Brackish water thecamoebians as an indicator of sea-level

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Foraminiferal Laboratory, Faculty of Education and Research Center for Coastal Lagoon Environments, Shimane

Interdisciplinary Faculty of Science and Engineering, Shimane University. Nishikawatsu, Matsue, ,

Received September , . Accepted May , .

*

University. Nishikawatsu, Matsue, , Japan.

*

Japan.

* Department of Civil Engineering, Tottori University. Koyama-cho-minami, Tottori, , Japan.

* a Corresponding author : [email protected]. ac. jp

We report on , a brackish water thecamoebian, from low oligohaline LakeKoyama-ike. Its occurrence in a -cm sediment core taken from the southeast side ofthe lake enables us to reconstruct historical salinity changes. , representedby one species, consistently occurred during the th century, but the number of in-dividuals per gram sediment gradually increased. Before the s was notabundant ; it occurred prominently around the s and then increased with largefluctuation during the last years. The prominent occurrence in the s correlateswell with the decreased chlorine content, which may have been caused by the closureof a sluice gate to stop the backward flow of marine water into lake. The low numbersof in the s and in the early s were well in accordance with the in-creased chlorine content of sediment core, suggesting the presence of more saline lakewater. We confirmed that these periods of reduced thecamoebian counts coincidedwith sea-level rise in the s and early s.

Our reconstructed salinity model indicates a gradual decrease in lake salinity andan increase of toward the present, despite global sea-level rise. This can beunderstood when considering that the salinity of Lake Koyama-ike has been arti-ficially and strictly controlled in order to utilize lake water for large-scale agriculturalirrigation.

; Burbidge and Schroder Adams, ),and Scott, ; Scott and Medioli, ; Medioli

soils and tree bark (Scott ). Some forms

are also found in very low salinity brackishwater (Scott ).

Thecamoebian is a testate amoeba The thecamoebian test is made of varied ma-in the subphylum Sarcodina. Its test morphol- terials, including secreted organics, calcareousogy is similar to that of agglutinated foramini- and siliceous matter, and cemented xenogene-fera. The ecology of thecamoebians is not well ous mineral particles. The thecamoebian test isunderstood, but individuals are commonly preserved in lake sediments and has been stud-found in freshwater lakes, rivers and ponds, as ied mainly by Canadian researchers (Medioliwell as in moist environments on mosses, wet

Ritsuo Nomura* , Shigenori Kawano* and Hiroshi Yajima*

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Keywords : sea-level change, thecamoebians, radioactivity dating, th century,artificial salinity control, Lake Koyama-ike

I. Introduction

Brackish water thecamoebians as an indicator of sea-level

changes in Lake Koyama-ike, Tottori Prefecture,

southwest Japan, over the last years

The Quaternary Research p. Oct.

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� �Fig. Map of Lake Koyama-ike and core sample location

Sold circle indicating sampling location : N ; E ; m depth. Two

double circles indicating the site for monthly examination of water quality. S : Sakai

Tidal Observatory, T : Tajiri Tidal Observatory, M : Maizuru Tidal Observatory.

Nomura, R., Kawano, S. and Yajima, H.

ofTottori Prefecture has monitored the monthly

from the viewpoint of eutrophication and heavy

Asioli ( ). In Japan, however, thecameo-

chlorine contents of surface water ( m depth)

understand the impact on existing lake organ-metal pollution in the lakes (Patterson isms. Environmental changes in response to

; Patterson and Kumar, ). Thecameo- the sea-level variations during the latter halfbians in some Italian lakes were studied by th century have been reported from Lake

Nakaumi and Kumihama Bay (Nomura, ;bians have not been studied extensively, and Nomura ).thus their taxonomic and ecological informa-tion is very limited.

