Carbon dioxide in surface seawaters of the Seto Inland Sea,Japan

Observations were made of time variations of carbon dioxide in seawater, pCO₂, and in the atmosphere, PCO₂, in the Seto Inland Sea of Japan. The pCO₂ data showed well defined diurnal variation; high values at nighttime and low values during daylight hours. The pCO₂ correlated negatively with dissolved oxygen. These results denote that the diurnal variation of pCO₂ is associated with effects of photoplankton's activity in seawater. The pCO₂ measured in the Seto Inland Sea showed higher values than the PCO₂ during June to November, denoting transport of carbon dioxide from the sea surface to the atmosphere, and lower values during December to May, denoting transport of carbon dioxide from the atmosphere to the sea surface. The exchange rates of carbon dioxide were calculated using working formula given by Andriéet al. (1986). The results showed that the Seto Inland Sea gained carbon dioxide of 1.0 m-mol m^–2 d^–1 from the atmosphere in March and lost 1.7 m-mol m^–2 d^–1 to the atmosphere in August.     

Measurements of carbon dioxide in the Seto Inland Sea of Japan

The carbon dioxide in seawater (pCO₂) was measured in the Seto Inland Sea of Japan using newly developed equilibrator instrument designed to be free from the correction for addition or extraction of the carbon dioxide from carrier gas. The temperature dependence of pCO₂ was about 4.5%pCO₂/°C for a single seawater sample which was processed as free from biological activity and change in total carbon dioxide content during an experiment. The decrease in pCO₂ during daylight hours due to the photosynthetic fixation was about 30% of the daily mean of pCO₂ during warm months and about 15% during cold months. The effect of carbon dioxide exchange between air and seawater on pCO₂ was about 0.6 ppm in August and about 0.1 ppm in March. This is negligible small compared with the daily oscillation of carbon dioxide in seawater.     

Horizontal distribution of particulate matter and its characterization using phosphorus as an indicator in surface coastal water,Harima-Nada,the Seto Inland Sea,Japan

To study the horizontal distribution and characterization of suspended particulate matter (SPM) in the coastal environment, the distribution and seasonal variation of particulate phosphorus (PP), divided as organic (POP) and inorganic (PIP) fractions with POC, PON, and Chl.a, in 4 seasons throughout Harima-Nada of the Seto Inland Sea, Japan, were investigated. A high concentration of PIP was observed not only in the northern coastal area, which received much freshwater discharge, but also in the channel parts. In winter, when Δσt was low, the difference between surface and bottom σt, resuspension of sinking particles or/and surface sediment which has a high PIP/PP ratio seems to have occurred, and then a high concentration of PIP in surface water was observed. In spring and summer, PIP/PP ratios (0.24) of surface waters were close to those of phytoplankton (0.1–0.2). On the other hand, PIP/PP ratios in autumn and winter, when Δσt was low, were similar to those of surface sediment in Harima-Nada (0.32–0.39). SPM in Harima-Nada showed higher PP contents than other coastal environments, and its mean value of 9.10 mg P/g was close to the phytoplankton P content of 9.75 mg P/g. This suggests that SPM in Harima-Nada was strongly affected by phytoplankton. The C/N ratio in Harima-Nada in spring was high (11), although high Chl.a and low PIP/PP were observed, possibly resulting from preferential decomposition of nitrogen in comparison with carbon. We consider that the PIP/PP ratio is a good indicator to characterize SPM and to predict its origin as well as the C/N.     

Open Ocean Originated Phosphorus and Nitrogen in the Seto Inland Sea,Japan

The ratios of phosphorus and nitrogen originating from land and the open ocean in the Seto Inland Sea, which is the largest semi-enclosed coastal sea in Japan, have been estimated from data on total phosphorus and nitrogen loads and observed concentrations of total phosphorus and nitrogen. The ratios of land-origin and open-ocean-origin total phosphorus and nitrogen in the Seto Inland Sea are 0.28:0.72 and 0.19:0.81, respectively.     

