FATE OF SILTS AND CLAY FROM RIVER AND ITS CONTRIBUTION TO
TRANSPARENCYTetsuji Okuda, Ryukoku University, Japan
Satoshi Sekitou, Hiroshima University, JapanAkira Umehara, Hiroshima University, Japan
Satoshi Asaoka, Kobe University, Japan
Satoshi Nakai, Hiroshima University, Japan
Wataru Nishijima, Hiroshima University, Japanokuda@rins.ryukoku.ac.jp
Phytoplankton plays a key role as primary producer, forming the base of marine foodwebs. Knowledge in relation to permeability of light in water is important for theunderstanding of phytoplankton growth in the euphotic zone. In this study, we conductedlaboratory experiments in relation to light attenuation using inorganic particle (silicaparticle) and field investigations in Osaka Bay. There was a positive correlation between
the concentrations of the silica particle and integral values of the absorbance at
photosynthetic active radiation (PAR: wavelength 400-700 nm) in the laboratory
experiments. The highest integral value of the absorbance at PAR was observed for theparticle size of 1.0 μm. In Osaka Bay, high contribution of the inorganic particle to lightattenuation was observed compared to the organic particles. Multiple linear regression
analysis using the particle size and the amount of total suspended solids (TSS: consistingof three component fractions; organic/inorganic tripton and phytoplanktons) showedthat the particle size was an essential factor controlling the light attenuation in thecoastal sea.
Key words: light attenuation, CDOM, LISST-100X, Osaka Bay, particle size
I. INTRODUCTION
Riverine supply of inorganic nutrients to estuaries makes these areas of high primary
production potential [1]. In shallow areas including estuaries, not only pelagic small organisms
representing phytoplanktons but also benthic plants (e.g. seagrasses) and benthic microalgae are
principal primary producers. For their photosynthesis, light condition in the water column
represented by transparency (Secchi depth) is one of the important factors. The light (PAR)attenuation in the water column was typically divided into the following components: the TSS,
chromophoric dissolved organic matter (CDOM), and seawater [2]. The TSS was divided further
into three component fractions (inorganic tripton: TSS IT , organic tripton: TSS OT , and1

phytoplanktons: TSS PHY ) [3][4].
Our study area, Seto Inland Sea (area: 23,203 km2) is located in the western part of Japan.
In this area, eutrophication occurred during high economic growth period of industrialization andurbanization in the 1950s-70s. Therefore, organic matter and nutrient (nitrogen and phosphorus)
loadings have been reduced according to the Water Pollution Control Law (1970) and the Law
Concerning Special Measures for Conservation of the Environment of the Seto Inland Sea (1973).
Impacts of these control policies were effective, and the transparency in Osaka Bay which wasoriginally a highly eutrophic water area, was dramatically improved. For a better understandingof the light condition supporting primary production, the controlling factor of the transparency inthe coastal area needs clarification.
In this study, laboratory experiments in relation to the light attenuation using inorganicparticle (silica particle) and field investigations of PAR attenuation factor in Osaka Bay wereconducted. From these results, the particle size strongly contributing to the light attenuation wasclarified, as well as its impact to the ambient light condition in the coastal area.
II. MATERIALS AND METHODS
Study area
For evaluation of the relationship between TSS (including <1 µm in the particle size) andlight attenuation in the coastal area, field investigation was carried out in the Osaka Bay (Fig.1).
Osaka Bay is located at the eastern part of the Seto Inland Sea, Japan. Osaka Prefecture has apopulation of 8.8 million, and organically polluted waters are discharged into the bay through thetwo large rivers named Yodo River (flow distance: 75 km, basin area: 8,240 km2, population inthe basin: 11.2 million) and Yamato River (flow distance: 68 km, basin area: 1,070 km2,population in the basin: 2.5 million) [5][6].

