Sediment Pore-Water Dynamics of Little Rock Lake, Wisconsin ...
Sediment Pore-Water Dynamics of Little Rock Lake, Wisconsin: Geochemical Processes and Seasonal and Spatial Variability Author(s): Leslie A. Sherman, Lawrence A. Baker, Edward P. Weir, Patrick L. Brezonik Source: Limnology and Oceanography, Vol. 39, No. 5 (Jul., 1994), pp. 1155-1171 Published by: American Society of Limnology and Oceanography Stable URL: http://www.jstor.org/stable/2838479 . Accessed: 09/05/2011 19:06 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=limnoc. . Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].
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Limnol. Oceanogr.,39(5), 1994, 1155-1171 ? 1994, by the American Societyof Limnologyand Oceanography,Inc.
Sedimentpore-water dynamicsofLittleRock Lake, Wisconsin: Geochemicalprocessesand seasonaland spatialvariability LeslieA. Sherman
Department ofSoil Science,1525Observatory Dr.,University ofWisconsin, Madison53706
LawrenceA. Baker
Department ofCivilEngineering, ArizonaStateUniversity, Tempe85287
EdwardP. Weir
MinnesotaPollutionControlAgency, Rochester 55904
PatrickL. Brezonik
Department ofCivil& MineralEngineering, 500 Pillsbury Dr. SE, University ofMinnesota, Minneapolis55455 Abstract
The natureofsediment alkalinity generation processesand thetemporal and spatialvariability ofthe pore-water chemistry of an experimentally acidifiedseepagelake (LittleRock Lake, Wisconsin)were determined. Analysisof verticalgradients of solutesnearthe sediment-water interface indicatesthat sulfatereduction and base cationproduction A werethemajormechanisms of alkalinity generation. ofsurficial accumulation ratesand burialratesindicatesthatthemajorsourceofcationsto comparison theporewateroccurred byreleaseoforganically boundandexchangeable cationsthrough decomposition. Pore-water measurements also revealsignificant seasonalchangesin solutefluxes,including a sudden in particledea springtime changein sedimentmetabolism following algalbloom.Spatialdifferences fluxes ofammonium andalkalinity tobe almostan orderofmagnitude positioncausedpore-water higher at a hypolimnetic 2 yrofacidification, sitethanat epilimnetic ofsulfate, sites.After pore-water gradients showedonlyminorchanges, andthepore-water basinremained calcium,andalkalinity pH intheacidified within0.5 pH unitsofpreacidification pH.
Sedimentsare major sites of alkalinitygenSediment pore-waterdynamics have been eration for the water columns of soft-water studiedas a means ofunderstandingalkalinity lakes (Cook et al. 1986; Brezonik et al. 1987; generation in lake sediments (Kuivila and Rudd et al. 1986). This process is important Murray1984; Schiffand Anderson1986; Rudd ra- et al. 1986). However, detailed investigations in lakes withsmall watershed-to-lake-area tios (Schindler 1986) and especially for pre- of these dynamics have not been conducted. cipitation-dominatedseepage lakes (Brezonik Althoughsome investigatorshave noted difet al. 1987). The latterconstitutea major frac- ferencesin pore-waterchemistryin soft-water tion of the low alkalinitylakes in the United lakes between sites and across seasons (Cook Statesand are especiallyprevalentin theupper et al. 1987; Rudd et al. 1990; Carignan and Lean 1991), studieshave not been specifically midwestand in Florida (Baker et al. 1991). designed to evaluate the spatial and temporal variabilityof thepore-waterchemistryin softAcknowledgments byU.S. EPA ERL-Corvallis waterlakes. Pore-waterdata most oftenhave Thisworkwassupported been collectedat low temporaland spatial resCRS-11540-01. Agreement Cooperative WethankNaomiDetenbeck, JaniceTacconi,CarlMach, olution, which may conceal significanttemassistanceand poral variabilityin response to temperature, and ScottKing forfieldand laboratory data.We the 1985 pore-water Todd Perryforproviding and lake mixingpatternsor may extendspecialthanksto Noel Urbanforcriticalfeedback sedimentation, resultingfrom different trends mask spatial andwriting theresearch processandaregratethroughout Cameroon,fortheuse sedimentationrates or overlayingwater-colfulto HeiferProjectInternational, facilities ofcomputer duringthestayofL. A. Shermanin umn concentrations.In addition,althoughcatCameroonwiththeU.S. Peace Corps.Finally,we thank well as S042- reduction) FranklinShermanforbeingthecruciallinkin commu- ion production (as to sedimentalkalinity of therevisionof thismanu- contributessignificantly nicationforcoordination generation (Schiffand Anderson 1986), descript. 1155
1156
Shermanet al.
