THE SULPHUR CYCLE IN LAKES AND CONTINENTAL RESERVOIRS 5.1 I:"TRQDUCTIQS

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The GloboJ Biopocl emril Sl
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The GloboJ Biopocl emril Sl lphor Cyt!. Edit d Or M V. I.. and J. R. F..j' ~ 1981 Sci otili ConImn _' of til EoYironm . (SCOPE) CHAPTER 5 The Sulphur Cycle In Continental Reservoirs Part I THE SULPHUR CYCLE IN LAKES AND CONTINENTAL RESERVOIRS M. v IVA~OV 5.1 I:TRQDUCTIQS A quanti!ativ( a!i6(:m:m(n\ of biog :odv:mical pro«~s in conti..nlal ~S(:rvoif'S is ~quir :d 1 mood tll( global sulphur qcl(. Ibl: m'll important link in tl\( sulphur eye i' the discharg( of sulphur compounds in rirs-tl\( main transport m(chanism suppl)'ing the world o«ans 'ilh sulphur from the ro.. of suba(rial W(athenng of oonti..n!s. As sulphur;' transport(d 1 the ocean (Fig. 5.1) ils oonntration in riv(r.wat(f's is raid by dry and..t atmospheric deposition. and Jov..red by biological and g(ochemical proce!i6(:s in wat(r and bottom S(dinlS of ri...is fr :shwat(r lak(s. and wa\(:r res(r voir.; Considnabl( amounls of sulphur an: also lost to tl\( soil and uppnmost horizons of the lithospher(. and are remod from tlv: global C)'de in inland dr1lina~ basins (Fig. 5.1). This schematic picture: of he Iransport of sulphur oompounds by nalural pros is complicat(d by anthropo& nic influences. It is into conti..ntal wa\(:r bodies and primaril)' into the rir sysl(ms lhal the bulk of induslrial and urban S(:wage wain. with ils variety ofsulphur compounds. is dischargc:d. Drainag( Wat(r.; of agricultural gicns also supply sulphur compounds from f(rtiliuis. peisist(nl peslicides. and liock..sidtj(s to river s)'sl(ms (Fig.5.1). In Chapter 4 il was sho'n Ihaltl\( bulk of gaseous sulphur r(sulting from m(tal (lting. combustion of fossil ftj(ls and other processes inid in lhe (conomic ulilizalion of sulphur entaining mi..rals. is r :lum(d 1 the oonli..nts by atmospheric deposilion processes. Quit( of!(n the dissold oxides of sulphur in a!mosplv:ric pr(cipitalion acidify w(akl)' buff(red soils and freshwat(r laku. n.~ Glob41 Bii:JgwcMmicul Sldphur Cyck SOURCE NATURALIANTHROPOGEN C SUlPHUR AOD'TION FRO' LITHOSPHERE AND PEOOSPHERE SuIO' o~ I« I~ 1 .. Ell'''''' r 1 oo~... ~. SUL...UR ADOIT'ON FRO' AT'OSPHERE sea ~ SULPHUR REIoIOVAL FRO' R'VERS O.~o'.... I... ~... ~ ito...'onl' So 4' Fi!_ S I OCEAN Sulphu. ytk ~... in continen.1...u.-bodie. However rhe amhropogenic influence On he.ulphut cyde in re~ryoirs i no' confined solely to rhe ia... in.ulphu. co~mraionand acidity of the...tet. In some ea... ttt combined pollulion of wae. 'i'h sulphu. compounds. otganic matter. ni'rogen and phophoru lead. to the activation of a rlu! Sulphur Cyd~ in O ~liM~Itl! R~suvCJirs '''' chain of biological processes including tik:!'tima') prodoction of organic maner oxyg.n oonsump1ion during IK: oxidation of e~ce organic mau. and prodoction of hydrogen sulphide by putfaetion and suiphat. red oct ion In tho majority of publishod 'je.. on the pt xesses of the global sulphur cycle in wator...serv irs. emphasis is placed on que'tion-tik: assessm.nt of ulphur flu. in river diseharge to tl\ ocean Ho'eOf me anempts ha'.. al boen made to divide river ulphat. lnt nalural and anthropogenic phases and to assess tl\ m separately (Berner 1911; Friend 1913; Granal ~r of. 1976) Among tho anthropogonir: processes most.tt.ntion has been paid to the aci .!iflc.tion of I'h...t.r I.k.s by acid rain Our 'ork att.mpts to..iimat. Guantitativdy.lIlh. basic proces 1 tho sulphur..