This study would be the first to apply the Lake Koyama-ike is located in the westernthecamoebian thanatocoenosis in a lake sedi- part of Tottori City, Tottori Prefecture, andment core, to understand recent environmental is connected to the Sea of Japan through thechanges in Japan. The water of Lake Koyama- km long River Koyama (Fig. ). The sur-ike is very low in salinity, and the salinity is face area of this lake occupies km and itsartificially controlled in order to utilize the average water depth is m with a maximumwater for irrigatation. We found a large num- depth of m near Ryugasaki in the northernber of thecamoebians in Lake Koyama-ike. Sa- part (Takayasu ). The Governmentlinity control in Lake Koyama-ike was startedwith the construction of sluice in , water characters of this lake since the earlyand in most of the th century this lake has s. According to the recent -year reportsbeen frequently influenced by backward ma- (April, March, ) from o Fuse (Fuserine water flow during the periods of high sea site), southeastern side of Lake Koyama-ike,level. Moreover, there have been several floodsand typhoons in this century. By applying the were very regularly varied, alternating be-brackish water thecamoebian as an indicator tween high salinity seasons ( Cl mg/l)of salinity changes in this lake, we are able to and low salinity seasons ( Cl mg/l) (Fig. ).

et al.

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Water character of Lake Koyama-ikeII.

Oct.306

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Fig. Variations of monthly chlorine content in Lake Koyama-ike

Chlorine contents at the central site in the s s and are unpublished data

from Tottori Prefectual Institute of Public Health and Environmental Science, and those

at Fuse site in the s are opened data from the Government of Tottori Prefecture.

No data in the intervals of and .

Table Major historical events related to Lake

Koyama-ike

Brackish water thecamoebians and salinity changes in Lake Koyama-ike

salinity seasons (April to November), and opened

studies using core sediments recognized a num-

Lake salinity has been rigidly controlled fromon for the purpose of agricultural water

use. Thus the sluice is usually closed in low

in high salinity season (December to March).

The oligohaline water of Lake Koyama-ikehas been utilized for both fisheries and agricul-ture, including irrigation water. Salinity varia-tion of lake water is critical for these indus-tries. Furthermore, this area has experienced anumber of flood events during the typhoonseason, because the lake and its surroundingarea are topographically depressed. Previous

ber of flood-related sediment layers (Takayasu; Hamada ). The major

flood events in the th century are indicatedin Table . In order to improve the drainagesystem and also to coordinate the fisheries re-source and agricultural water, a sluice was con-structed in , and later repaired inand . Since the construction of the sluice,the flow of upstream marine water into thelake became restricted. Although fishery is tural purpose. In addition, the river mouth ofcommon throughout the year, lake water is the River Koyama was largely reorganized inused only in low salinity season for agricul- ; and thus Lake Koyama-ike and the envi-

et al., et al.,

. Human activities

III. Historical background of LakeKoyama-ike

307

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Fig. Topographic maps of Lake Koyama-ike and the environs in , , and ,

published by the Geographical Survey Institute of Japan


positions are recognized in three periods : Earlyelement analysis using a CHN analyzer (Yanako

tained from three tidal observatories : Maizuru,

rons have been artificially changed, as shown In June , a -cm sediment core was ob-in Figure . tained by pushing cm diameter acrylic pipe

into the bottom sediment at m depth in theThe sea-level data used in this study were ob- southeastern part of Lake Koyama-ike (Fig. ).

We cut the core at cm intervals.Sakai and Tajiri (Fig. ). Maizuru and Sakai All samples for geochemical analysis wereobservatories have recorded the long-term sea- measured for wet weight and then dried atlevel variations since the latest s and Tajiri in hours. To determine the water con-Observatory began recording in . Annual tent, sample dry weight measurements werechanges in the sea-level recorded at these ob- then obtained. The dried block samples wereservatories show a similar pattern from the ground into powder and mixed well for organic

s to the present. Relatively high sea-levelMT ). Approximately g of powdered sample

s, early s and the late s to the was pressed to make a tablet for X ray analy-present. Such annual changes of the sea-level sis (Horiba XGT ). One square centimeterare well in accordance with the general varia- of the flat tablet was scanned by an X ray oftion pattern recognized on the southwestern m diameter with an acceleration of kV.side of Japan (Sakurai and Konishi, ). The chlorine X ray intensities (cpm/mA) were

expressed as the ratio of every downcore sam-ple to the intensity of the topmost core. The

. Sea-level variation in

IV. Methods

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: The depth dis-

EGPC P ; FWHM resolution keV at

a mixture of the supported and excess Pb. Anexcess Pb, which is produced by Rn through

top cm of the core (Fig. a and Table ). Low

intensities, water content and organic elements.