Fig. 1. Sampling stations in Osaka Bay. Stns 1-3 and Stns 4-6 indicate Yodo River line andYamato River line, respectively.
Field investigation and sample treatment
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The investigation at ten sampling stations in the bay was carried out on 10 November
2015. At each station, the transparency of the water with a Secchi disk was measured, and thebasic water quality (temperature, salinity, and photon flux and others.) and the vertical profiles ofwith a probe (AAQ176, JFE Advantec, Kobe, Japan) were determined. A laser scattering andtransmissometry probe (LISST-100X Type-B, Sequoia Scientific, Inc., WA, USA) was used tomeasure in situ particle size distribution (thirty-two different size classes) and particle volumeconcentration in the range of 1.25-250 μm (diameter). For determination of the CDOM
absorbance (440 nm), chlorophyll a (Chl.a) concentration, TSS concentration, loss on ignition(IL), the surface water was collected using a plastic bucket.
For CDOM analysis, the surface water samples were filtered through disposal disk filters(0.45 μm, DISMIC-25cs, ADVANTEC, Tokyo, Japan). Absorbances at 440 and 750 nm of thefiltrated samples were measured using a double beam spectrophotometer (UV-1800,SHIMADZU, Kyoto, Japan). For the determination of Chl.a concentration, 300 ml of water
samples were filtered through glass-fiber filters (GF/F, Whatman, Maidstone, UK), and theresidues on the filters were extracted with 10 ml of 90% acetone in dark conditions at -20°C for16-24 hours. The extracts were sonicated for 20 min, and their concentrations were determined
fluorophotometrically (10-AU-5, Turner Designs, Sunnyvale, CA, USA), according toWelschmeyer method [7]. 500 ml of water samples were filtered through GF/F, the residues on
the filters were freeze-dried, and weighted for determination of the TSS and IL at 550℃ for 2hours.
Laboratory experiments and Data analysis
To find out the relationship between the particle size and the K TSS , laboratory
experiments using spherical silica particles (sicastar, COREFRONT Co., Ltd., Tokyo, Japan) of0.1, 1.0, 3.0, 5.0, and 10 μm in diameter were conducted. The particles were suspended inultrapure water (varying in concentration between 1 and 10 mg/L), and the turbidity and theabsorbances in PAR were analyzed.
For evaluation of the light attenuation in the water column, vertical downward attenuation
coefficient of light (K d , PAR: 400-700 nm) was used. The K d (m-1) was obtained according to theLambert-Beer Law expressed as follows:(1)
where z indicates the depth in meters, I Z and I 0 are photon flux (µmol/m2/s) at z in depth andbelow the water surface, respectively. To estimate the relative contribution of each component toK d , K d was partitioned into three fractions:(2)
where K W , K CDOM , and K TSS are attenuation coefficients of the seawater (0.038 m-1 [8]), CDOM
(0.221 × CDOM 440 m-1 [9]), and TSS, respectively. In this study, K TSS was divided into three3

component fractions:(3)
where K IT and K OT are the attenuation coefficients of inorganic tripton and organic triptonestimated from the IL analysis, respectively. The attenuation coefficient of phytoplankton (K PHY )was obtained from the dry weight (µgDW/L) of the phytoplankton. It was calculated from theChl.a concentrations (140 × µg-Chl.a/L), which was previously reported in the Seto Inland Sea[3].
For the determination of the in situ particle size distributions and particle volumeconcentrations, the dataset of LISST-100X between 3.1 and 165 .0 µm was used due to removalof doubtful values. Multiple linear regression analysis was performed to determine whichvariables (the particle size, the particle volume, and the amount of TSS consist of threecomponent fractions) affected the light attenuation coefficient of TSS (K TSS ).
For evaluation of the particle size effect for the K TSS , the TSS consists of threecomponent fractions was weighted by the results of the laboratory experiments using silica
particles. According to the Beer-Lambert law, the absorbance A is expressed by the following
equation:(4)
where ε is the molar extinction coefficient, C indicates the particle concentrations (mg/L), L isthe cell length (0.1 m).
In the laboratory experiments, the absorbances of the PAR in each particle size (1, 3, 5,and 10 µm) were determined, and the ε was calculated from the formula (4).
III. RESULTS AND DISCUSSIONS
Spatial variations of the attenuation coefficients of TSS (K TSS ) and CDOM (K CDOM ), andtheir contributions to the K d in the Yodo River line (Stn. 1-3) and the Yamato River line (Stn.
4-6) are shown in Fig. 2. The K TSS values were constantly high compared to the K CDOM , andtended to increase toward the river mouth in both river lines. The contributions of the K TSS to theK d were relatively high, and it reached 80% at Stn. 1 (mean value at all stations: 63%).
Relationship between salinity of the surface water and the attenuation coefficients (K TSS andK CDOM ) at all stations in the bay was found (Fig. 3). The K TSS dramatically increased inassociation with a decrease in salinity. Therefore, it was likely that the TSS supplied from theriver was an important factor in decreasing the transparency in the waters in the coastal area.
Distribution patterns of the particle size showed two peaks, one for the fine particles(10-30 µm) and another for the large particles (>100 µm) in Fig. 4. In any particle size, volumeconcentrations of the particles in the water tended to decrese from the river mouth to the offshore
areas (e.g. Stn. 1 to 3). In the areas near the river mouth (Stn. 1 to 2 and Stn. 4 to 5), the large
particles (decrease ratio of the volume concentrations: 55% and 81%) quickly depositedcompared to the fine particles (34 and 46%). While in the offsore areas (Stn. 2 to 3 and Stn. 5 to46), the fine particles (94% and 44%) quickly deposited compared to the large particles (32 and1.2%). The phenomen indicated that in the areas near the river mouth, the fine particles could be
difficult to deposit due to a fast current velocity (Stokes' Law). Therefore, it is assumed that thefine particles was important in controlling the light conditions in the estuary where a highcontribution of the K TSS to the K d was observed.

