Atmospheric Nitrogen Deposition to the Eastern Indian Ocean during Boreal Autumn
Yoko Iwamoto1, Katsuhiro Kawamoto2, Fumikazu Taketani3, Yugo Kanaya3, Takashi Sekiya4, Maki Noguchi Aita5 and Kazuyo Yamaji6, (1)Hiroshima University, Graduate School of Integrated Sciences for Life, Higashi-Hiroshima, Japan, (2)Kobe University, Graduate School of Maritime Sciences, Japan, (3)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Research Institute for Global Change, Yokohama, Japan, (4)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Research Institute for Global Change, Kanagawa, Japan, (5)Japan Agency for Marine-Earth Science and Technology, Research Institute for Global Change, Yokosuka, Japan, (6)KOBE UNIVERSITY, Graduate School of Maritime Sciences, Kobe, Japan
Abstract:
Anthropogenic nitrogen emissions to the atmosphere have increased significantly the deposition of nitrogen compounds to the open ocean, with potential impacts on marine primary productivity and the global carbon cycle. Economic developments in South Asia will lead to further changes in nitrogen deposition pattern over the Indian Ocean. However, few atmospheric observations covering a wide area of the Indian Ocean have been carried out, and the effects of atmospheric nitrogenous compounds resulting from human activities in the South Asian region on the marine ecosystem are not accurately understood. In the R/V Hakuho Maru KH-18-6 cruise, gas, aerosol and rain water samples were collected along the 88 degree east transect to evaluate the ocean fertilization effect of atmospheric nitrogen deposition to the Bay of Bengal and the Southeast Indian Ocean.
Backward trajectory analysis showed that the north of the tropical convergence zone (ITCZ) was under the influence of continental air masses coming from India and Bangladesh, and the south of ITCZ was under the influence of pristine marine airmasses. Of the nitrogen compounds in the aerosol, NO3- was mainly present as coarse particles, and NH4+ was mainly present as fine particles. The average concentration of nitrogen compounds in the aerosol showed a clear difference between north and south of ITCZ. This suggests that the Bay of Bengal, north of the ITCZ, was strongly influenced by anthropogenic pollutants even in the open ocean. The average particulate dry deposition flux of nitrogen compounds in the north of ITCZ was estimated to be 59 μmol m-2 d-1 (NO3-) and 23 μmol m-2 d-1 (NH4+). This is an order of magnitude larger than those reported in the North Pacific Ocean, and as for the nitrate ions, it was even larger than those reported in the East China Sea, where the anthropogenic influence is relatively strong.