opening size) and then stained with rose-bengal

radioactivity analyses for Pb and Cs werecarried out using a well-type system of ultra- In the present study, we will apply the con-low background ray spectrometry (Canberra stant rate of Pb (excess) supply model (CRS

model) for the estimation of the sedimentationkeV). A styrene tube containing g well-mixed rate (Appleby and Oldfield, ), instead of thesediment powder was used for the radioactiv- commonly used constant model for both fluxity measurements. They were in radioactive and sedimentation rate ( Oldfield and Ap-secular equilibrium for Rn and its daughters pleby, ). Usefulness of the CRS model was

Pb and Bi with Ra, after being sealed in examined by Appleby ( ) and Applebyan airtight tube for more than three weeks. and Oldfield ( , ), and has been com-The ray counting time was seconds. monly used for various lakes. The concept of

Sediment samples for thecamoebian analy- this model is :ses were washed on a -mesh screen ( m

and ,

solution down to cm depth to distinguish where ( ) is the total residual Pb (excess) inlive forms from dead forms. We counted the the whole core sediments and is the cumula-thecamoebians with m test size in the tive residual Pb (excess) beneath sediments ofwashed and dried residues. depth ( ) is the Pb (excess) concentra-

tion in each sediment layer. The age of sedi-ments of depth satisfies the following equa-tion :

: Measured Pb concentration is,

where is the decay constant. The dry-massdry and/or wet deposition into the water and sedimentation rate at time can be calculatedsubsequent sedimentary processes in the water using the equation :column, is used to obtain the sedimentation

.rate. The excess Pb is obtained from themeasured concentration of Pb by subtracting The sedimentation rate in dry mass of thisthat of Ra. The Ra concentration is esti- core resulted in g/cm /year for themated from the measurements of Pb concen-tration at keV (Table ). sedimentation rates occurred in the top cm

Figure a shows a discontinuous distribu- and at cm. On the contrary, much in-tion of Pb (excess) with core depth. We recog- creased sedimentation rate occurred at cm,nized abnormally low concentrations at sev- cm and cm. Figure b shows ageeral sediment intervals, such as cm, cm, (years before sampling date) versus core depth,cm, cm, cm, cm, cm and cm, based on the sedimentation rate in each sedi-that may have resulted from the dilution of ment layer. In general, the errors in the CRSconstantly formed Pb (excess) by increased model increase with core depth (Appleby, ).sedimentation rate. It is possible that the ap-parent low concentrations are due to post- tribution of Cs concentration showed excep-depositional redistribution of the sediment by tionally high values in top cm interval (Fig.physical mixing or bioturbation (Robbins and b). This distribution pattern is unusual fromEdgington, ). However, we are aware that the viewpoint of declined total Cs depositionthese low concentration intervals are stra- in recent years in Japan (Igarashi ).tigraphically well correlated with those of the However, observed variability is common duedecreased chlorine ratios and low water con- to spatial variations in Cs inventories (Owenstents, which suggest the original sedimentary and Walling, ). Taking account of the ob-processes related to flooding events, as will be served high concentration for the core top, wementioned in the sections of chlorine X ray need further information on local inventory of

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. Radioactivity of Pb and Cs

V. Results

309

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Table Results of radionuclide measurements

Nomura, R., Kawano, S. and Yajima, H. Oct.310

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Fig. Distribution of Pb (Bq/g) and Cs (Bq/g) in the sediment core

Error bars are calculated based on two sigma. The arrow in the distribution of Cs marks early s.

Fig. Sedimentation rate and accumulated ages to core depth

(a) changes in sedimentation rate (g/cm /year) vs. core depth. (b) age-depth relationship.

Reference date is June .


Cs in the catchment area of Lake Koyama- (Krishnaswamy ). The most promi-ike. This is beyond the scope of this paper. nent deposition of Cs occurred in (Wall-

A basic premise in the use of Cs in radioac- ing ), which is a good age marker intivity dating is that Cs fallout can be treated Japan (Kanai and Ikehara, ; Kanaias spatially homogeneous. The detectable con- ). A clear shift of the concentration wascentration of this radionuclide has been rec- not found in this core, but a slight shift mayognized in the sediment from onwards have occurred at cm from the top of sedi-with significant release of Cs in ment core (Fig. b). If we assign this core

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Table Calculated age and sedimentation rate

based on CRS model of radioactivity


low, suggesting unusual sedimentological events

show nearly equal sedimentation rates, and thus

bution pattern with that of per sediment weight

around except for the topmost cm depths,

Cs in each sample shows small di erentials.Thus the Pb dating by means of the CRSmodel guarantees the analyses of sedimentaryevents with known historical data. The cmlong core covers over years of history. Tak-ing higher dating errors with increasing coredepths, however, we prefer the top cm of thecore that would be reliable for this age model.

