We collected settling particles by time-series sediment trap deployed at station KNOT (44°N 155°E, trap depth 770m) from 25 October 1999 to 20 June 2001, and from 25 June 2002 to 11 May 2006, and measured particulate organic carbon content and its carbon isotopic ratio (δ13C-POC) for each sample. δ13C-POC generally showed the trend that minima occur in spring, and maxima occur in fall. The range of the values was from about -27.0 to -23.00/00. To interpret the mechanisms controlling such variations quantitatively, we developed a one-dimensional ecosystem model including carbon isotopes. In our model, we considered the isotope effects by photosynthesis, degradation of particulate organic carbon, precipitation of calcite and gas exchange. This model successfully reproduced the seasonal variations in dissolved inorganic carbon (DIC) concentration, δ13C of DIC in the surface water, and δ13C-POC. We analyzed the mechanisms controlling δ13C-POC in our model results.
