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Large quantities of anthropogenic nitrogen (N) in the marine environment are detrimental for coastal ecosystems and understanding the magnitude and distribution of this N opens paths to remediation. One area of concern is Cocos Lagoon, in the south of Guam, Northern Mariana Islands, which hosts a diversity of corals including Acropora species listed in the Endangered Species Act. Degradation of Guam's coral reefs may be influenced by untreated sewage effluent. To gain insight into the spatial extent of human N pollution, we deployed 50 macroalgal thalli (Padina sp.) as bio-indicators in a grid pattern over the entire lagoon for one week and then measured the stable N isotopes of bio-available N ([delta][sup.15]N). We observed relatively low [delta][sup.15]N values (~1-2[per thousand]) in the shallow areas in the east suggesting coastal currents bringing in fixed N from the open ocean or local [N.sub.2] fixation. The comparatively high [delta][sup.15]N values (~6 [per thousand]) in the deeper channel of the lagoon suggest sewage as a major source of N. The distribution of [delta][sup.15]N values was not determined by the distance to N sources, such as river outlets or the tourist resort on Cocos Island but by the bathymetry, which affects the residence time and influx of oceanic water. Keywords: N stable isotopes, algal bioassays, tropical lagoon, Padina sp., The increasing impact of human activities on the nitrogen (N) cycle is manifested at various temporal (long vs. short term) and spatial (global vs. local) scales. Fossil fuel combustion, the [...]

Quantification of sediment and carbon (C) fluxes in rivers with strong seasonal and inter-annual variability presents a challenge for global flux estimates as measurement periods are often too short to cover all hydrological conditions. We studied the dynamics of the Tana River (Kenya) from 2012 to 2014 through daily monitoring of sediment concentrations at three sites (Garissa, Tana River Primate Reserve and Garsen) and daily monitoring of C concentrations in Garissa and Garsen during three distinct seasons. In wet seasons, C fluxes were dominated by particulate organic C (POC) and decreased downstream. Dry season fluxes of dissolved inorganic C (DIC) and POC had a similar share in total C flux at both locations while POC fluxes increased downstream. The dissolved organic C (DOC) flux did not show strong spatial nor temporal variations. The construction of constituent rating curves with a bootstrap method in combination with daily discharge data (1942-2014) provided potential sediment and C flux ranges as a function of annual discharge. At low annual discharge, our estimates generally predict a net decrease of sediment and C storage between the upstream and downstream site. As the annual discharge increases, our simulations shift toward net retention. This analysis allowed us to infer how variations in discharge regime, related to climate or human impacts, may affect riverine fluxes. Overall, we estimate that retention was dominant: integration over all simulations resulted in an average net retention of sediment (~2.9 Mt yr-1), POC (~18000 tC yr-1), DOC (~920 tC yr-1) and DIC (~1200 tC yr-1) over the 73 years of discharge measurements. [ABSTRACT FROM AUTHOR]

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 (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δ 15 N of TSP, δ 15 N-NO 3 in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δ 15 N 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 NO x driven by the East Asian Monsoon. It was found that most NO x 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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)