Thecamoebians occurring in this lake arecomposed of sp. (Fig. ). This speciesconsisting of a single test with fine-grainedquartz particles and the test is generally globu-lar, but rarely elongate. A small rounded open-ing rimmed with granular cemented materialis observed. The features of this species is ap-parently similar to those of foraminiferal gen-era Carpenter andSchulze, but the latter foraminirferal taxa havebeen reported from the ocean (Loeblich andTappan, ) and never known from oligoha-line brackish lake.

This species appeared throughout the coresediments, suggesting core sediments weremainly deposited in non-marine or very low-salinity brackish environments. In general, theindividual numbers per gram sediment gradu-ally decreased with core depth, but there areprominent occurrences at the core depthscm ( individuals per gram) (Fig. a). Theindividual occurrences based on the sedimen-tation rates (Fig. b) showed a similar distri-

(Fig. a). Stained forms did not occur in thecore sediment.

The chlorine ratio to the core-top value indi-cates a general increase of chlorine contentwith increasing core depth, but there are flu-ctuations in some depth intervals (Fig. a).The water content of the core sediments is

depth to the early s when many nucleartests were carried out, then it is well in accor- but that in several intervals such as cm,dance with the age calculated by Pb (excess), cm, cm and cm is relativelywhere cm in depth indicates about yearsbefore the present, June (Fig. b). (Fig. a). The stratigraphic distribution of

The results of both Pb and Cs dating these fluctuated chlorine ratios and water con-tent is in accordance with each other, particu-

the calculated age di erence between Pb and larly for the depths below cm. Figure b

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Saccammina Psammosphaera

. Thecamoebian and its occurrence

. Chlorine X ray intensity and the watercontent in the sediment core

Oct.312

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c is a close-up of b, and c is a close-up of b.

Fig. Distribution of in core sediment

(a) individual number per one gram of dried sediments. A smoothed thick line is

applied a geometric fitting to the individual occurrence. (b) the number is given

by the sedimentation rate ( ) and error bars indicate the numbers using the

standard errors in


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Fig. Distribution of chlorine and water content in the sediment core

(a) distribution of chlorine and water content. (b) ratio of the chlorine ratios to

water content. The ratios are standardized to the top of core.


The distribution of total organic carbon (TOC)

contain terrestrial organisms and the sediments

sedimentary condition was unstable, and might

shows the relationship between chlorine ratios intervals of cm and cm. On theand water content (Cl/WC) with core depth, contrary, the TOC/TN ratios change fromindicating the variable nature, particularly in to , with higher values of at cmthe top cm depths. Several positive shifts of and lower values of at cm. Higherthe Cl/WC in the depth intervals of cm plants in terrestrial and nearshore areas haveand cm are clearly caused by the high high TOC/TN ratios, while both phyto- andchlorine ratios, setting aside the questions zoo-planktonic organisms have low TOC/TNwhether they are original chlorine or not. A ratios ( Sampei and Matsumoto, ). Thehigh peak of the Cl/WC around cm depth is sediments at core depth of cm mainlydue to lowered water contents.

in the top cm are mainly composed of plank-tonic-derived organic matter.

and total nitrogen (TN) shows wide variation Both TOC and TN in the depth intervals ofthroughout the core, but it is less varied in the cm, cm and cm suggest thattop cm of the core (Fig. ). TOC varies be-tween and except for some abnormal be related to flooding events caused by ty-values. TN varies around . in the top cm phoons in the s , s and s (Ta-and in the interval of cm, but two higher ble ). These depth intervals are not alwaysvalues ( in average) appear at the depth characterized by a high sedimentation rate ;

e.g.,

. TOC and TN content

Oct.314

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Fig. Relative abundance of total organic carbon (TOC) and total nitrogen (TN),

and their ratios (TOC/TN) in the sediment core


ported. Moreover, planktonic species

but they indicate a remarkable variation of in the middle s. The Bureau of Tottorihigh to low sedimentation rates (Fig. a). Construction ( ) explained the social condi-

tion with regard to the sluice constructed inthe s and improved it in in thefollowing ways :