tailed studiesofthemechanismsofcation pro- layer of flocculentorganic matteror macroductionin lake sedimentshave notbeen made. phytes. In the acidificationexperiment,the north This paper examines the sedimentpore-water chemistryof Little Rock Lake, a precipi- basin was acidified with electrolyte-grade tation-dominatedseepage lake that is the site H2SO4, and the south basin provided a refof an experimentalacidificationstudy. Pore- erence (see Brezonik et al. 1993). The porewaterconcentrationsof alkalinity,major ions, water study reportedhere was conducted in and nutrientsweremeasured seven timesdur- the first2 yrofacidification(April 1985-April ingthefirst2 yrofacidificationat fivedifferent 1987), duringwhichthe pH of the northbasin sites in the lake. Results reveal a spatiallyand was lowered fromits preacidificationvalue of temporallydynamic sedimentpore-wateren- 6.1 to a level of 5.6. vironmentcontrolledby multiplerelated factors.Coupled withsolid-phasecarbonand cat- Methods Field- Pore-water equilibrators were detheprofilesprovideinsights ion measurements, into the mechanisms of sediment alkalinity ployedforseven samplingperiods:April,June, generationin a seepage lake. This studydem- and August 1985, 10 and 30 May 1986, Seponstratestheimportanceand dynamicsof cal- tember 1986, and January1987. The equilicium release fromorganic matterin the sedi- bratorswere installedat fivesites in the lake: at 1I-mand 5--mdepthsin the south basin and mentsof such lakes. 1-, 5-, and 9-m depthsin the northbasin. The Studysite 1-m depths were sampled only in 1985 and Little Rock Lake (Vilas Co., northeastern the 9-m depth only in 1985 and May 1986. The design of the equilibratorsclosely reWisconsin, 49060'N 89050'W) is an oligotrophic seepage system.Precipitationcomprises sembled thatof Hesslein (1976); each consist98-100% of the total water input to the lake ed of a 2.0 x 10 x 79-cm Plexiglas bar into (W. Rose unpubl. data). The lake has two ba- whichweremilled7-ml cells at 1-cmintervals. sins connected by a narrow (70 m) constric- A Nuclepore polycarbonatemembranewitha tion. The total surfacearea is 18 ha, and the 2.0-,umpore size covered thecells,whichwere maximumdepthsare 10.5 m in thenorthbasin filledwithdistilleddeionized water(DDW). A and 5 m in the south basin. The northbasin Plexiglasfaceplate withholes cut to matchthe stratifiesthermally,creating an oxygen-de- equilibratorcells was used to hold the mempleted hypolimnionthat constitutes 8% of brane filterin place. A DDW-soaked, 0.5-cmthe total basin volume. Before experimental thickpolyurethanefoam filterrestedbetween acidification,the lake had a low ionic content the membrane and the top plate, protecting (sp cond. = 11 ,uScm-' at 250C), low nutrient the membrane from punctureduring instaland chlorophyllconcentrations(total P = 11 lation.The faceplate was fastenedto theequil,ugliter-'; Chl a = 2-5 ,ugliter-'), and low ibratorbody withnylonscrews.Beforeinstallation, a 1 x 1-m plastic gratingwith a 2-cm color (-15 PCU) and turbidity( 90% sand and cies (XH2S, Fe2 , total Mn) were sampled first have a water content of - 70%. The organic and preserved as described below. Samples contentis variable,rangingfrom5 to 20%. The were withdrawnfromeach cell in the region littoralsedimentsare oftenoverlainwitha thin extendingfrom 5 cm above to 10 cm below
chemistry Sedimentpore-water the interface,withsamplingat widerintervals outside this range.Analyses of the pore water forall desired chemical constituentsrequired >7 ml (the volume of one cell). In 1985, two neighboringequilibrator cells were sampled (but not combined) to collect a sufficient volume: one cell was sampled formajor ions and the next forthe remainingspecies. Therefore, a resolutionin depthofonly2 cm was obtained foreach chemical constituentin the regionof the interface.In the two May 1986 sampling periods, fewer species were measured (only Ca2+, Mg2+, S042-, and Fe2+), and a 1-cm resolutionwas achieved. In September 1986 and January1987, two or threeequilibrators were deployed at each site to measure many species and obtain a 1-cmresolution.The average time required to remove samples from an equilibratorwas -0.75-1 h. Analytical-The