Hydrodynamic Instability of Ionization Front in HII Regions: From Linear to Nonlinear Evolution

A sample of 54 6.7-GHz methanol maser sources was monitored at HartRAO for 4 years, and 11 12.2-GHz methanol masers for 3 years. The majority of the maser features display a significant degree of variability but with a wide range of timescales and behaviors. Some maser features remained unvarying throughout the monitoring programme, while others showed sporadic flares or sudden drops in flux density. Yet another group show quasi-periodic and periodic variations. In some cases the maser features dropped below the detection limit for a significant length of time before increasing in intensity and reappearing.     

Rates of net assimilation and respiration of amino acids by marine bacteria

The selectivity of amino acid assimilation by marine bacteria was examined using seven kinds of¹⁴C-amino acids and the acid hydrolysate of¹⁴C-labelled proteins. It was found that the net assimilation and respiration by marine bacteria followed MICHAELIS-MENTEN kinetics for all of amino acids used in our experiments. Maximum velocities of amino acids were 0.01 to 0.19g carbon/hour per 2×10⁷ cells for net assimilation and less than 0.18g carbon/hour per 2×10⁷ cells for respiration at 20C. The velocity of gross assimilation was found with the following order: phenylalanine>valine, glutamic acid>serine, arginine>tryptophan>glycine. The assimilation velocities of amino acids in these laboratory works showed almost the same order as those in field experiments. The assimilation velocity of an amino acid was influenced by coexisting another amino acids or glucose. The assimilation velocity in lower substrate range of amino acids was directly proportional to the number of bacterial cells in the range from 6×10² to 3×10⁴ cells per ml. No linear relation between the assimilation velocity of amino acids and reciprocal of absolute temperature was found, but a marked bending was observed at 15 to 20C. The velocity at the optimum temperature was three to six times of that at 5C.     

Standing Crops of Planktonic Ciliates and Copepod Nauplii in the Subarctic North Pacific and the Bering Sea in Summer

Abundances and biomasses of planktonic ciliates and copepod nauplii, major components of the microzooplankton community, were investigated in the subarctic North Pacific and the Bering Sea in summer of 1997. Their regional variation was illustrated by demarcating the entire area into five regions. Ciliates always predominated both in abundance (>94%) and biomass (>78%) over nauplii. Regional means of ciliates in the water column were higher in the Alaskan Gyre (120 × 10⁶ cells/m²) and the Western Subarctic Gyre (110 × 10⁶ cells/m²) in terms of abundance, and rich in the Bering Sea Gyre (360 mgC/m²) and the Western Subarctic Gyre (340 mgC/m²) in terms of biomass. By contrast, standing crops of ciliates were poor in the Oyashio Region (67 × 10⁶ cells/m²; 170 mgC/m²) and the Transition Region (64 × 10⁶ cells/m²; 160 mgC/m²). The values of biomass reported here are generally in agreement with the values reported previously from the Bering Sea Gyre and the Alaskan Gyre but are considerably higher than the previous value found in the Western Subarctic Gyre. No significant correlations could be found between chlorophyll a crop and standing crops of ciliates and copepod nauplii over the entire subarctic North Pacific and the Bering Sea during this summer.     

The roughness height and drag law over the water surface based on the hypothesis of local equilibrium

A series of measurements of winds and wind-waves were carried out in wind-wave flumes. A data analysis based on the hypothesis of local equilibrium yielded a new empirical formula on the controversial quantity of roughness heightz ₀ over the water surface: , where the nondimensional roughness height is defined bygz ₀/u _* ² and the wave-wind parameterũ byω _p u _*/g, g being the gravitational acceleration,u _* the friction velocity of air,ω _p the peak frequency of wind-wave spectra. The obtained formula is compared with Charnock's (1955) and Toba's (1979) proposals; is constant in the former and inversely proportional toũ in the latter. As in Toba's, this formula immediately leads to a practically important conclusion that the drag coefficientC _d depends not merely on the usual variableU ₁₀ (wind velocity at 10m height over the water surface), but also on the surface state represented by wind-waves. An explicit expression is provided for the drag coefficient incorporating the wave-wind parameter; it covers the range ofC _d calculated from most of the previous drag formulas, by varying the wave-wind parameter.     

