Study on interaction of marine snow dynamics and biological processes within the vicinity of the pycnocline of upper ocean.

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Although marine snow plays important role on the transport of organic carbon produced at the upper ocean into the ocean interior, its biogeochemical studies have been restricted to the coastal area, because of the access problems. In this study, we focused on the interaction of biological processes and physical environments in the vicinity of upper layer, to understand the production and vertical transfer of marine snow by using newly developed marine-snow camera. In FY2004, improvements of the camera systems and their performance test have been conducted, i.e., use of new LED light source and addition of clear sight and others. Main field campaign was performed during the Hakuho-Maru Cruise (July-September, 2005) to the central Pacific transect (10°S - 53°N, 160°W), which includes from oligotrophic region to rather nutrients rich sub-arctic region. In this long transect, no survey has been done before using marine snow camera. Followings are major results obtained from the field studies. (1) Presence of peak of marine snow abundance was confirmed at the pycnocline of the nutrients rich sub-arctic area. (2) Integrated amounts of chlorophyll a at the 0-200m depth along the transect showed about 2.5 fold difference, while the same treatments of marine snow volume gave about 50 times difference. This comparison suggests that several factors including physical environments, size of phytoplankton, and food web structures in this transect control abundance of marine snow in the upper layers. (3) The depth of pycnocline located between 30-110m depths in the study areas. The ratio of integrated chlorophyll a amounts between 0-100m and 100-200m depths along the transect showed a good correlation with the ratio of marine snow volume obtained by the same treatment. Since primary source of marine snow (>0.5mm size) is phytoplankton of 0.8-100 micron size, aggregation processes to produce marine snow may occur without any large vertical movement of these source materials.