Q of the crust beneath southwestern Honshu,Japan, derived from explosion seismic waves

The specific quality factor Q of the crust was derived from explosion seismic waves by measuring the change of spectrum ratio with distance from the source. The spectrum for two wave groups, which are supposed to be transmitted entirely in the upper crust and transmitted primarily in the lower crust, respectively, were used. The source function could be represented by a simple function with one parameter. The value of Q and the parameter were determined simultaneously by the least-squares method. The same Q value was obtained for the upper and the lower crust and was estimated to be in the range Q = 1000–2000.     

One Dimensional Ecosystem Model Simulation of the Effects of Vertical Dilution by the Winter Mixing on the Spring Diatom Bloom

A one-dimensional ecosystem model has been used to investigate the processes relevant to the spring diatom bloom which play important roles in the biogeochemical cycle in the western subarctic Pacific. The model represents the plankton dynamics and the nutrient cycles in the spring diatom bloom; its results show the importance of dilution by deep mixing in winter. It is supposed that the vertically integrated biomass of phytoplankton decreases in the winter due to the decrease of photosynthesis, because the deep mixing transports phytoplankton to a layer with a low light level. However, the observed integrated diatom biomass increases as the mixed layer deepens. This is because the decrease of concentration due to dilution by mixing causes the diatom grazed pressure to be less significant than diatom photosynthesis. In other words, the effect of dilution on the grazed rate is more significant than the effect on the photosynthesis rate because the grazed rate depends on the concentrations of both diatom and grazer, whereas the photosynthesis rate depends only diatom concentration. The average specific diatom grazed rate, defined as grazed rate divided by diatom biomass, decreases by 35% associated with the deepening, while the average specific photosynthesis rate of diatom decreases by 11%. As a result, the average specific net diatom growth rate during the deep mixing is about 70% of its maximum during the spring diatom bloom. The deep mixing significantly affects the amplitude of the spring diatom bloom not only by the supply of nutrients but also by the dilution which drastically decreases the grazed pressure. This revised version was published online in July 2006 with corrections to the Cover Date.     

An ecological-physical coupled model applied to Station Papa

A vertical one-dimensional ecosystem model was constructed and applied to Station Papa. The model has seven compartments (phytoplankton, nitrate, ammonium, zooplankton, particulate organic matters, dissolved organic matters, dissolved oxygen) and was coupled with a mixed layer model for calculating diffusion coefficient which appears in the governing equations. The mixed layer model was driven by SST, SSS data observed at Station Papa in 1980 and ECMWF wind data for 1980, and the ecosystem model was driven by fixing nitrate concentration in deep layer to an observational value. The phytoplankton maximum in March was reproduced by the model although the maximum in fall-winter could not be reproduced. The model also suggests the importance of studying nitrification. As a whole, the model could reproduce characteristic features at Station Papa such as the summer ammonium maximum at 50 m depth, the summer dissolved oxygen maximum at 70 m depth and the absence of remarkable phytoplankton bloom.     

A lower trophic ecosystem model including iron effects in the Okhotsk Sea

We applied a three-dimensional ecosystem-physical coupled model including iron the effect to the Okhotsk Sea. In order to clarify the sources of iron, four dissolved iron compartments, based on the sources of supply, were added to Kawamiya et al.'s [1995, An ecological-physical coupled model applied to Station Papa. Journal of Oceanography, 51, 635-664] model (KKYS) to create our ecosystem model (KKYS-Fe). We hypothesized that four processes supply iron to sea water: atmospheric loadings from Northeastern Asia, input from the Amur River, dissolution from sediments and regeneration by zooplankton and bacteria. We simulated one year, from 1 January 2001 to 31 December 2001, using both KKYS-Fe and KKYS. KKYS could not reproduce the surface nitrate distribution after the spring bloom, whereas KKYS-Fe agreed well with observations in the northwestern Pacific because it includes iron limitation of phytoplankton growth. During the spring bloom, the main source of iron at the sea surface is from the atmosphere. The contribution of riverine iron to the total iron utilized for primary production is small in the Okhotsk Sea. Atmospheric deposition, the iron flux from sediment and regeneration of iron in the water column play important roles in maintaining high primary production in the Okhotsk Sea.