Microzooplankton standing crop in the western subtropical pacific off the Bonin Islands in winter, 1980

Quantitative data on the vertical distribution and biomass of microzooplankton are presented for the western subtropical Pacific west of the Bonin Islands in winter. Microzooplankton other than foraminifera and radiolarians showed similar vertical distribution to chlorophylla. Among microzooplankters, naked ciliates, tintinnids and copepod nauplii were dominant components both in number and volume. Naked ciliates were the most dominant, comprising 82.2 % and 47.7 % of the total microzooplankton by number and volume, respectively. Copepod nauplii, occupying less than 10 % by number, accounted for 35.7 % of the volume of all microzooplankton. Tintinnids contributed less to the microzooplankton standing crop, 12.6 % of the total by number and 17.3 % by volume. Calculated microzooplankton wet weight varied from 0.84 to 1.80 g m^?2 and corresponded to 9.9–18.1 % of net zooplankton weight. The relative abundance of microzooplankton to net zooplankton in this study is comparable to that previously reported in the tropical and subtropical Pacific Ocean, but a little higher than in oceanic subarctic areas.     

The coriolis effect against frontogenesis in steady buoyancy-driven circulation

The Coriolis effect on frontogenesis in the transitional region between the inner bay and the outer ocean is studied by running several three-dimensional numerical experiments. The aim is to clarify what external parameters, including the transverse scale of the basin, control the phenomena. The characteristics of the front in each case are described by defining three diagnostic quantities, namely, the sharpness of the front, the intrusion of the front, and the buoyancy difference between the minimum buoyancy region and the surrounding part, as the existence of the minimum buoyancy region is closely related to frontogenesis. The three quantities are shown to depend on two nondimensional numbers,R _of (=f ⁻¹ L ^−2/3 F _b ^1/3 : defined as flux Rossby number) andR _ef (=ν _v ⁻¹ L ^−2/3 F _b ^1-3 H ²: defined as flux Reynolds number).     

Wind-wave spectra based on the hypothesis of local equilibrium

Wind-wave spectra measured in a wind-flume are analyzed according to the hypothesis of local equilibrium. The gross relation between the wave height and the frequency is reexamined to yield the basic validity of the 3/2-power law of Toba orE～? in the range of 0.4≦?≦1, where? is the wave-wind parameter defined by?=ω _p u _*/g; ω _p denotes the peak frequency of the wind-wave spectra,u _* the friction velocity andg the gravitational acceleration. Noticeable deviation is found, however, for?<0.4 or?>1. In particular, the data for large? suggest the existence of an upper limit of the wave nonlinearityE at about 5×10^?2, whereE=Eω _p ⁴ /g² withE the total power of the wind wave spectrum. Then, the spectral form is investigated in detail. As? decreases, the normalized spectrum becomes more gradual as a whole, but its forward (low frequency) part tends to show a steeper profile. In the high frequency region ( \(\tilde \omega \) >2.6), the spectrum is found to have a functional form likeu _* ² ω ^?3, which differs from the usualω-dependence asω ^?5 orω ^?4. It suggests weak dependence of the high-frequency spectra on the gravitational accelerationg and on the peak frequencyω _p ; spectral density at high frequencies may be saturated, so that its magnitude may be dominated by the frequencyω, the friction velocityu _*, the surface tension and the viscosity.     