Today, the environment of Lake Koyama-ikeis very artificial. The water of this lake is uti-lized for the purpose of both inland fisheriesand agricultural irrigation, and so the salinityis artificially controlled. Salinity control of thelake water started in with the construc-tion of a sluice preventing the backward flowof marine water into the lake (Table ). Actu-ally, a sluice regulated the flow of water be-tween coast and the lake (Fig. ). Although thesluice was built in , it was not functionalsoon after. Hada ( ) reported the surfacewater salinity of Lake Koyama-ike was

when he examined the foraminifera inOctober, . At that time foraminiferal spe-cies are characterized by brackish species suchas Heron Allen and Ear-land, (Brady) and varieties of

(Brady), and some ma-rine taxa such as quinqueloculinid, triloculinid,bolivinid, buliminid and elphidid were also re-

was found in the lake. His report This document indicates that Lake Koyama-shows the evidence of upstream marine water ike experienced a high salinity period in sev-

Globigerina

“The sluice of m height and mwidth, to protect the inflow of marine water,was made of wood, and thus it changed todeteriorate and meanwhile it did not workwell to control the flooded water and up-stream marine water. In and the neigh-boring ages, marine water easily flowed up-stream through the sluice into the lake, andthus agricultural products was damaged byseawater for several times. A school of ma-rine fish was observed at the time of high tidein these years. In , such a salt damagebecame more prominent at the time of theIsewan Typhoon attacked. Furthermore inthe s, it became more activated to pro-duce agricultural products in the sand dune

Miliammina obliqua area near the lake, which needed the lakeMiliammina fusca water as a source water of irrigation. In

Proteonina di ugiformis order to resolve the problems mainly camefrom salt damage, a new system of sluice wasconstructed. (a short summary of Japanesedocuments, p. ).”

bulloides

. Salinity changes in

VI. Discussion

315

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Fig. Decadal correlation among Cl/WC (a), number of individual (b),

and sea-level variation (c)

The lightly hatched area indicates prominent occurrence corresponding to low

sea-level and low salinity of reconstructed lake water. Dark hatching indicates decreased

occurrences of corresponding to high sea-level and high salinity of reconstructed

lake water. Sea-level data are from the Japanese Meterological Agency, and include revised

local reference data. Tajiri Tidal Observatory (TO) is located at N E, Sakai TO

is at N E, and Maizuru TO is at N E.


correlated with the high sea-level period (Fig. ).

increased temporarily around (Fig. ). The

eral times. It is noted that the early s was s was substantially caused by climaticallyinduced sea-level rise. The high Cl/WC around

The chlorine ratios relative to the water con- the s correlates with the early s sea-tent (Cl/WC) was higher before the late s, level rise.but it decreased during the s and then The River Koyama was originally a tribu-

tary of the River Sendai ( in Fig. ). Becausetime series variation of the Cl/WC was well in large-scale flood damages occurred in this riveraccordance with that of sea-level variations at system, the Bureau of Tottori Construction im-Maizuru and Sakai Tidal Observatories. Back- proved the river mouth for both the Riverward flow of upstream marine water would be Koyama and the River Sendai in . Theeasily accelerated when high tide condition river mouth of the River Koyama was dividedwas formed. We believe the salt damage in the from the main body of the River Sendai and it

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Oct.316

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simultaneously characterized by rather decreased

prefers Cyanophyta and diatoms as food sources

individuals per gram sediment (Fig. b). Thisand attacked the sluice from this time on (Yomi-

material in the sediment and Canadian

In early s before the above-mentioned im-

creased occurrence of in Lake Koyama-

Brackish water assemblage occurredthecameobians in Lake Koyama-ike have larger

became closer to the Sea of Japan through the major meiobenthos of this lake. Our ageTottori Port (Fig. ). As a consequence of the model suggests the marked occurrence ofimprovement of the river mouth area, back- was found around , showing overward flow of marine water commonly occurred

period is correlated with decreased Cl/WC, in-uri Newspaper, January, ). According to dicating the lower salinity of lake water (Fig.the unpublished data from the governmental a). As mentioned earlier, a sluice was con-laboratory of Tottori Prefecture, salinity of sur- structed and improved as measures for theface water over , Cl mg/l was appeared in backward flow of marine water. This promi-

(Fig. ). However, the improved sluice nent occurrence of provides strongrigidly prevents the easy flow of upstream ma- evidence for artificially induced declines inrine water from that time on, and thus the lake salinity. Otherwise, the productivity ofsalinity of lake water has been kept on in lower is controlled by the type of organiclevel, despite the continued increase of sea-level rise in recent years.