-ycl. in continent.1...ser\'oirs.nd.s..s tl\ nalur.l and.nthropog.nic contribulion to tbe sulphur cyde 5.2 THE SULPHUR CYCLE l'l LAKES Th. mosl widely occurring fenn of sulphur in both f...shwater and brackish lake''''aters and muds is sulphate. rkpending on tho origin of 11K: lake. the fonn of lhe lake oosin. climatic condlllons composllion of II\ surrouriding roch.nd hydrologic.1 r.gime. II\ sulphate cont.nt ollake-'''er may '.ry from se\'eral milligr.ms per lit' in lake' of glacial origin in tho humid zone. 1. completely 'turated lution ('ilh respect to odium sulphale) in the It l.k.. of the arid zone (HutChmn 1957; Kuzoet'. 197; Posokhov. 1912; Nriagu and Hem 1918). Fresh'aler lakes.re of primary import.nce for tbe ec momic activilies of man and. naturally =;.. special emphasis ;n lhe 'orks of limnologists The Sulphur Cy'de in Fresh'r Monnmi tlr L.ak.. Some infonnalion on the sulphur composition of fresh..'ater monomictic lake.te.. and muds is gi... n in Table 5.1. Act;'.. circulation of 'ater mas is observed in all such lak.s in spring and autumn and thus the water i. 'turaled..';th orygen twice y'ead)'. T'he...for. ana.robic coridilions de'-iop. if at all only ;n Ihe bypolimnion of deep-waler merrophic and eulrophic lah. at II\ end of the summer stagnation period (Fig. 5.1) or. in some ca.e. during wlnter 'hen tho lakes are oo...'d 'ith ice DeSPIte lbe low sulphate oontent of fshwaler lake.nd rho inlqnl short periodsof an...robiesis. lhe hy-drogen sulphide concenrration may reach 's much as mg litre- 1 (Table 5.1) during rhe summer and winter stag nation period. Large amounts 1 redoced sulphur a fourid in the bollom sedim.nts of freshwater lakes and rhe numbe.. of sulphal....ducing bacteria in the muds of such lakes may'...ach hund'ds of thousands of cells per gram of wet silt (Table 5.1) ~' d IQ...rd nt ' body 1Urn ...' r Table SI SUlphate hyd'ogen.ulphide.oj ulphlltc-rwucog I'c' in he wale. and...jme' 1 f'cwa' lale. Watc ~. Sui1'1 ulphu HydroSC n Lake Type of I.ke m (ms litre ').ulphidc H.il.1 Olit'phic BOO. Sclsahl... Mehic.- 'kolsh p Kich Mesotrophic 14.S 1l.2 lleloyc Mlmphic f lI.l Yugocm Me' If ph' '1'. EUlrophic U U IlOI: En'roph' ' ~ bhes_ ulmph g.f gli Ki. U Eut'phlC Sow Enlrophic TkI oI I)n En'mphi llydrosen.ulph~1c (~g S I wel'il') u S_3 '2()-4 B' fi. (1' rell. S 'wet ;It) ' Ref.reflre. Gom. (1~75) Oh1e (1954) Cl ::oot.''''v ( 1974n) SokIu. 'kl S'uk i (1957) KUlnet (1'152) Ohk: (I'1S4) lihm;roo (1931) Ollie (193~) 'Ibuk. (1979) Tezuka (1979) Te uk. (1919) 1'M SuJphUT Cycle ill COt1lWmal R~~rvoin T.p.otu (Cl ~ SO'- os.- I -. I ::.._._._._'---'...- /...S... I I. I o ~ 2 'Y9.n ond hyd9u ulphd. cont.not;on. ('9 hlre.) '' 2 3 Sulphot.oon tnuOHon ('9 SO:-Iltrt-'l FIC. S.2 V.nic.1...lificOlion of o.ygtn. tempt' tuft hy lr p:n.u1i hidt. ond ll!phatt in.t... of tho.utroplli : lab PIOn.. J() Stpttmhor 1952 (Ohi 1954) Depitt 'uch dala a vit'poinl prevail. in limoological lileralu thllt sulphale-ducing bacleria in f1te bottom ~dimenls of freshwaler lak.s play an insigniflcanl rol. in the sulphur cycl. bttauso llteir aclivily is limilt l 1»' a low conlenl of dissolvt l sulphate (KIl1Jt.uov ). In cent ynrs halo-ever. data ha accumulaled which cause us 1 revise this cot c :pt. For exampl. it 11M liten.hown by lite u~ of s1- IhIlt sulphale-ducing bacl.ria a live in the ~dimenls of freshwaler lak.s.nd in SOme cases. lhey produce significant amounls of h~'drogtn.ulphidt (Tabk 5.2). 