Studies on oceanic carbon cycles have developed dramatically in the past decade, as enhanced by the social demands to understand, and if possible to control, anthropogenic climate changes. This lecture summarizes the recent studies and the circumstances of our community related to the carbon cycle and the multi-decadal scale climate changes in the North Pacific.

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Primary production is one of the key factors in understanding the carbon cycle in the ocean. Gross primary production is defined as total photosynthesis, whereas net primary production denotes gross primary production minus phytoplankton respiration. It is very difficult to estimate the rate of phytoplankton respiration in natural community. In incubation experiments for 24 hours, ¹⁴C method measures net primary production while H₂¹⁸O technique estimates gross primary production. However, it has been reported that bottle incubations suffer from artifacts associated with enclosed seawater in the bottle, or poor simulation of the light field from that of in situ environment. Further, the traditional technique must carry out bottle incubation and need many time and labor. It is expected that a new method, which measures primary production in situ with short-time experiment, will be developed in the near future.

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Larvae of Nihonotrypaea harmandi (Deapoda: Thalassinidea: Callianassidae) are released from the Tomioka tidal flat, grow to decapodid offshore and recruit to the tidal flat. The purpose of this study is to clarify what physical and behavioral processes play important roles in the recruitment of Nihonotrypaea harmandi. At first, we calculated the current field using a numerical model. Second, we calculated the horizontal distribution of larvae using a tracer model. We calculated 5 cases and obtained the following results. Larvae are released from the Tomioka tidal flat at the spring ebb tide. The horizontal shear of the tidal current expands the distribution area of larvae. Larvae in the early period of their growth stages are distributed at the center of the Tachibana Bay by the effect of residual circulation. In the latter period of their growth stage, larvae begin to move to the bottom layer. The effect of residual circulation on larvae becomes weak, because the circulation exists in the upper layer. Larvae go back to the Tomioka tidal flat as a result of the flood tidal current, after they have grown to the decapodid stage. Larvae need to increase their feeding opportunities by extending their distribution area by tidal currents. If larvae continue to stay near the hatched area, their recruitment becomes ineffectual. Larvae are distributed near the Tomioka tidal flat by residual currents. It is clear that the present current field is suitable for the recruitment strategy of the larva. Larvae perform the diurnal vertical migration in order to increase feeding chances and to escape from the predators. Moreover, the diurnal vertical migration increases the recruitment ratio of larvae.

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Averaged data were used to contrast the observed TF (total phosphorus) and TN (total nitrogen) concentrations in the surface and bottom layers of the Seto Inland Sea. These data were collected during the winter from 1981 to 2000, as part of a monitoring program of the Ministry of Environment, Japan. By estimating the origins of TP and TN to clarify the characteristics of bays and basins, it was found that the ratios of land originated TP and TN were over the six-tenths and seven-tenths in Osaka Bay, respectively. We suggest that this was due to additions from land, because there was one third of the total load from land in the Seto Inland Sea. On the other hand, the ratios of open ocean originated TP and TN were over eight-tenths in the western part of the Seto Inland Sea (e.g., Iyo-Nada and Suo-Nada). There were therefore contrasting characteristics between the east and the west of the Seto Inland Sea. We have discussed how Kii or Bungo Channels affected the variations of open ocean originated TP and TN concentrations in each region of the Seto Inland Sea. Consequently, it was shown that the TP and TN in the western side of Bisan-Seto were affected mainly by the open ocean via the Bungo Channel, whereas those in the eastern side of Bisan-Seto were affected mainly via the Kii Channel. This is partly due to the structure of the density driven current in the Seto Inland Sea.

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There has been much discussion about the cause of the decrease in M₂ tidal amplitude that has resulted from the reclamation of Isahaya Bay in Ariake Bay. As it has not been clarified yet, we tried to solve this problem using two approaches: (1) by analysing observational data at tide stations near Ariake Bay and (2) by numerical calculations using a three-dimensional variable mesh model. Applying these two approaches to tidal amplitude at three stations in Ariake Bay from 1981 to 2001, it was shown that the 25% decrease in M₂ tidal amplitude at Oura from 1981 to 2001 is caused by the Isahaya dike, and the remaining 75% decrease is caused by the attenuation of tide in the East-China Sea. This conclusion is verified by both approaches. Although the decrease of tidal amplitude in Ariake Sea responds linearly to the external effects, the tidal amplitude changes caused by internal effects such as the Isahaya dike depend on the location; that is, there is a decrease at Oura, no change at Misumi and an increase at Kuchinotsu. This is explained by the change in the tidal oscillation pattern in Ariake Bay due to the reclamation of Isahaya Bay.

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