(Burbidge and Schroder Adams, ). Thusprovement, the surface water of Lake Koyama- both of salinity and food sources are significantike fluctuated around the level of Cl mg/l. in the ecological preference of the thecamoe-Chlorine content in the lake water was arti- bian species. The slightly lower TOC/TN ra-ficially controlled at that time, but salinity con- tios in the early s, as shown in Figure ,trol, as that the sluice is opened in winter and is suggest that bottom sediment was rich inclosed in agricultural seasons, was not so rigid, plankton-derived organic materials. The in-compared with that from the late s on (Fig.

). Figures and a show a very changeable, ike would be intimately related to the supply ofbut a well-regulated nature for salinity from these organic materials. This period was alsothe late s on ; indicating certain evidencefor artificial control in the sediment core. The lake salinity (Fig. ). In the early s, theca-sediment samples down to cm depth ( . moebian numbers abruptly declined, whichin year) involve about -year time-interval re- may have been caused by sea-level rise, despitespectively (Table ). Thus the seasonal salinity that there were lowered TOC/TN ratios in thischanges of the lake water were not able to period (Fig. ). The thecamoebian occurrencereconstruct, because of the resolution limits of during the s to s clearly reflects a highour dating model over -year ( year/ sensitivity to salinity variations. Again, theca-cm interval for the top cm core depths). moebian numbers increased in the s, ac-However, a -cm sediment interval was suf- cording to the decrease of salinity resultedficient to find the artificial control of the lake from artificial control of the sluice gate.salinity. Ecological information of this species has not

Based on these salinity changes, it is clear been obtained, but thecamoebian taxa showthat permanent opening of the sluice gate a very varied ecological distribution rangingwould change the water quality to more saline from brackish to freshwater (Scott ).water with active inflow of costal water, and Species belonging to the genus arethus would result in a two-layered water struc- usually reported from freshwater lakes andture (Yajima and Hayakawa, ). We believe brackish species of thecamoebian are com-that change in the lake environment would be monly associated with foraminifera (Scottmore severe because the present river condi- ). Japanese has been also re-tion easily leads sea-water to Lake Koyama-ike. ported from freshwater areas ( Lake Biwa-

ko : Tsugeki ; Ichinose )and their test size and texture are small andfine. Compared with these reported taxa, the

consistently in the th century and represents

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e.g.,et al., et al.,Thecamoebian as an indicator of salinity

variation.

317

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Appleby, P.G. ( ) Chronostratigraphic techniquesin recent sediments. Last, W.M. and Smol, J.P. (eds.)Tracking environmental change using lake sedi-ments : Basin analysis, coring and chronologicaltechniques : , Kluwer Academic Publishers.

Appleby, P.G. and Oldfield, F. ( ) The calculationof lead- dates assuming a constant rate of sup-ply of unsupported Pb to the sediment. Catena, ,

.Appleby, P.G. and Oldfield, F. ( ) The assessment

of Pb data from sites with varying sedimentaccumulation rates. Hydrobiologia, , .

Appleby, P.G. and Oldfield, F. ( ) Application ofPb to sedimentation studies. Ivanovich, M. and

Harmon, R.S. (eds.) Uranium-series disequilibrium :Applications to earth, marine and environmentalsciences : , Oxford University Press.

Appleby, P.G., Olderfield, F., Thompson, R., Huttunen,P. and Tolonen, K. ( ) Pb dating of annuallylaminated lake sediments from Finland. Nature,

, .Asioli, A., Medioli, F.S. and Patterson, R.T. ( ) The-

camoebian as a tool for reconstruction of paleoen-vironments in some Italian lakes in the foothills ofthe southern Alps (Orta, Varese and Candia). Jour-nal of Foraminiferal Research, , .