1lH: ubscqutnl fale of Ihis hydrogen sulphide m.y vary; pan of il may be oxid~d al the sedimemj-.ter interface and pan may diffuse up the w.ter column. In lite lauer case oxidalion of hydrogen sulphide occun.1 lite inlerface belween the aerobic and.n.erobic zones of lite 'aler column and may resull in local in~reases in sulphate conc ::nlr/ltion in lite lake_waler (see Fig 5.2) Hower il.ppears from lhe few geochemical analyses available lhat a considerable part of lhe hydrogen sulph~ fonned in lhe bollom sedimenls undergoe diagene.is 'ilh lhe prodoclion of Pl'rile and organically bound sulphur and Ihus is 'ilhdrawn from lhe y le. Recenl in'''stigalions on Lake Baikal have clarified SOme aspt 'is of wlphur eycling in freshwaler rese-oirs. Typical concrelions of pyrile and pyr_ rhalile have liten found in lhe bonom sediment' of lhis oligoirophic freshwater lake (Bondarev. 1974). In addilion qullnlilies of reduced sulphur. bolh 32 in the form of3 :1d ;olul: l. wlpllidei and fl}'rire-sulphur...ere present in nearly all of tile sediments sludied in Ihis lake (Laro ) (.'en in deep water-regions don ( m; Table 5.3) These geochemical findingi. IOgethe wilh microbiological data and the results of labelling techniques (Table S.2). clearl) poim to the prc ;ellce of acti~ sulphate I't'duc\ion in 1M sedimems of Lake Baikal. where the sulphat. oonntration in water is known to be greater than 2 mgs Iitre-' s.eraj ~udies have en made on the oontent! of the various forms of reduced sulphur in bonom 'diments of freshwater lakes in Japan and Nonh America. Wht n oomparing these data ;1 liould be borne in mind thai 11.. 'arious authors used diffurm method' of analysis and oomputations for particular form' of reduced sulphur. and only the content of add soluble sui phide seemed 1 be estimated by similar metho' Sediment~of f~hwater lake~ wnh differem deg~esof contamination oonlain appredable amounts of acid-soluble ~ulphur (Table 5.4): lolal reduced sulphur in the muds of the mesotl'qphic Lake Nakatosuno-Ko amounts to.11~ in term of d...' silt (Sugaara e al. 1953). The bottom sediment~of brackih lakes in Japan a~ characterized by a markedly higher content of both acid-soluble and pyrite.ulphur. The ~duced sulphur content of the sediments of continental brackish lakes in Japan i. 2~. and i. comparable 'ith the amount found in the sediments 1 the highly productive ~gion. of sea. and ocuns (See Chapler 6). Nriagu (l ) Nriagu and Coke' (1976). Nriagu and Harvey (I978) and Nriagu and Hem (1918) determined the different form~ of reduced sul phur in lake sedimems of Nonh America. In the upper I orizons of silt deposits of the eutrophic Lake Mendota (Table 5.5) acid-soluble sulphide i~ the main form of sulphur. TIle con~ntrations of acid-soiuble sulphide and total reduced ~ulphur are much higher in the sediments of the deep part of the lake than in lhe ohallow-water muds. This is explained by the annual gener ation of anaerobic conditions in the hypolimnion of this lake (Nriagu. 