pore waterwas analyzedfor S042-, Ca2+, Mg2+, and Fe2+ on all the samplingdates and foralkalinityand NH4+ on all dates except the May periods; pH, DIC (dissolved inorganicC), DOC (dissolved organic C), NO3-, SiO2, XH2S, MnT (total Mn), and AlT (total Al) were measured on one or two occasions. All samples, except those for pH and DIC, wereplaced in small plasticvials and refrigerated.The following were also preserved:XH2S (0.09 N ZnAc and 0.18 N NaOH), Fe2+ (0.3 N HCI), and MnT and AlT (0.04 N UltrexHNO3). Analyses forthese parameters were performedin a laboratoryat the Universityof Minnesota: S042- by ion chromatography;major cations by flame atomic absorptionspectrophotometry (AAS); MnT and AlTbygraphitefurnaceAAS; and NH4+, SiO2, Fe2+, 2 H2S, and N03- by colorimetricmethods (Brezonik et al. 1993). Alkalinitywas determinedby Gran alkalinitytitration.Holding times for preserved Fe2+ and XH2S samples were 100 ,ueqliter-'). alsocausedthezoneofS042- reduction toshift The amountof2H2S produceddidnotequal upward. theamountofS042- reduced,suggesting that - Profilesof redox-sensi- ZH2S was not the finalproductof S042- reSulfatereduction tivespecies[Fe(II), MnT, and SO42-] indicate duction.Saturationindices(the ratioof the thataerobicdecompositionwas confinedto productofmeasuredion activitiesto thesoltheupper1 cm of surficial sedimentsor floc- ubilityconstant)were calculatedfor amorand pyrite griegite, culentlayer.This findingis consistentwith phous FeS, mackinawite, of microelectrode oxygenmeasurements, which fromthemeasuredpH and concentrations Fe2+ andZH2Sconcentrations with indicatethatoxygenpenetration is confined to pore-water theupper2 cm ofthesedimentcolumneven K,ovalues fromGiblinand Howarth(1984). in highlyoligotrophic undersaturated with systems(Carletonet al. Theporewaterwashighly 1989).Fe(II) increasedrapidlywithin1 cm of respectto thefirstthreeminerals,suggesting ofsulfideintotheseformsdid theinterface (Fig.3), and XH2Swas presentin thatthefixation on the Pyrite, the pore waterin thisregion,thoughat low notoccurwithinthesediments. in the concentrations indicate otherhand,was highlyoversaturated (6.0 in Ontario and New York and found that Ca2+ release into the pore water is a major componentofalkalinityproductionin thepore water,whereas Mg2+ and NH4+ are minor contributors.In lakes with pH
American Society of Limnology and Oceanography is collaborating with JSTOR to digitize, preserve and extend access to Limnology and Oceanography.
http://www.jstor.org
Limnol. Oceanogr.,39(5), 1994, 1155-1171 ? 1994, by the American Societyof Limnologyand Oceanography,Inc.
Sedimentpore-water dynamicsofLittleRock Lake, Wisconsin: Geochemicalprocessesand seasonaland spatialvariability LeslieA. Sherman
Department ofSoil Science,1525Observatory Dr.,University ofWisconsin, Madison53706
LawrenceA. Baker
Department ofCivilEngineering, ArizonaStateUniversity, Tempe85287
EdwardP. Weir
MinnesotaPollutionControlAgency, Rochester 55904
PatrickL. Brezonik
Department ofCivil& MineralEngineering, 500 Pillsbury Dr. SE, University ofMinnesota, Minneapolis55455 Abstract
The natureofsediment alkalinity generation processesand thetemporal and spatialvariability ofthe pore-water chemistry of an experimentally acidifiedseepagelake (LittleRock Lake, Wisconsin)were determined. Analysisof verticalgradients of solutesnearthe sediment-water interface indicatesthat sulfatereduction and base cationproduction A werethemajormechanisms of alkalinity generation. ofsurficial accumulation ratesand burialratesindicatesthatthemajorsourceofcationsto comparison theporewateroccurred byreleaseoforganically boundandexchangeable cationsthrough decomposition. Pore-water measurements also revealsignificant seasonalchangesin solutefluxes,including a sudden in particledea springtime changein sedimentmetabolism following algalbloom.Spatialdifferences fluxes ofammonium andalkalinity tobe almostan orderofmagnitude positioncausedpore-water higher at a hypolimnetic 2 yrofacidification, sitethanat epilimnetic ofsulfate, sites.After pore-water gradients showedonlyminorchanges, andthepore-water basinremained calcium,andalkalinity pH intheacidified within0.5 pH unitsofpreacidification pH.