A numerical study of the seasonal circulation in the Seto Inland Sea,Japan

The seasonal variation of water circulation in the Seto Inland Sea is investigated using a high resolution, three-dimensional numerical ocean model. The model results are assessed by comparison with long-term mean surface current and hydrographic data. The simulated model results are consistent with observations, showing a distinct summer and winter circulation patterns. In summer the sea water is highly stratified in basin regions, while it is well mixed near the straits due to strong tidal mixing there. During this period, a cold dome is formed in several basins, setting up stable cyclonic eddies. The cyclonic circulation associated with the cold dome develops from May and disappears in autumn when the surface cooling starts. The experiment without freshwater input shows that a basin-scale estuarine circulation coexists with cyclonic eddy in summer. The former becomes dominant in autumn circulation after the cold dome disappears. In winter the water is vertically well mixed, and the winter winds play a significant role in the circulation. The northwesterly winds induce upwind (downwind) currents over the deep (shallow) water, forming a “double-gyre pattern” in the Suo-Nada, two cyclonic eddies in Hiuchi-Nada, and anticyclonic circulation in Harima-Nada in vertically averaged current fields.     

Purine and pyrimidine bases in marine particles in the Seto Inland Sea, Japan

Purine and pyrimidine bases in marine environmental particles collected in Harima-Nada, the Seto Inland Sea, Japan, were investigated by high performance liquid chromatography.Purines and pyrimidines concentrations varied from 0.3 to 9.3 μg l⁻¹ (n=20) for suspended matter, and 0.3 to 0.6 mg g⁻¹ (n=10) for sinking particles. A good correlation was found between chlorophyll a and purine+pyrimidine bases in suspended matter, indicating that these bases contained in suspended matter originated from phytoplankton.A comparison between several compositional data of the suspended matter and the sinking particles, namely CN ratio, composition of purines and pyrimidines, and percentages of the nitrogen bases relative to total particulate organic nitrogen, demonstrates that the sinking particles were different from suspended matter. Also, from the variety of purine and pyrimidine concentrations of marine particle samples, it was estimated that the decomposition rate of these bases seemed more rapid than decomposition rates of amino acids reported in our earlier study.     

Evidence of microphytobenthic roles on coastal shallow water of the Seto Inland Sea,Japan

This study describes the temporal variation of microphytobenthic biomass and its controlling factors, as well as the impact of microphytobenthic activities on coastal shallow sediment in the eastern Seto Inland Sea, Japan. The chlorophyll a (Chl a), phaeopigments and sedimentary biophilic element (C, N, P and Si) contents in surface sediments, as well as nutrient concentrations at the sediment-water interface (overlying water and pore water) were observed monthly during November 2003 to May 2005 at one site in Shido Bay (water depth ca. 7 m) and at one site in Harima-Nada (35 m). No correlation was observed between the sedimentary biophilic elements and other parameters. The maximum chlorophyll a content of 48.2 μg g^–1 was found in surface sediments under the photon flux reaching the seafloor of 537 μmol photon m^–2 s^–1 during the winter period when water transparency was the highest at station S (Shido Bay). Our results suggest that higher chlorophyll a content in surface sediment was due to the fresh microphytobenthic biomass (mainly benthic diatom). We also found a significant negative correlation between Chl a and inorganic nutrients in pore water during the low temperature period, especially silicic acid. This result suggests that the silicic acid was assimilated largely during the increase of microphytobenthic biomass in surface sediment. This study suggests that the microphytobenthic roles may have a great effect on nutrient budgets during the large supply of irradiance (winter periods) for their photosynthetic growth in shallow coastal waters.     

The effect of a tidal cycle on the dynamics of nutrients in a tidal estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH₄ ⁺−N, (NO₃ ⁻+NO₂ ⁻)−N, PO₄ ³⁻−P and Si (OH)₄−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r ²=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r ²=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.     