1968). Figure 5.3 ~lumates the distribution of reduced ulphur in a seclion of the Lake Mendota sedimem column. The upper pan of the sedimem accumulated during the pa~t 15 l'eals and differs markedly in composition and sulphur content from lhe marl deposits that are lower don lhe section. Nriagu amibute. the change. in the nature of the sedimem. and the higher coc 'ntralion~of sulphur in the upper pan of sedn.nts. to the influec ' of urbanization and economic development of the Mendota basin that began in the fill1 half of the nineteemh ~ntu... (Nriagu. 1968). Data on the sediments of two of the Great Lakes-mcsotl'Qphic Lake Ontario and eutrophic Lake Eric-are shon in Table 56. l1ie reduced sulphur contem in the surface horizons of Lake Otario is O.16~. while at a depth of 2-6 em it i! creases 1 O.2~ and then drops 1.13 and 'emain practically constant don to a depth of 5 em. In the uppetm5t mud horizon Sulph.1Ie 1I'e;' (~gs k ' (1' II k~. we (gh~~ S I w.' I...k. 5 Xlimenl) Jncn') Jimen' I Jay) Ok '.11OO2-OOCI11 ~ Bolsi Kichie' ' 4.8_ _19.3. ' '1c bc.ru 6J-127 2_4.16~.16 Somioo S.w.Ilkoye 1111 '.11.\ (hahn-.l l )IN) ''.1. Vahkol 1117 I).(NI6 Ryumnkokoye ' W.1115 Baik.I ofm.. aion !.O S;'c Lake ' Rd. Sokolo..11 1 Sorukin (1951) C''''''ar.o (l'l14a) ;.:'. (1975) 1JncItC (1914) Thirkoc~ SII h~1c F...ulpllu I'y'i'. O'g.nir ulphu ~l\i~'g I)1 II of Jinn' To1 'o... c1...11'11 k..~ Iml (.) (.S. d'y \1.1\11' b'i ) (. f ulphu') S ' ' ' 24 7SO S.IO 76.4 I UN) ' 55 I1.H 41 lui. no.69 2S.SS MO '' m ' ~. 4 'M S 21.1 I Tobk 5.4 FOrni of.olph' in Me'lyi _'... 'n1 sediments of Ja; a lakes f''''''ili t I.k. Auk;'Ko. 3.7 oligolropic K'zak'_Ko. 2.1 mc olrophic Nk.'.ro-K. 1. ' ' '1''' ' lidjjh Ik Nakaumi. pokk' '1'8. pokk' Lake Aoo'agafochi Ovelying ate Slphate Chlor.1c (~ hi ') ll Te«1 ilt hin (en) 5ed'm nb Soll'e!\ei ! lol ic R...lual.ulphuf ulp' ('S in dry..it). ~' O.26---{).44 '1'1.ulpo _2 O.19_.n Suso.. I. (195.1) ~ ( ~ \.2.~ 2. Yd I'd Kouchi ~' (1972) LO\I.18---{).2J.43_117 O.5---{).l\ SS.o. Ill. ~ (1953) ~ Tile Sulphur Cycle in C1!lwnrol Resuwin 35 Table 5.S O..tributio l of wlphur n t... 1'P' horizon' of L.ake Mendo.. jnru.gu. 1968) Depth (m) Total (~S. dry... eigllt bou) Ek: oms !l6 12 (116 1S \ S ocid_solubk sulphur has a light sulphur isotopic composition:... ithi. the mud column It become' heavier than pyrite-sulphur by 5-11'1 (Fig. 5.4). This suggests that. in he mud column. acid soluble sulphur is the product of reduc 'ion of heavier sulphate present in pore...a'n. From data on the rontent of reduced sulphur in dimenl .nd an assumed sedimentaion rate of.397 mgs em ' j'e.r-' (Kemper a/. 1972; Nriagu and Coker. 1976) he annual accumulalon of reduced sulphur in Lake Ontario sediments is estim.ed to be -.82 Tg. more lhan '...o-ihirds of... hich...s produced by sulphate-reducing bac'eri. E4 ' a o - t;'. ' :' 2~ o ' '3; ~ ' ~ ZoO '~' to 2. 3 S'o~ur (mils gm.-') Fig. S.3 D'stribution of aoid-loluhle (6) &lid total (.I.) ulphur in 'dimeo of Lake Mendoa (Nriiu. 1968). DIe stla kd.re. indicat.. tim: column of marl 'dime Ioclot d under tim: I'J'tr of dark reduced OO Imcnl (A. !JaJ n:pon B. C. deep n:gion of late) TM GlobDl Biogwchemil:u Sulphur Cycle Table 5.6 POte-....ulphate odd rrduce l sulphur in '..dimeo' of Lake Ontario (:-:riagu and CokeT 1976) Pore 'ater Add-solubl Filled Tool reductd Inleal Ulphal (em) ('i li'.