A highly sensitive and detailed vertical distribution of nutrients in surface oceanic waters as a basis for evaluation of carbon dioxide absorption flux

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A highly sensitive method was developed for colorimetric analysis of nutrients in seawater by combining a continuous flow analysis system and a long path-length capillary flow cell (LWCC). With a 2-m light path cell, the detection limit of phosphate in seawater was 2.8nM. Interference with silicate and arsenate was examined and arsenate was to found to affect significantly unless a reduction reagent for arsenate was added. During several cruises of R/V Umitaka-maru and R/V Shinyo-maru, detailed vertical distributions of nutrients within surface or euphotic layers were determined. For stratified and oligotrophic waters, the highly sensitive colorimetric method was applied in phospate analysis. Nitrate (+nitrite) was analyzed by a highly sensitive chemiluminescence method. Detailed profiles of nutrients were obtained by conventional colorimetric analysis in the Southern Ocean during a Umitaka-maru cruise in January-February 2003. In these observations, the depth at which nutrient concentration begins to increase (or the depth of top of the nutricline) was found to be shallower for phosphate than for nitrate within the mixed layer. These features may reflect the general pattern of difference between nitrogen and phosphate dynamics in shallow oceanic waters. The highly sensitive colorimetric analysis of these nutrients can be used in high sensitive surface monitoring of nutrients and onboard incubation experiments.