Seasonal dynamics of the phytoplankton community in Sendai Bay,northern Japan

Sendai Bay is located on the Pacific coast of northern Japan and suffered serious damage following the 2011 off the Pacific coast of Tohoku earthquake and tsunami in March 2011. To assess the impact on the marine ecosystem, information was needed on the phytoplankton communities and their seasonal variation. However, such information was limited. Therefore, an intensive monitoring of the phytoplankton was carried out from March 2012 to April 2014. Seasonal variation of the phytoplankton community was similar at coastal and offshore stations. Total phytoplankton biomass, based on Chl a concentration, peaked in spring and then decreased to a minimum in summer, before gradually increasing during early winter and peaking again in the following spring. This seasonal pattern was consistent with previous studies conducted before the earthquake and tsunami. Also, size structure of the phytoplankton community and its four main groups was estimated from the size-fractioned samples of Chl a. Our results also showed that the spring bloom consisted of large diatoms, with their growth ceasing due to nitrogen depletion. The bloom was followed by a summer period where cyanobacteria and picoeukaryote became dominant, with high cell densities in spite of low nutrient concentrations. In addition, sporadic environmental changes, such as those following typhoons, were observed. These resulted in large increases/decreases in individual phytoplankton groups.     

Statistics of Wind and Waves off Tsuyazaki, Fukuoka, in the Eastern Tsushima Strait

The variability of the sea surface wind and wind waves in the coastal area of the Eastern Tsushima Strait was investigated based on the hourly data from 1990 to 1997 obtained at a station 2 km off Tsuyazaki, Fukuoka. The annual mean wind speed was 4.84 m s⁻¹, with strong northwesterly monsoon in winter and weak southwesterly wind in summer. Significant wave heights and wave periods showed similar sinusoidal seasonal cycles around their annual means of 0.608 m and 4.77 s, respectively. The seasonal variability relative to the annual mean is maximum for wave heights, medium for wind speeds, and minimum for wave periods. Significant wave heights off Tsuyazaki turned out to be bounded by a criterion, which is proportional to the square of the significant wave period corresponding to a constant steepness, irrespective of the season or the wind speed. For terms shorter than a month, the significant wave height and the wave period were found to have the same spectral form as the inshore wind velocity: white for frequencies less than 0.2 day⁻¹ and proportional to the frequency to the −5/3 power for higher frequencies, where the latter corresponds to the inertial subrange of turbulence. The spectral levels of wave heights and wave periods in that inertial range were also correlated with those of the inshore wind velocity, though the scatter was large. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differences in the structure of the lower trophic levels of pelagic ecosystems in the eastern and western subarctic Pacific

To illustrate areal differences in the structure of lower trophic levels of the pelagic ecosystems in the subarctic Pacific, data collected in the quasi-steady state summer/fall conditions were analysed for five areas, i.e. the Bering Basin, Western Subarctic Gyre, the area south of the Aleutians, the Gulf of Alaska, and the Oyashio Region. Average values of stock size of four components of the lower trophic levels showed a clear difference between areas with ranges of 7.5-fold for nitrate, 3.0 for chlorophyll a, 9.9 for microzooplankton, and 2.4 for mesozooplankton. Such differences were more striking when the structure of the lower trophic levels was expressed as a biomass pyramid. In the Gulf of Alaska, Western Subarctic Gyre, and south of the Aleutians, the relative biomass of microzooplankton to phytoplankton is large and large amounts of nitrate remained unused. In addition to possible iron limitation, grazing control by the microzooplankton on small phytoplankton must be substantial in these areas. Conversely, in the Oyashio Region, the nitrate stock is very small indicating higher efficiency of nitrate consumption by phytoplankton. However, since phytoplankton and zooplankton stocks are not particularly large, their products are likely to be transferred, also efficiently, to the higher trophic levels such as planktivorous pelagic fish. The situation in the Bering Basin is intermediate between the Oyashio Region and the other three areas. Inter-annual fluctuations in stock size of the planktivorous fish which migrate into the Oyashio Region in summer/fall were quite large. However, the inter-annual variation of mesozooplankton biomass was small, suggesting the existence of certain mechanisms to stabilize plankton abundance under increasing predation pressure. As a result, the increasing fish stocks likely keep the transfer efficiency from nitrate through to fish higher, at least in the Oyashio Region.