-') ('los. dty wcigllt ba'i ) H 95.1 O19~ '-' .83 MS6.139 g~lo .78 OOSI oms O.13S ' 18-2 SO ' The ~ulphur distribution in 1he bot1om sediment' of some ba;n of Lake Erie is shown in Fig In sediments of thi lake. just as in other I'onh American lake. pronounced increase in rom:enlral;n of redd sulphur compounds is obscryed in the surface _ Fig cemrll basin) or subsurface rirons (!;«Fig. 55-easlern and 'eslem basins of Lake En.). According to Sriagu (1975). the an concenl1-ation of reduced sulphur in the surface borizonsof Lake E~ sediments is las (dry '~I basi') From this value and lhe 'dimenlation rate (Kemp II al ) the amount of sulphur annually illc(}rporaled into the lake dmen in reduced form can be calculated 1 be Tg (Nriagu. 1975). From this reiew of Ihe sulphur cycle in freshwaler lake it is concluded ~ 1 E 2 l.o'opc compos.lon ls' S. 'fo.) o +to +2 I I '! 4 F~. 54 Varia'; in ;op;e comlion of..id--oolu... (6) and onlc ~ Pl'e (&) sulphu in a li«ion I be: Lak. On.!ediment . (Nriagu and Coke. 1976) Su'pnQt. Slpnu' (... 1 hog s ~. -') R.5.5 Varia1ion i.. am junl. of dill...n' form. of Ipllu' n secl of danen! from. m (A). conn! (B) and '.m (C) basins of Lak. Er; e Sulpha... in po'.... acid lub~o.~...nioi; (.f... l'riaju.1915) lha1. in spit. of lhe low COnr.nl of dissol'd sulphales. lhe sulphur cycl. in lhese lak.s is quil' subslantial. Usually sulpha1' reduction lakes pta«only in U... sedim.nts. although il can o«u in It... hypolimnion of deep-water and highly producti'.. lakes. The geochemical result of sulphale reduclion in sediments of fresh...-al lakes is lhe accumulalion of anou insoluble form' of reduced sulphur. Ho. 1he amounts accumulaled ' II}' 'idely (Table 5.7). Thi sulphur is removed from lhe cycle. and lherefor. obould be considered as a 1s ilem in ' Table S.7 TM GlolMl BiogwcumkoJ Sulphur Cycle Total t :doccd sulphu in 'dim nts of fre5ll... lake. Sulphu U' ('!los. dry..eighl bui» Rd... Olig olt )p~ichlk.. B.ikal O.18-{).I8 La' (198) H~ O.lS- l38 K.mp~ ai. (1972) M'Wlrophk lake. Nak1uoo-Ko SUg.lnrae oj. (1953) Ontario Nri.ogu and Coker (1976) Eutrophic luk M.Ddot. O.l86..{).425 Nri_! (1968) Err O.OSO-lJ.34 Nn.gu (1975) the limation of ooth regional and global sulphur budgets. Based on the rew dala give io Table 5.7 the.rage sulphur comem in oontemporary sedimen. of mesotrophic and eutrophic lake. is.1 5'Mo (dry sediment basis). This value is ;cd in OUT calculation. of sulphur flux into lake and re.rvoi dimenls Brinkmann and santos (1974) investigated hydrogen sulphide production in high_flood reservoir.; of the Amazon River. The idth of the stagnant floodplain in the middle reaches of lhe Amazon calkd 'igapo' in Bruil. is 2 km c!'. now' ~... ~~ ~ FlO a.. T...'.. FIO.. I Wi I I -\ I 'J 1/ lj M j i ' l-hs-! O.IND.J F LO'. ~om r---.j (2) 1 ' /I' \971 M Fig. S.6 V.rialion. in 1..'.1 of flood.. (I)nd Ul'Pr limi' of dissol...d H5 (2) io t fioodpl.in of... Amazon Ri .. V.nic.I.ITO.. 1 ' be time. of HS omiss.ion to tbe.unosp''' (Brinlm.nn and Sa.ntos 1974) Tit - Sulphur Cyclt ill COf1/;/Itmal RtUrvOlrs gl~'. ' I I I '. mq...-' so:- O. ''''.'.' HS./. :\ '227 I 241.'.. /j : \ '\ '647 I I o. 1 ' o.~ If:Qh t Loke bol'om Fig. S.7.Variltioo' in ')'8en. hl'dros n.ulpbide. and.ulphot. nt'..~ during ming fiood'ater \ ..1 in he floodplain of the AmaMn RIV.' (Bnnkmann and Son 1974) and this area is flooded 'ith f...shet from January to late (ktober (Fig. Hi). Hydro~n sulphiok is produlxd inhe kll' horimn' of Ire'het reservoirs in the period of.ttr rise (Fig. 5.7). 