SOURCES AND FATE OF ATMOSPHERIC NUTRIENTS OVER THE REMOTE OCEANS AND THEIR ROLE ON CONTROLLING MARINE DIAZOTROPHIC MICROORGANISMS

Abstract

Atmospheric deposition can be a major source of nutrients to the remote ocean where these nutrient species can play a critical role in major biogeochemical cycles (e.g. carbon). Atmospheric input of Fe controls phytoplankton growth in high nitrate low chlorophyll regions. Fe can also be a rate-limiting nutrient to diazotrophic microorganisms and control the nitrogen fixation in the oligotrophic ocean. Due to low solubility of aerosol Fe in the seawater only a small fraction of atmospheric input of Fe may be bioavailable. This dissertation developed an aqueous sequential extraction procedure to measure the labile Fe species in aerosols. The measured labile Fe species were compared to the photo-reducible Fe under the ambient sunlight and to the bioavailable forms of aerosol Fe to a diazotrophic microorganism. The diazotroph showed a large capacity of luxury uptake of aerosol Fe, and the uptake amount was less than the total labile Fe measured in aerosols. Labile and total aerosol Fe was found to be highly variable in time and space over the North Atlantic and North Pacific oceans. The labile aerosol Fe was mostly associated with mineral dust transported from North Africa or Asia, although it can also be associated with anthropogenic sources and atmospheric processing. Major nutrients (soluble phosphate, nitrate and ammonium) in aerosols were also found to be temporally variable over these two oceanic regions. Mineral dust transported from North Africa or Asia was a major source for soluble phosphate only during a certain season. Soluble phosphate in aerosols was sometimes strongly associated with anthropogenic tracers. Anthropogenic activities were major sources for both aerosol nitrate and ammonium. It was also found that marine biogenic source of NH3 could be significant during the spring and summer over the remote oceans. Ratios between the atmospheric inputs of labile Fe, N and P also varied seasonally, which may result in a various nutrient limitation to the water column. The residence time of dissolved Fe in the upper Pacific was estimated longer than those in the Atlantic and the Indian oceans.