Sedimentsare major sites of alkalinitygenSediment pore-waterdynamics have been eration for the water columns of soft-water studiedas a means ofunderstandingalkalinity lakes (Cook et al. 1986; Brezonik et al. 1987; generation in lake sediments (Kuivila and Rudd et al. 1986). This process is important Murray1984; Schiffand Anderson1986; Rudd ra- et al. 1986). However, detailed investigations in lakes withsmall watershed-to-lake-area tios (Schindler 1986) and especially for pre- of these dynamics have not been conducted. cipitation-dominatedseepage lakes (Brezonik Althoughsome investigatorshave noted difet al. 1987). The latterconstitutea major frac- ferencesin pore-waterchemistryin soft-water tion of the low alkalinitylakes in the United lakes between sites and across seasons (Cook Statesand are especiallyprevalentin theupper et al. 1987; Rudd et al. 1990; Carignan and Lean 1991), studieshave not been specifically midwestand in Florida (Baker et al. 1991). designed to evaluate the spatial and temporal variabilityof thepore-waterchemistryin softAcknowledgments byU.S. EPA ERL-Corvallis waterlakes. Pore-waterdata most oftenhave Thisworkwassupported been collectedat low temporaland spatial resCRS-11540-01. Agreement Cooperative WethankNaomiDetenbeck, JaniceTacconi,CarlMach, olution, which may conceal significanttemassistanceand poral variabilityin response to temperature, and ScottKing forfieldand laboratory data.We the 1985 pore-water Todd Perryforproviding and lake mixingpatternsor may extendspecialthanksto Noel Urbanforcriticalfeedback sedimentation, resultingfrom different trends mask spatial andwriting theresearch processandaregratethroughout Cameroon,fortheuse sedimentationrates or overlayingwater-colfulto HeiferProjectInternational, facilities ofcomputer duringthestayofL. A. Shermanin umn concentrations.In addition,althoughcatCameroonwiththeU.S. Peace Corps.Finally,we thank well as S042- reduction) FranklinShermanforbeingthecruciallinkin commu- ion production (as to sedimentalkalinity of therevisionof thismanu- contributessignificantly nicationforcoordination generation (Schiffand Anderson 1986), descript. 1155
1156
Shermanet al.
tailed studiesofthemechanismsofcation pro- layer of flocculentorganic matteror macroductionin lake sedimentshave notbeen made. phytes. In the acidificationexperiment,the north This paper examines the sedimentpore-water chemistryof Little Rock Lake, a precipi- basin was acidified with electrolyte-grade tation-dominatedseepage lake that is the site H2SO4, and the south basin provided a refof an experimentalacidificationstudy. Pore- erence (see Brezonik et al. 1993). The porewaterconcentrationsof alkalinity,major ions, water study reportedhere was conducted in and nutrientsweremeasured seven timesdur- the first2 yrofacidification(April 1985-April ingthefirst2 yrofacidificationat fivedifferent 1987), duringwhichthe pH of the northbasin sites in the lake. Results reveal a spatiallyand was lowered fromits preacidificationvalue of temporallydynamic sedimentpore-wateren- 6.1 to a level of 5.6. vironmentcontrolledby multiplerelated factors.Coupled withsolid-phasecarbonand cat- Methods Field- Pore-water equilibrators were detheprofilesprovideinsights ion measurements, into the mechanisms of sediment alkalinity ployedforseven samplingperiods:April,June, generationin a seepage lake. This studydem- and August 1985, 10 and 30 May 1986, Seponstratestheimportanceand dynamicsof cal- tember 1986, and January1987. The equilicium release fromorganic matterin the sedi- bratorswere installedat fivesites in the lake: at 1I-mand 5--mdepthsin the south basin and mentsof such lakes. 1-, 5-, and 9-m depthsin the northbasin. The Studysite 1-m depths were sampled only in 1985 and Little Rock Lake (Vilas Co., northeastern the 9-m depth only in 1985 and May 1986. The design of the equilibratorsclosely reWisconsin, 49060'N 89050'W) is an oligotrophic seepage system.Precipitationcomprises sembled thatof Hesslein (1976); each consist98-100% of the total water input to the lake ed of a 2.0 x 10 x 79-cm Plexiglas bar into (W. Rose unpubl. data). The lake has two ba- whichweremilled7-ml cells at 1-cmintervals. sins connected by a narrow (70 m) constric- A Nuclepore polycarbonatemembranewitha tion. The total surfacearea is 18 ha, and the 2.0-,umpore size covered thecells,whichwere