Long-term variability of shelf water intrusion and its influence on hydrographic and biogeochemical properties of the Seto Inland Sea, Japan

The variability of the pathway of the Kuroshio is known to control that of the intensity of upwelling of the cold and nutrient-rich deep shelf water at the shelf-break point. This study has investigated the influences of the long-term variability of the hydrographical and biogeochemical conditions at the shelf-break point on those in the inner Seto Inland Sea. Results from observations conducted since 1972 by prefectural fisheries observatories showed a temporary decrease of temperature in the lower layer of the entrance of Kii Channel in summertime in the specific years when the Kuroshio pathway favored deep shelf water intrusion. A corresponding change in temperature was also found in the inner shelf sea, such as western Osaka Bay and Harima-Nada. The model developed in this study covers a 30-year period and well reproduces the temporal change in temperature. It also shows sufficient reproducibility in both seasonal behavior of hydrographical and biogeochemical properties and the inter-annual variability of hydrographical properties throughout the eastern Seto Inland Sea. It was found that the inter-annual variations of annual total flux through the section at the entrance to Kii Channel affect the inter-annual variation of the observed concentration of Dissolved Inorganic Nitrogen (DIN) in Harima Nada, but the effect was smaller than those of river input and other parameters.     

Analysis of primary production in the Seto Inland Sea, Japan, using a simple ecosystem model

A simple model of lower trophic level ecosystem has been created to analyze possible environmental control of primary production in eight sub-areas of the Seto Inland Sea. The primary production rates observed by Hashimotoet al. (1997a) in these sub-areas are well reproduced by the model, including horizontal processes such as horizontal transport of nutrients and vertical processes such as vertical mixing, light intensity and sinking of particulate matter. Without taking account of horizontal processes the model also successfully reproduces the observed primary production rates in some areas, but if fails to reproduce those in the others. This shows that the relative importance of the horizontal transport on the primary production differs are by area. Two time scales,T _z andT _H, are introduced to explain this difference.T _z is a vertical cycling time of material, which is defined as the time during which the stock of the material in the water column is utilized for primary production;T _H is the horizontal transit time of the material. The relative importance of the horizontal process is well explained by theT _H/T _z ratio; that is, the horizontal transport process is important in the areas where this ratio is small. Further the possible mechanisms of nutrient supply for the primary production in each sub-area are investigated using this model.     

Dynamics of microphytobenthic biomass in a coastal area of western Seto Inland Sea, Japan

This study focused on the causes of the variation in microphytobenthic biomass and the effects of this variation on macrobenthic animals in the western Seto Inland Sea, Japan, where the importance of microphytobenthos as the primary food source for benthic animals has been recently reported. We investigated the microphytobenthic biomass together with light attenuation of seawater, phytoplanktonic biomass, macrobenthic density and biomass at eight stations (water depth = 5–15 m) during four cruises in 1999–2000. The increased light attenuation coefficient of the water column associated with increased concentration of the phytoplanktonic Chl-a caused a decrease in light flux that reached the seafloor. The biomass of the microphytobenthos within the upper 1 cm of the sediment, 1.9–46.5 mg Chl-a m⁻², was inversely correlated with the phytoplanktonic biomass in the overlying water column, 10.9–65.0 mg Chl-a m⁻². Thus, interception of light by phytoplankton is considered to be a main cause of the variation in the microphytobenthic biomass. The microphytobenthos biomass showed a significant positive correlation with the macrobenthic density (78–9369 ind. m⁻²) and biomass (0.4–78.8 gWW m⁻²). It appears that the increase in oxygen production by the microphytobenthos allowed macrobenthic animals to become more abundant, as a consequence of oxygenation of the organically enriched muddy sediments (14.5 ± 2.69 mg TOC g⁻¹). This study suggests that the variation in the microphytobenthic biomass is influenced by the phytoplanktonic biomass due to shading effect, and the balance between these two functional groups might affect the variability in the macrobenthic density and biomass.     

Heavy metals and accumulation rates of sediments in Osaka Bay,the Seto Inland Sea,Japan