Fig. 2. Spatial variations of (a, c) the attenuation coefficients of TSS and CDOM, and (b,d) its contributions to the K d in the Yodo River line (Stn. 1-3) and the Yamato River line (Stn. 4-6)

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Fig. 3. Relationship between salinity of the surface water and the attenuation coefficientsat all stations (Stn. 1-6)

Fig. 4. Particle size distributions in (a) the Yodo River line and (b) the Yamato River line
Simple linear regression analysis between the amount of TSS and K TSS showed nosignificant correlation (r2 = 0.01) as indicated in Table.1. The TSS was divided further into threecomponent fractions (TSS IT , TSS OT , and TSS PHY ) since the particle size affected the lightattenuation even with the the same amount of TSS [10]. The coefficients of determination (r2) inmultiple linear regression analysis using the three component fractions relatively increased to0.50 (K TSS = 0.003TSS IT + 0.16TSS OT + 0.37TSS PHY - 0.13). This result demonstrated that thecontribution of the phytoplanktons was relatively high in the three component fractions. The r2
between the total particle volume concentrations (sum of 3.1 and 165.0 µm in particle size) andthe K TSS relatively increased (r2 = 0.23) compared to that of the amount of TSS (r2 = 0.01). Thus,
the result showed the possibility that we could estimate the accurate K TSS by accommodating theeffect of the particle size.
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Table. 1. Coefficients of determination (r2) in multiple (simple) linear regression analysis
for the K TSS at all sampling stations in the bay

Parameter
Coefficient of determination r 2TSS
0.01
Total volume concentration
0.23
TSS consisting of three component fractions
0.50
To clarify out the relationship between the particle size and the K TSS , laboratory
experiments using spherical silica particles were conducted. There were significant positive
correlations between the TSS concentrations and the turbidity as shown in Fig. 5(a). The
turbidity of the particle of 0.1 µm size fluctuated extremely at low levels even at various TSS
concentrations, and the slopes increased with increasing particle size. Positive correlations
between the TSS concentrations and the average absorbances in the PAR were found as shown inFig. 5(b), and the slopes increased with decreasing particle size. It should be noted that small
particles, round 1 µm, possessed the potential for attenuation of light in the water.

Fig. 5. Relationship between (a) the TSS concentrations and the turbidity, and between(b) the TSS concentrations and the average absorbance of the silica particles in the PAR
For evaluation of the particle size effect for the K TSS , the TSS consisting of threecomponent fractions was weighted from the results of the laboratory experiments using silica
particles. The volume concentrations in thirty-two different size classes measured by theLISST-100X were multiplied by their respective molar extinction coefficient (ε), which wasobtained from the extrapolation formula (ε = 2.47 exp(-0.15D)) between the particle size (3, 5,and 10 µm) of the silica particles and the ε due to fit of the measurement range of theLISST-100X (3.1-165 µm). The products in different size classes were summed, and the value
was multiplied by the amount of TSS consisting of three component fractions (TSS IT , TSS OT ,TSS PHY ).
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Fig. 6. Relationship between the particle size (D) of the silica particles and the molar
extinction coefficient (ε)
The results of the multiple (simple) linear regression analysis for the K TSS at all samplingstations in the bay are shown in Fig. 7. From the regression analysis, the coefficient ofdetermination (r2) between the amount of TSS consisting of three component fractions weighted
by the particle size and the measured K TSS was highest (Fig. 7(c)). In summary, not only thecomponent of the TSS (organic/inorganic tripton or phytoplanktons), but also the particle size
could be an essential factor in controlling the light attenuation in the coastal sea.

Fig. 7. Relationship between the measured K TSS and the estimated K TSS , by (a) theamount of TSS, (b) the amount of TSS consists of three component fractions (TSS IT , TSS OT ,TSS PHY ), and (c) the amount of TSS consisting of the three component fractions weighted by theparticle size.
V. CONCLUSIONSThe highest light attenuation in PAR was observed for the particle size of 1.0 μm basedfrom laboratory experiments using spherical silica particles. From field investigations in Osaka
Bay, the importance of the particle size effect in light attenuation was clarified and it wasindicated that not only the component of the TSS (organic/inorganic tripton or phytoplanktons),
but also the particle size could be an essential factor in controlling the light attenuation in the8coastal sea.
VI. ACKNOWLEDGEMENTS
The authors show appreciation to the Mr. Yasuhiro Ushihara and the members of theKobe University Research Center for Inland Seas (KURCIS) for piloting the survey ship andhelp of the sampling in the Osaka Bay. We would like to express our thanks to Dr. Masaki
Nakajima, Dr. Keigo Yamamoto, Dr. Satoshi Akiyama, and the members of Research Institute ofEnvironment, Agriculture and Fisheries, Osaka Prefecture for their help in the investigation inOsaka Bay. We acknowledge Dr. Cervinia V. Manalo (Hiroshima University) for her English
check, Mr. Hiroshi Baba who was a staff of EMS CO., LTD, and all the people who supported
us.

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