Seasonal Variations of Atmospheric Black Carbon Concentrations and Implications for Nutrient Inputs and Organic Carbon Partitioning in the Marine Coastal Ecosystem of Halong Bay, North Vietnam

CHU Van Thuoc, Institute of Marine Environment and Resources, Vietnam Academy of Science and Technology, Dept. of Marine Plankton and Microorganism, Hai Phong, Vietnam, Xavier Mari, Institute of Research for Development, Mediterranean Institute of Oceanography (MIO), Marseille, France, Benjamin P Guinot, CNRS, Paris Cedex 16, France, Justine Brune, Institute of Research for Development, Laboratoire Ecologie des Systèmes Marins Côtiers (ECOSYM), Montpellier, France, Jean-Pierre Lefebvre, Institte of Research for Development, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS), Toulouse, France, Patrick Raimbault, Mediterranean Institute of Oceanography, Marseille Cedex 09, France, Jutta Niggemann, University of Oldenburg, Research Group for Marine Geochemistry (ICBM-MPI Bridging Group), Oldenburg, Germany and Thorsten Dittmar, Carl von Ossietzky University Oldenburg, Institute for Chemistry and Biology of the Marine Environment (ICBM), Oldenburg, Germany
Abstract:
Black Carbon (BC) is an aerosol emitted during biomass burning and fossil fuel combustion. On a global scale, BC deposits on the ocean at a rate of 12-45 Tg per year, with higher fluxes in the northern hemisphere and in inter-tropical regions, following the occurrence of hotspots of atmospheric BC concentration. In the present study conducted in a coastal site located in a regional hotspot of atmospheric BC concentration, North Vietnam, we monitored the seasonal variations of atmospheric and marine BC during an annual cycle. Atmospheric BC followed a seasonal pattern characterized by high concentrations during the dry season, i.e. from October to April, and low concentrations during the wet season, i.e. from May to September. This trend is linked to a change in wind regime, with air masses originating from the North during the dry season and from the South during the wet season. On average, the contribution of BC to the particulate and the dissolved organic carbon pools was 43% and 3%, respectively. The concentration of particulate BC (PBC) was on average 50 times higher in the surface microlayer (SML) than in the water column. In the water column, the concentration of PBC was higher during the dry season than the wet season, which is consistent with variations of atmospheric BC concentrations. On the contrary, the concentration of dissolved BC (DBC) was lower during the dry season than the wet season. This seasonal pattern suggests that PBC concentration in coastal marine systems depends upon atmospheric BC concentration, while increased DBC concentration is linked to rainy conditions. The deposition of BC during the dry season was concomitant with a strong enrichment of organic phosphorus in the SML. During the annual cycle, the POC:DOC ratio was positively correlated with the concentration of PBC, suggesting adsorption of DOC onto BC particles and formation of POC via stimulation of aggregation processes.
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