The metal load into sediments and the change in the sedimentary environment of Osaka Bay in the Seto Inland Sea have been studied through geochemical analysis of core sediments, using both Pb-210 dating and a selective chemical leaching technique. Analytical results from a 6-m core of sediment show that copper and zinc pollution started in the late 1800's and the present enrichment ratios of copper and zinc, relative to background levels (20 mg kg^–1 for Cu and 94 mg kg^–1 for Zn), are 2.8 and 4.1, respectively. The present anthropogenic copper and zinc loads into Osaka Bay sediments, are 47 and 368 ton yr^–1, while natural copper and zinc loads are 40 and 186 ton yr^–1, respectively. Osaka Bay sediment at the present day is considered to be seriously polluted by zinc, now. The vertical profiles of copper and zinc in four successively separated fractions (10% acetic acid soluble fraction: F-HAC, 0.1M hydrochloric acid-soluble fraction: F-HCl, hydrogen peroxide-soluble fraction: F-H₂O₂ and hydrofluoric acid-soluble fraction: F-HF) from the core sediments indicate that enrichments of copper and zinc in the upper layer of the sediment are dependent on increases in the metal contents of the F-HAC, F-HCl and F-H₂O₂ fractions. Copper in F-HAC, and zinc in F-HAC and F-HCl, seem to be of anthropogenic origin.Results of sequential studies of the whole Seto Inland Sea can be summarized as follows: At the present time, the sedimentary loads of copper and zinc over the whole Seto Inland Sea area are 630 and 3,500 ton yr^–1, respectively, while the natural and anthropogenic loads are 320 and 310 ton yr^–1 for copper and 1,800 and 1,700 ton yr^–1 for zinc, respectively.     

Reciprocal sound transmission measurement of mean current and temperature variations in the central part (Aki-nada) of the Seto Inland Sea,Japan

A 30 km-range reciprocal sound transmission experiment was carried out on the line connecting Honshu and Shikoku (the first and fourth biggest main Japanese islands, respectively) in the central part (Aki-nada) of the Seto Inland Sea, Japan, during March–May 2010 to measure the mean current and temperature variations over the sea. The range-averaged current along the sound transmission line was estimated to have a mean and standard deviation of (3.8–4.4) ± (1.7–1.8) cm/s after converting the travel time difference data into currents, including a fortnightly tidal variation in the range of ±30 cm/s. The positive mean current implies slow water movement from the west to east through Aki-nada. The range-averaged speed of sound was estimated by converting from the mean travel time or one-way travel time into the speed of sound, and further converted into temperature for fixed values of salinity and depth, according to the standard speed of sound formula. Besides the precise measurement (to an accuracy of 0.01°C) of semidiurnal and diurnal tidal variations and seasonal warming, the temperature data showed periodic variations with periods of 7.0 and 21.1 days that had never been observed in Aki-nada before. This study suggests that reciprocal sound transmission is a powerful technique for the long-term accurate measurement of mean current and temperature variations in coastal and inland seas.     

Standing stocks and production rates of phytoplankton and abundance of bacteria in the Seto Inland Sea, Japan

Standing stocks and production rates of phytoplankton and abundance of bacteria were investigated at 39 stations in the Seto Inland Sea, Japan during four crulses in October 1993, January, April and June 1994. Primary productivity was measured by the¹³C tracer method. Photosynthetic rate varied from 0.41 to 32.1 μgC/1/h with an average value of 4.67 μgC/l/h. Annual primary production was estimated to be 218 gC/m²/year. Annual primary production in this study was 1.8 times as high as the values which were previously reported at same area. The reason for the disagreement between our primary production value and previous values is not thought to be due to the difference of methods used for measuring primary production or the different Chl.a concentrations but to the method of estimating the primary production in the euphotic zone from thein vitro measurements. The distribution of bacterial cells in surface seawater was examined during the same cruises. Bacterial cell density ranged from 0.32 to 3.4×10⁶ cells/ml. The density was relatively high in the eutrophic regions of Hiroshima Bay and Osaka Bay In addition, a high density of bacteria was also observed in an area within Suo Nada where Chl.a was relatively low. The disparity between Chla and bacterial density in Suo Nada suggests that bacterial abundance can be controlled by the availability of substrates other than phytoplankton exudate.     

Seasonal variation of water temperature in the Seto Inland Sea

The seasonal variation of water temperature in the Seto Inland Sea, Japan is examined using data analysis and numerical experiments and is shown to be controlled by heat exchange through the sea surface and horizontal heat dispersion from the Pacific Ocean. The average water temperature goes down from the Pacific Ocean to the center of the Seto Inland Sea indicating that 4.0 to 6.0×10¹⁵ cal day^?1 (1.6 to 2.5×10¹⁶ joule day^?1) of heat is transported from the Pacific Ocean to the Seto Inland Sea and is lost through the sea surface. The amplitude of seasonal variation of water temperature is large at the center of the Seto Inland Sea and the maximum water temperature is reached first at Bisan Straits and last at Iyo-Nada.     