11 e hydrogen sulphiok concentration gr.dually ;'-'5. and by the end of the high flood period..ache. I{\-I7 mg li're-' (Fig. 5.8) In the middle of Augu ! t! e level of f! et f.lls and hydrogen.ulphide containing.'e are exposed to the.urface. Th.. result. in! e degassing of hydrogen sulphide to Ite atmosphe... usu.lly during Stp'tmbtr and Octobtr (Fig. 5.8). Based on the duralion of tbe degassing proce... and the aa of floodplain (75 km' according to l.ub.hova. 1958). tbe total emission of hydrogen sulphide to the atmmphe... as estimated to bt Tg (Ivanov. 1979). During the freshet in floodplain reservoirs tbe 'ater layer containing h)'d rogen 5Ulphiok is separated from tbe atmosphe' by a layer of 'ater containing o'}'gen (Fig. 5.6). Ho'..''tr. under conain atmospheric conditions. for example duting tbundtrstonns. tbe upper water I orimn5 of t! e Arnown freshet..rvoirs m.y cool don: l! en duting l! e period 'hen freshel is al' . o ~2 o FOII'9...lOr_ '9;w.- o TN Global Biogeochemit: d SuJplr~r --- Cyd~ Fig. S8 Vanat; in 'l.o:en and hydrogen 'ulphlde co=noti.on. dunn~ falling _-aler Ic~h in Ooodpl'n of Am...n R' r (Brinkmann and Sant 1974) high le 'ol tllesc r st'voirs ar : med. and hydrogen sulphide escapes to.he almo.pbere (Fig. 5.6). This uample sho... that h)'drogen sulpbide can be emilled to Ihe mpliel'(: in significant amounls from freshwater bodies. ~.2.2 The Sulphur C)'rle in ~luomktir Labs The monimolimnions of stratified meromictic lake. can conla;n appro';. able concentrations of h)drogen.ulphidc. up mg 1;lre-' (Cbebolarev. 1914b: Hutchinson. 1957). In the.modine in these lakes. intensi»e chemical and microbiological oxidation of hy'drogcn sulphide 1 sulphur and sulpha.e oceurs. Tbe contration of photoautolrophi ; bacteria in lhis to is so high lhat lhe waler lakes on an intense red or green colour (Ku ;neISOv. 1952: Sorokin. 191: Takahashi and lchimura. 197; Gcno'-e.. and Bruni. 1915: GorlenkotltJi ) Two wpes of meromictic lakes OCCur mosl widely: lakes of coaslal regions and lakes of karst origin. The first lype of meromictic lake is represented by lake Mogilnoye on Hildin Island (in the Baren Sea) described atlhe beginning of this century (halshenko. 1914). lhe wellsludied Solar lake. Sinai. lake Faro in Italy (Geno'-ese and Bruni. 1915: Sorokin and Donalo. 1915). and a number of lakes in lho coaslal reg;n of Antarctic. (Matsuba~'aer at..... ''Om I... ~ th t ~ p.on~ d.j in ~ '... ~'V '.. ~ in lop 1.)' . BolIOnl tay ...f J'W ud... 1 T~5~ s. Ch~ic f too 11 ''11 : i '''''''K'lic I~ SlrOO'~ rcduc'.m ''''~ 1' 1 '~' Moo'i ChO'''''''j''... (mgs litre ' day ) (mgs m 'day ') HydmllC Mal Thicb''' Sulplla'c ulpllidc I_.~c ret'.lepth (n) ('its ') (ng.~ '''e ') We. Scdn Wate Scd Refe'.. fh' Iik... S'. Vok'gda 1('. I 1.11 'A' ' I 1.(1 IUli/. ' ' J'II) I.rv (IY7~) Ko. Mi lclk: '.7 OOOO~ S2 Chde. (1~7S) Volga I{h'h J.' JOO ~ Chchol'''' 1 (1~75) MNl J1c Volg.' 1\Ik I{ichic 1JI B J li.olb IJ.J AA 1JJl) ClI :botorc. (I ~Ha) Mklk: Volt'. V I S; 2UO M 1 ' II oon lian CI no 11.nen J (1'l7h) H_ki.h I~~r Kepnyc 65 I ~ ~ Ukn ' (197) Vdso Ukri Ib..~ 1 W) 'W OOJ 1 15.b 15 ~ Chcho'''rco I OJ n Ctoobo'rcy (1975) 1919; Bunon and Barker. 1919). 1l c: monimolimnions of lhese lakes uually ronlain modified sea 'ate
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