Burbidge, S.M. and Schroder Adams, C. J. ( ) The-camoebians in Lake Winnipeg : A tool for Holo-cene paleolimnology. Journal of Paleolimnology,

, .Bureau of Tottori Construction ( ) Salt damage


s and s. Misses M. Himei and M. Tanabe

Grant in Aid for Scientific Research (A) (Ritsuo

posed solely of This form consistently

chlorine content, suggesting that the sluice gate

changed salinity that was controlled artificially

and thicker tests with larger cemented mineral . Our reconstructed salinity model indi-grains. Although no comparable species has cates a gradual decrease in lake salinity and abeen reported elsewhere in Japan, we are cer- consequent increase in counts. Thistain that this thecameobian is useful for recon- salinity model is obviously against the globalstructing the salinity variations in oligohaline sea-level rise. However, the salinity decrease isto freshwater lakes. due to strict control of marine water inflow to

Lake Koyama-ike.

Brackish water thecamoebians from low oli-Tottori Prefectural Government kindly sup-gohaline Lake Koyama-ike, Tottori Prefecture,

plied the recent water quality data of Lakesouthwest Japan, were studied and their occur-Koyama-ike. Dr. Y. Miyamoto, Tottori Prefec-rence in the sediment core make it possible totual Institute of Public Health and Environ-reconstruct the salinity changes of this lake.mental Science, presented salinity data of theThe following is the results :

. The studied sediment core was cmhelped our field and laboratory works. Thankslong, but it covered over years. We appliedare extended to Dr. H. Takata, Shimane Univ.,a CRS model for the Pb radioactivity dating.for his constructive comments and to the edi-The sedimentation rate varies betweentor, Dr. S. Sato of Tohoku Univ., for improvingg/cm /year down to cm core depths.the manuscript. This work was supported byThe sedimentation rates in flood event are gen-

erally high, but flooding is not always fullyNomura, no. ).correlated with high sedimentation rate.

. The thecamoebian assemblage was com-

occurred during the th century, but the indi-vidual number per gram sediment distinctlyincreased in the s and commonly occurredin the s early s, excepting the mostrecent years.

. The prominent occurrence during thes was well correlated with the decreased

was closed to stop the backward flow of marinewater into the lake. Simultaneously, sea-levelpositions during the s were lowered in gen-eral. Both factors might have influenced theincreased occurrence of Althoughthe water salinity decreased much in recentyears, did not increase in response tothe decreased salinity. We guess seasonally

is a major cause for this unconformable occur-rence.

. Low numbers of in the s andthe early s were well in accordance withhigh chlorine ratios relative to water content,suggesting the presence of more saline lakewater. We confirmed that the decreased occur-rence of coincided with periods ofhigh sea level in the s and early s.

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VII. ConclusionsAcknowledgements

References

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Igarashi, Y., Aoyama, M., Hirose, K., Miyao, T., Nemoto,

sion : Decadal monitoring time series of the anthro-

and thecameobian indicators. p, Cambridge Uni-Loeblich, A.R., Jr. and Tappan, H. ( ) Foraminiferal

Medioli, F.S. and Scott, D.B. ( ) Holocene arcellacea

stratigraphic implications. Journal of Paleontology,

Foraminiferal evidence from Lake Nakaumi, south-

and the prevention of backward flow of marine Oldfield, F. and Appleby, P.G. ( ) Empirical testingwater. Bureau of Tottori Construction (ed.) The his- of Pb dating models. Haworth, E.Y. and Lund,tory of the River Sendai : , Chugoku Benefit J.G. (eds.) Lake sediments and environmental his-Association of Construction. (J) tory : , Leicester University Press.

Hada, Y. ( ) Studies on the foraminifera of brack- Owens, P.N. and Walling, D.E. ( ) Spatial variabil-ish waters : II Hachirou-gata, III Koyama-ike. Zoo- ity of Caesium- inventories at reference sites :logical Magazine, , . (J E) An example from two contrasting sites in England

Hamada, T., Sakuno, Y., Takayasu, K., Seto, K., Akagi, and Zimbabwe. Applied Radiation and Isotopes, ,S., Nishida, R., Tanaka, Z., Murata, Y., Asamura, H. .and Fukuta, K. ( ) Diatom remains and its verti- Patterson, R.T. and Kumar, A. ( ) Use of Arcel-cal change of the core samples from Lake Koyama- lacea (thecamoebians) to gage levels of contamina-ike, Tottori Pref., southwest Japan. Laguna, , . tion and remediation in industrially polluted lakes.(J E) Martin, R.E. (ed.) Environmental micropaleontol-

Ichinose, S., Morita, T., Wakabayashi, T., Kusuoka, Y. ogy : , Kluwar Academic Press.and Nishino, M. ( ) Disappearance and its past Patterson, R.T., Barker, T. and Burbridge, S.M. ( )distribution of the endemic protista Arcellaceans (thecamoebians) as proxies of arsenicKawamura, in Lake Biwa, central Japan. Re- and mercury contamination in Northeastern On-port of the Shiga Prefectural Institute of Public tario lakes. Journal of Foraminiferal Research, ,Health and Environmental Science, , . (J) .