Geotechnical properties of submarine sediments in the Seto Inland Sea

Abstract

Sampling of submarine sediments by an improved piston corer and a bucket dredger has been carried out since 1973. The length of the core samples ranged from 0.9 m to 5.4 m. The recovery ratio ranged from 39.1 percent to 98.9 percent. The physical and engineering properties of 16 sediment cores and the physical properties of 125 dredged samples were determined, and variation of these properties was analyzed.

Clayey silts and silty clays cover the floor of relatively wide bays and sea areas. Sands and sandy silts blanket the floor of channels that have swift currents, and are also found in the vicinity of estuaries.

The piston core samples showed considerable sample disturbance, which should be estimated quantitatively in the future.     

228Ra in surface water of the Seto Inland Sea

The concentrations of²²⁸Ra in surface waters of the Seto Inland Sea were determined. Surface waters from the central region of the Seto Inland Sea, Hiuchi Nada and Bingo Nada, contained concentrations of²²⁸Ra of 655–811 dpm/1000 l which were 100 times higher than those obtained in the Pacific Ocean. These high concentrations of²²⁸Ra must be supported by a²²⁸Ra flux from the bottom sediment. The lower limit of this flux was estimated to be more than 0.16 dpm cm^–2 y^–1. The²²⁸Ra concentrations decreased markedly from central regions of the Seto Inland Sea to about 18 dpm/1000 l in the Kii and the Bungo Channels as salinity increased. Using a box model and the²²⁸Ra data, the mean residence time of sea water in the Seto Inland Sea with respect to the exchange with the open ocean water was estimated to be less than 10 y, and the most probable value is the order of several years.     

Entrainment of bottom sediment in the Seto Inland Sea in summer

Time series of the vertical distribution of resuspended matter and bottom current were collected concurrently during summer at a few anchored stations in the Seto Inland Sea. The vertical distribution of resuspended matter was measured every hour for about one tidal cycle and the three components of current fluctuation were obtained at each sampling station. Current data at each sampling station show that the bottom is hydraulically smooth.Assuming that the averaged vertical distribution of resuspended matter for one tidal cycle shows a steady state distribution, the settling velocityWs of resuspended matter is estimated to be in the range of 1.2×10^–2 to 5.7×10^–2 cm sec^–1 from analysis of the averaged distributions.The relation between the erosion rate and the bottom shear stress for this study area is investigated and is compared with that for other areas. The results show that the erosion of sediment in the Seto Inland Sea during summer occurs even due to the low bottom shear stress which is considered as almost smooth hydraulically.     

Total organic carbon and nitrogen contents in surface sediments of Harima Nada,eastern Seto Inland Sea,Japan: a comparison under two different trophic states

Phytoplankton abundance in the surface waters of Harima Nada, eastern Seto Inland Sea of Japan, decreased after around the late 1970s following a reduction of terrestrial phosphorous inputs, and relatively low levels persisted thereafter. This symptom of de-eutrophication in recent decades might have reduced organic matter storage in surficial sediments. To test this possibility, we examined total organic carbon (TOC) and nitrogen (TN) contents of Harima Nada sediments in 2011, and compared the results to past data from more eutrophic states. In 2011, the TOC and TN varied horizontally within the basin, in relation to physicochemical properties (water and mud content) and phytoplankton remnants (pheo-pigments) of the sediments. These horizontal patterns were fundamentally similar to those of the early 1980s (1982–1983), although discrepancies between the periods were observed in some areas. Differences of mean TOC and TN in the 0–2 cm layer at 63 stations from 1982–1983 to 2011 were consequently <10 %. Hence, the recent de-eutrophication in Harima Nada was associated with little overall change of TOC and TN storage in the sediment. In Harima Nada, changes in water clarity and particle size composition of the sediment seem to have occurred during recent decades. The aforementioned paradoxical phenomenon may therefore be explained by factors that are affected by these environmental changes. Such factors include the ratio of export flux to the seafloor to phytoplankton production, phytoplankton biomass and production in the entire euphotic zone, and accumulation rate of surface sediment.