maximumdepthsare 10.5 m in thenorthbasin filledwithdistilleddeionized water(DDW). A and 5 m in the south basin. The northbasin Plexiglasfaceplate withholes cut to matchthe stratifiesthermally,creating an oxygen-de- equilibratorcells was used to hold the mempleted hypolimnionthat constitutes 8% of brane filterin place. A DDW-soaked, 0.5-cmthe total basin volume. Before experimental thickpolyurethanefoam filterrestedbetween acidification,the lake had a low ionic content the membrane and the top plate, protecting (sp cond. = 11 ,uScm-' at 250C), low nutrient the membrane from punctureduring instaland chlorophyllconcentrations(total P = 11 lation.The faceplate was fastenedto theequil,ugliter-'; Chl a = 2-5 ,ugliter-'), and low ibratorbody withnylonscrews.Beforeinstallation, a 1 x 1-m plastic gratingwith a 2-cm color (-15 PCU) and turbidity( 90% sand and cies (XH2S, Fe2 , total Mn) were sampled first have a water content of - 70%. The organic and preserved as described below. Samples contentis variable,rangingfrom5 to 20%. The were withdrawnfromeach cell in the region littoralsedimentsare oftenoverlainwitha thin extendingfrom 5 cm above to 10 cm below
chemistry Sedimentpore-water the interface,withsamplingat widerintervals outside this range.Analyses of the pore water forall desired chemical constituentsrequired >7 ml (the volume of one cell). In 1985, two neighboringequilibrator cells were sampled (but not combined) to collect a sufficient volume: one cell was sampled formajor ions and the next forthe remainingspecies. Therefore, a resolutionin depthofonly2 cm was obtained foreach chemical constituentin the regionof the interface.In the two May 1986 sampling periods, fewer species were measured (only Ca2+, Mg2+, S042-, and Fe2+), and a 1-cm resolutionwas achieved. In September 1986 and January1987, two or threeequilibrators were deployed at each site to measure many species and obtain a 1-cmresolution.The average time required to remove samples from an equilibratorwas -0.75-1 h. Analytical-The pore waterwas analyzedfor S042-, Ca2+, Mg2+, and Fe2+ on all the samplingdates and foralkalinityand NH4+ on all dates except the May periods; pH, DIC (dissolved inorganicC), DOC (dissolved organic C), NO3-, SiO2, XH2S, MnT (total Mn), and AlT (total Al) were measured on one or two occasions. All samples, except those for pH and DIC, wereplaced in small plasticvials and refrigerated.The following were also preserved:XH2S (0.09 N ZnAc and 0.18 N NaOH), Fe2+ (0.3 N HCI), and MnT and AlT (0.04 N UltrexHNO3). Analyses forthese parameters were performedin a laboratoryat the Universityof Minnesota: S042- by ion chromatography;major cations by flame atomic absorptionspectrophotometry (AAS); MnT and AlTbygraphitefurnaceAAS; and NH4+, SiO2, Fe2+, 2 H2S, and N03- by colorimetricmethods (Brezonik et al. 1993). Alkalinitywas determinedby Gran alkalinitytitration.Holding times for preserved Fe2+ and XH2S samples were 100 ,ueqliter-'). alsocausedthezoneofS042- reduction toshift The amountof2H2S produceddidnotequal upward. theamountofS042- reduced,suggesting that - Profilesof redox-sensi- ZH2S was not the finalproductof S042- reSulfatereduction tivespecies[Fe(II), MnT, and SO42-] indicate duction.Saturationindices(the ratioof the thataerobicdecompositionwas confinedto productofmeasuredion activitiesto thesoltheupper1 cm of surficial sedimentsor floc- ubilityconstant)were calculatedfor amorand pyrite griegite, culentlayer.This findingis consistentwith phous FeS, mackinawite, of microelectrode oxygenmeasurements, which fromthemeasuredpH and concentrations Fe2+ andZH2Sconcentrations with indicatethatoxygenpenetration is confined to pore-water theupper2 cm ofthesedimentcolumneven K,ovalues fromGiblinand Howarth(1984). in highlyoligotrophic undersaturated with systems(Carletonet al. Theporewaterwashighly 1989).Fe(II) increasedrapidlywithin1 cm of respectto thefirstthreeminerals,suggesting ofsulfideintotheseformsdid theinterface (Fig.3), and XH2Swas presentin thatthefixation on the Pyrite, the pore waterin thisregion,thoughat low notoccurwithinthesediments. in the concentrations indicate otherhand,was highlyoversaturated (6.0 in Ontario and New York and found that Ca2+ release into the pore water is a major componentofalkalinityproductionin thepore water,whereas Mg2+ and NH4+ are minor contributors.In lakes with pH
