To determine the quantitative relationship between phytoplankton production and zooplankton grazing pressure in Atsumi Bay, a eutrophic andpartially-mixed estuary, a series of investigations, including measurements of hydrographic conditions, dissolved oxygen, dissolved total nitrogen, particulate organic nitrogen, and phyto- and zooplankton biomass were conducted 13 times at intervals of 2-7 days in June and July 1984. Continuous measurements of water flow and salinity were also carried out to examine transverse flow and horizontal diffusivity. The supply of freshwater and nitrogen was estimated from given data. The changes of hydrographic condition, net photosynthetic rate and community primary production were calculated by a two-layered box model analysis. The grazing rate on phytoplankton obtained as the difference between net photosynthetic rate and community primary production was compared to the one estimated from zooplankton biomass and sardine, Sardinops melanosticta, biomass. The agreement between the data was remarkable in the upper layer, showing the grazing pressure on phytoplankton followed phytoplankton production, suggesting that a large part of produced phytoplankton was immediately grazed by zooplankton. Consequently, the community primary production was depressed to a fairly lower level. An important role of nutrient supply and water circulation, to limit phytoplankton production, was also confirmed. Dynamic response observed between the calculated grazing pressure and the biomass of phytoplankton and protozoa was also analyzed.

View full abstract

Long-term current measurements were carried out near the Soya Strait in the Okhotsk Sea during a period from February 1980 to September 1982. The data were divided into five segments, each being 150 days long, and the tidal ellipse parameters of major axis, minor axis, orientation, and phase for the four major constituents (M2, S2, K1 and O1 tides) were calculated at each segment. The major axis of the mean tidal ellipse averaged over five segments was 29.9cm sec-1 for O1 tide, 28.3cm sec-1 for K1 tide, 10.4cm sec-1 for M2 tide, and 3.7cm sec-1 for S2 tide. The phase and orientation of the tidal ellipse were much stable. But, the root mean square deviations of the major axis reached 20% of the mean values for all four constituents. The tidal currents estimated from the sea level records at Wakkanai and Esashi along the Hokkaido coast in the Okhotsk Sea show that their amplitudes and phases are in good agreement with the observed ones for all four constituents.

View full abstract

Variations of water and flow in Sagami Bay in relation to the Kuroshio path variations are examined by using 100m-depth temperature and salinity data from 25 stations as well as sea level data from five stations (Minami-Izu, Itô, Ôshima, Aburatsubo, Mera). In regard to temperature, anomalies from the mean seasonal variations are used. Results show that water properties are clearly different between the three typical paths of the Kuroshio. The difference is more remarkable in temperature than in salinity; temperature is higher during the typical largemeander (LM) path, and lower during the offshore non-large-meander (NLM) path, compared with the nearshore NLM path. Temperature anomaly and salinity distributions, as well as the Ôshima minus Minami-Izu and Ôshima minus Mera sealevel differences strongly suggest that the flows during the typical LM path are distributed as hitherto described in past studies, that is, water in the mouth region of the bay flows clockwise around Ôshima from the west channel to the east channel, and a counterclockwise eddy exists in the interior. On the other hand, flows during the nearshore and offshore NLM paths seem to be quite different from those during the typical LM path; velocities are very weak, and the directions of circulation is frequently reversed. This tendency also can be seen during parts of LM period in which the Kuroshio takes a non-typical LM path.
Water properties in Sagami Bay are most characteristic during transitions between nearshore and offshore NLM paths. During transitions from nearshore to offshore NLM paths, temperatures are extremely high as a whole in the bay, while during reverse transitions, both temperatures and salinities are very low in the entire region.

View full abstract

The effect of the earth's rotation on the behavior of heated water discharged from gigantic nuclear power plants was investigated numerically. It is shown that the effect is significant even for the existing scale of discharge. Horizontal spreading of the heated water is suppressed in offshoreward and northward directions (when heated water is discharged in an eastward direction from a westfacing coast in the northern hemisphere), but accelerated in the southward direction, and the shape of the formed warm water region becomes asymmetric. In this paper, several calculations were done using exaggerated Coriolis parameters (f) in order to demonstrate the effect of the earth's rotation. An increase of fenhances above effect, but the situation of the inner region of the produced warm water mass is much different from that in the outer region near its margin. The southward expansion of surface isothermal contours in the inner region increases with an increase in f, but those in the outer region do not. This results in an increase of the sharpness of the front which is generated near the southern edge of the warm water region. We have not found simple parameters which describe the temperature and velocity distributions in the warm water mass produced.

View full abstract

Tidal current ellipses formed by the Coriolis effect are investigated theoretically, taking account of the effect of horizontal boundaries. This study reveals that even in a narrow bay, the tidal current ellipse appears in the inside of the bottom boundary layer, although the ellipticity cannot be recognized in the outside of the boundary layer. In a wide bay, the ellipticity is observed even in the outside and it is larger in the inside. The rotation direction of these ellipses is counterclockwise in the northern hemisphere. And also, the Coriolis force has an effect to deflect the major axis of the ellipse in the inside of the boundary layer. These nature of the tidal current ellipse is well explained by the consideration of the formation mechanism of the ellipse.

View full abstract

We showed that, in all of the previous studies made of the horizontal circular “Tide-induced residual current (TIRC)”, the velocity field was far beyond the application limit of the TIRC theory and the discussions in previous studies had no physical reality, because in the circular TIRC, the pressure gradient force balances with the centrifugal force and a weak Coriolis' force. We also showed that the Euler number AP, which is defined by the ratio of the advection term to the pressure gradient term, is useful as a measure of nonlinearity of the temporal tidal velocity field in a numerical model experiment, and that the measure AP must be less than 0.05 or at least less than 0.1 for us to discuss the TIRC as a physical entity of “steady current” in physical space. We proposed another measure, KE, which is defined as the ratio of the sum of kinetic energies of compound- and over-tides to kinetic energies of diurnal- and semi=diurnal tides. We showed that this measure, KE, is also useful as an indicator of the magnitude of nonlinearity of tidal velocity field. We presented the distribution of KE in the Seto Inland Sea of Japan, showing that the nonlinearity is very large in many places of the basin, which must be considered when analyzing tidal current velocities and the dynamics of the current system or material transport in the Seto Inland Sea.

View full abstract