Field Evidences for the Positive Effects of Atmospheric Deposition on Primary Productivity in the Oligotrophic Ocean

Aerosol originated from Asian continent contain the rich inorganic nitrogen compounds, and those are transported from continent to the ocean surface as the wet and dry depositions. Such nutrient input via the atmospheric deposition expects to stimulate phytoplankton growth, because the surface layer in the most of tropical and subtropical regions are generally limited nutrients. In fact, several previous studies showed the positive effects on phytoplankton growth by atmospheric nutrient fertilization. However, it is difficult to detect the actual impact of atmospheric deposition on phytoplankton growth in nature. We conducted the shipboard observations at the several sites of the oligotrophic water in the northwestern subtropical Pacific during the summer, and we estimated the photosynthetic parameters by the experiments of photosynthesis-irradiance curve at the surface. Photosynthetic parameters showed a significant linear relationship between the observation sites. This indicates that the efficiency of primary production varies according to the observation sites. Primary productivity tended to be higher at the site where a rainfall event occurred just before the shipboard observation. It was shown that the wet deposition of aerosol had an impact to enhance primary productivity to three folds under the nutrient depleted condition, although it may be limited to the surface. In addition, a linear relationship of photosynthetic parameters was also observed in the oligotrophic eastern Indian Ocean at the surface. Aerosol may have been an impact on primary productivity there, too.