Robbins, J.A. and Edgington, D.N. ( ) Determina-K., Tomita, M. and Fujikawa, T. ( ) Resuspen- tion of recent sedimentation rates in Lake Michi-

gan using Pb- and Cs- . Geochimica et Cosmo-pogenic radioactivity deposition in Japan. Journal chimica Acta, , .of Radiation Research, , . Sakurai, K. and Konishi, T. ( ) Feature of long-

Kanai, Y. and Ikehara, K. ( ) Sedimentation rates term sea-level variation around Japan. Sokkou-of o shore deposits in the Sea of Japan o Niigata jihou, Special Volume, , S S . (J)Prefecture, using Pb- and Cs- radioactivity Sampei, Y. and Matsumoto, E. ( ) C/N ratios in ameasurements. Bulletin of the Geological Survey sediment core from Nakaumi Lagoon, southwestof Japan, , . (J E) Japan : Usefulness as an organic source indicator.

Kanai, Y., Inouchi, Y., Katayama, H. and Saito, Y. Geochemical Journal, , .( ) Estimation of sedimentation rate at the Lake Scott, D.B. and Medioli, F.S. ( ) AgglutinatedSuwa in Nagano Prefecture determined by Pb- rhizopods in Lake Erie : Modern distribution andand Cs- radioactivities. Bulletin of the Geologi-cal Survey of Japan, , . (J E) , .

Krishnaswamy, S., Lal, D., Martin, J. M. and Meybeck, Scott, D.B., Medioli, F.S. and Schafer, C.T. ( ) Moni-M. ( ) Geochronology of lake sediments. Earth toring in coastal environments using foraminiferaand Planetary Science Letters, , .

versity Press.genera and their classification. p and plates, Takayasu, K., Seto, K., Akagi, S., Nishida, R., Tanaka,Van Nostrand Reinhold Company. Z., Murata, Y. and Sakuno, Y. ( ) Bottom topog-

raphy and sediments of Lake Koyama-ike, Tottori(thecamoebians) from Eastern Canada. p, Cush- Prefecture, southwest Japan (general report). La-man Foundation for Foraminiferal Research, Spe- guna, ,cial Publication, no. , Cushman Foundation forForaminiferal Research.

Medioli, F.S., Scott, D.B. and Abbott, B.H. ( ) A casestudy of protozoan intraclonal variability : Taxo-nomic implications. Journal of Foraminiferal Re-search, , .

Nomura, R. ( ) Assessing the role of artifical vs.natural impacts on brackish lake environments :

west Japan. Journal of Geological Society of Japan,, .

Nomura, R., Ninagawa, K. and Nishido, H. ( )Significance of sea-level variations over the last

years, indicated by foraminiferal assemblagechanges in Kumihama Bay, Kyoto Prefecture,southwest Japan. Journal of Geography, ,

. (J E)

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. (J E)Tsugeki, N., Oda, H. and Urabe, J. ( ) Fluctuation

of the zooplankton community in Lake Biwa dur-ing the th century : A paleolimnological analy-sis. Limnology, , .

Walling, D.E., He, Q. and Appleby, P.G. ( ) Conver-sion models for use in soil-erosion, soil-redistribu-tion and sedimentation investigations. Zapata, F.(ed.) Handbook for the assessment of soil erosionand sedimentation using environmental radionu-clides : , Kluwer Academic Publishers.

Yajima, H. and Hayakawa, K. ( ) Water qualityvariation followed by seawater inflow in LakeKoyama during the winter season. Proceedings ofCivil Engineering in the Ocean, , . (J)

(J) in Japanese, (J E) in Japanese with English ab-stract.

Brackish water thecamoebians and salinity changes in Lake Koyama-ike 319

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Brackish water thecamoebians as an indicator of sea-level