Quantitative relationship on the behavior between organic matters and radionuclides in seawater

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Mesocosm experiment using the mesopelagic water from 400m depth of Suruga Bay was carried out from 20 November to 10 December 2002. Autotrophic biomass increased from 0.2μg C1^<-1> to 1.3mg C1^<-1> during the first 7 days of incubation. Nitrate and phosphate were depleted rapidly during Days 7 to 12 of incubation. It was thought that both nutrients-rich condition and low grazing pressure due to the low concentration of predator in the mesopelagic water at initial time caused bloom of autotrophic components. Biomass peak of bacteria, heterotrophic nanoflagellates (HNF) and ciliates occurred between Days 9 and 13 following the peak of autotrophic biomass. These results suggest that organic materials were transferred from autotrophic components to bacteria and subsequently to HNF and ciliates after nitrate was depleted, and then organic materials flowed through the microbial loop. POM (particulate organic matter) was divided in to the different size (>500μm,500〜105,105〜25,and 25〜0.4), and TH-234,Pb-210,Po-210 were measured in each fraction. Th-234 was not detected in dissolved form, while 80% of Th-234 in particulate form was found in size fraction from 0.6 to 500μm. The behavior of Th-234,Pb-210 and Po-210 was due to the behavior of POM with size of 0.4 to 25μm. The size distribution of P-32 is corresponding to the size distribution of phytoplankton, and its turnover time is about one week.