-Holocene decreased river outflow, leading to a relative increase of marine nitrate. This source shift from riverine-to-ocean dominance was overprinted by anthropogenic N. During the late 1980s to early 1990s, human development and associated effluent inundated the coastal system, contributing to the decline of coral communities.

Elucidating sources of atmospheric NOX pollution in a heavily urbanized environment using multiple stable isotopes.

Atmospheric deposition of nitrogen (N) from rain and aerosols can be a significant non-point source - particularly in urbanized coastal areas and contribute to coastal eutrophication and hypoxia. Here, we present geochemical and isotopic data from surface waters coupled with an 18-month time series of geochemical and isotopic data measured on wet and dry deposition over Hong Kong from June 2018. Dual stable isotopes of nitrate (δ15N-NO3- and δ18O-NO3-) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δ15N of TSP, δ15N-NO3 in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δ15N varied seasonally with higher values in winter and lower values in summer. This variation can be explained by a change in the sources of atmospheric NOx driven by the East Asian Monsoon. It was found that most NOx comes from coal burning in winter and a mix of vehicle emissions, fossil fuel combustion and lightning in summer. Moreover, the estimated dry and wet deposition of nitrate and ammonium in Hong Kong is around 18 kg N ha-1 annually, which is of the same order of magnitude as N released by sewage effluents and groundwater. This implies that atmospheric N deposition over the N-limited waters of the eastern side of Hong Kong could contribute significantly to the N budget. Therefore, atmospheric N deposition may alter the local N marine cycling, thus monitoring its impact is crucial for water quality in Southern China.

Substantial decrease in CO2 emissions from Chinese inland waters due to global change.

Carbon dioxide (CO2) evasion from inland waters is an important component of the global carbon cycle. However, it remains unknown how global change affects CO2 emissions over longer time scales. Here, we present seasonal and annual fluxes of CO2 emissions from streams, rivers, lakes, and reservoirs throughout China and quantify their changes over the past three decades. We found that the CO2 emissions declined from 138 ± 31 Tg C yr-1 in the 1980s to 98 ± 19 Tg C yr-1 in the 2010s. Our results suggest that this unexpected decrease was driven by a combination of environmental alterations, including massive conversion of free-flowing rivers to reservoirs and widespread implementation of reforestation programs. Meanwhile, we found increasing CO2 emissions from the Tibetan Plateau inland waters, likely attributable to increased terrestrial deliveries of organic carbon and expanded surface area due to climate change. We suggest that the CO2 emissions from Chinese inland waters have greatly offset the terrestrial carbon sink and are therefore a key component of China's carbon budget.

Investigating the link between Pearl River-induced eutrophication and hypoxia in Hong Kong shallow coastal waters.

We present geochemical analysis of 75 surface water samples collected in 2016 in Hong Kong coastal waters. We found that nitrogen distribution around Hong Kong can be characterized by two regimes driven by the influence of the Pearl River: 1) a regime where nitrate is the dominant species of nitrogen, associated with lower salinity and more faecal coliform and 2) a regime where dissolved organic nitrogen is dominant, associated with higher salinity and fewer faecal coliform. While the impact of the Pearl River on Hong Kong coastal waters is well characterized, we used the sharp contrast between the nitrogen regimes to produce new evidence about the role of the Pearl River on the generation of local hypoxia in Hong Kong. The impact of nitrate originating from the Pearl River on the generation of hypoxia in Hong Kong might be less important than previously thought, as no sign of eutrophication was found within the zones dominated by dissolved organic nitrogen and an historical decoupling of surface processes and bottom water oxygenation was observed. Moreover, we measured elevated ammonium levels and rapid cycling of ammonium and dissolved organic nitrogen in Victoria Harbour suggesting local sources, such as wastewater, might be rapidly oxidized and thus play an important role in the consumption of oxygen locally. A first-order calculation highlighted the potential for wastewater to drive the observed seasonal decline in oxygen. Taken together, these evidences suggest that eutrophication might not be the primary driver in the generation of seasonal hypoxia and that oxidation of ammonium released locally might play a bigger role than initially thought.

Nitrogen sources and cycling revealed by dual isotopes of nitrate in a complex urbanized environment.

Elevated nutrient inputs have led to increased eutrophication in coastal marine ecosystems worldwide. An understanding of the relative contribution of different nutrient sources is imperative for effective water quality management. Stable isotope values of nitrate (δ15NNO3-, δ18ONO3-) can complement conventional water quality monitoring programs to help differentiate natural sources of NO3- from anthropogenic inputs and estimate the processes involved in N cycling within an ecosystem. We measured nutrient concentrations, δ15NNO3-, and δ18ONO3- in 76 locations along a salinity gradient from the lower end of the Pearl River Estuary, one of China's largest rivers discharging into the South China Sea, towards the open ocean. NO3- concentrations decreased with increasing salinity, indicative of conservative mixing of eutrophic freshwater and oligotrophic seawater. However, our data did not follow conservative mixing patterns. At salinities <20 psu, samples exhibited decreasing NO3-concentrations with almost unchanged NO3- isotope values, indicating simple dilution. At salinities >20 psu, NO3- concentrations decreased, while dual NO3- isotopes increased, suggesting mixing and/or other transformation processes. Our analysis yielded mean estimates for isotope enrichment factors (15ε = -2.02‰ and 18ε = -3.37‰), Δ(15,18) = -5.5‰ and δ15NNO3- - δ15NNO2- = 12.3‰. After consideration of potential alternative sources (sewage, atmospheric deposition and groundwater) we concluded that there are three plausible interpretations for deviations from conservative mixing behaviour (1) NO3- uptake by assimilation (2) in situ NO3- production (from fixation-derived nitrogen and nitrification of sewage-derived effluents) and (3) input of groundwater nitrate carrying a denitrification signal. Through this study, we propose a simple workflow that incorporates a synthesis of numerous isotope-based studies to constrain sources and behaviour of NO3- in urbanized marine environments.