hypolimnetic methyl mercury and its uptake

7/29/2019 Hypolimnetic Methyl Mercury and Its Uptake

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Limnol. Orerrnogr., 4X(7), 1998, 1476-14860 1998, by the American Society of Limnology and Oceanography. Inc.

Hypolimnetic methylmercury and its uptake by plankton during fall destratification:A key entry point of mercury into lake food chains?Russell T. HerrinWater Chemistry Program, University of Wisconsin, 660 N. Park St., Madison, Wisconsin 53706Richard C. Lathrop and Patrick R. GorskiWisconsin Department of Natural Resources, Research Bureau, 1350 Femrite Dr., Monona, Wisconsin 53716Anders W. AndrenWater Chemistry Program, University of Wisconsin, 660 N. Park St., Madison, Wisconsin 53706

AbstractSeasonal concentration changes in monomethylmercury (MMHg) and total Hg (T-Hg) were determined inDevils Lake, Wisconsin, a lake with a mercury consumption advisory for walleye. Mercury dynamics werestudied in water and several lower food chain fractions during the ice-free seasons n 1994 and 1995, and limiteddata for Hg in water were collected in 1993. MMHg concentrations increased in hypolimnetic waters beforeturnover each year, although maximum concentrations declined from 1993 to 1995. As the hypolimnion eroded,

MMHg concentrations increased n both particulate matter and Daphnia. Maximum concentrations were obtainednear the time of complete mixis. The magnitude of this increase correlated with the mass of hypolimnetic MMHgthat had built up prior to turnover in 1994 and 1995. Hg concentrations in yearling planktivorous fish exhibiteda decline corresponding to the decline in hypolimnetic MMHg from 1993 to 1995. Our results suggest that falldestratification represents an important time period for entry of MMHg to the food chain of lakes exhibiting ahypolimnetic MMHg buildup.

As concern regarding Hg contamination has increased, sohas understanding of the mechanisms of monomethylmer-cury (CH,HgX or MMHg) production within lake systemsand the movement of Hg and MMHg through various com-partments of these systems. Atmospheric wet and dry de-position are important sources of Hg to lake systems (Masonet al. 1994), and groundwater can act as a significant sourceor sink, depending upon hydrological considerations (Hurleyet al. 1994~~). River and stream inflow can be a source ofHg to lakes as well. The most important sources of MMHgin lake systems that contain a significant amount of this formare currently a source of debate. It has been shown that Hgis methylated by bacteria, and that methylation rates arehighest where sulfur-reducing bacteria are present (Gilmouret al. 1992; Matilainen 1994). Mass balances on northernWisconsin lake systems calculated by Watras et al. (1994)have led to the conclusion that inputs of MMHg are notsufficient to explain in-lake masses; methylation of Hg with-in the lake must be invoked. A mass balance created for~Palette Lake in northern Wisconsin, however, requires verylittle or no in-lake methylation (Benoit et al. in prep.). What-AcknowledgmentsFunding was provided by the Wisconsin Department of NaturalResources.Becky Hayes was instrumental in the collection and pro-ccssing of samples.Jani Benoit and Chris Babiarz provided invalu-able technical advice related to Hg collection and analysis tcch-niqucs; in addition, Jani aided in the 1993 Hg profile sampling.Dave Dreikosen, Jerry Wegner, and Greg Quinn assisted n the col-lection of planktivorous fish. WC are grateful to Jim Hurley and theUniversity of Wisconsin Water Chemistry Program for allowing usto use their clean facilities and Hg analysis equipment.

ever the principal source of MMHg to a lake may be, it habeen observed in many different lakes that concentrations oMMHg in lake water are significantly higher in anoxic hypolimnetic waters than in oxic epilimnetic waters (Verta eal. 1994; Watras et al. 1994; Bloom et al. 1991).Predators and herbivores in lake systems appear to receivethe vast majority of their MMHg body burdens from theifood source rather than directly from the water, as indicatedby the fact that MMHg bioconcentration factors (Bf = lo[C,lC,], where C, = wet-weight concentration in biota, anC,, = water concentration) are higher in fish than in zooplankton, and in zooplankton than seston (Watras and Bloom1992; Back and Watras 1995). Concentrations of Hg iaquatic biota can vary seasonally as well as on a speciesspecific basis. Slotton et al. (1995) observed pronounced increases in Hg body burdens of zooplankton and juvenile basat the time of thermal destratification of a seasonally anoxicreservoir that is strongly impacted by effects of impound-ment and contamination from a nearby abandoned mercurymine.The observed MMHg buildup in the hypolimnia of lakesin conjunction with high MMHg bioconcentration factorscreates questions regarding the role of hypolimnetic MMHgin lake food web dynamics. During destratification, hypolim-netic MMHg may transfer a detectable mass of MMHg tphytoplankton or other sestonic particles. Through zooplankton grazing, followed by predation by planktivorous and piscivorous fish and fish-eating birds, this pulse of MMHgcould subsequently bioaccumulate at higher trophic levelsA seasonal pattern of increase could lead to strategies foreducing concentrations of Hg in aquatic organisms if hy1476


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Methyl-Hg in food chains 147

Fig. 1. Bathymetric map of Devils Lake, Wisconsin. Depthcontours are in meters.

polimnetic MMHg buildup can be reduced through tech-niques such as aeration or early destratification. The goal ofthis study is to determine if the annual introduction of hy-polimnetic MMHg to the mixed zone during fall hypolim-netic erosion and destratification represents an importantsource of Hg to the lower food chains of lakes.Materials and methods

Study site-Devils Lake is located approximately 65 kmnorthwest of Madison, Wisconsin. The lake has a simplebath-tub hydrography (Fig. 1); most of its bottom area isprofundal. Its surface area is 149 ha, and its maximum andmean depths are approximately 14 m and 9 m, respectively.Two small springs, one on the east side and one on the west

side of the lake, represent the only surface inputs, and nsurface outflows exist. The west-side spring flows through small wetland before entering the lake. The lake is dimictic,with fall turnover usually occurring in mid-October. From1986 to 1995, epilimnetic chlorophyll a (Chl a) concentrations were less than 7 pg liter-l between April and Augusand peaked in September or October at 4-25 pg liter- (Wisconsin Department of Natural Resources unpubl. data). Fothis study, depths at which the dissolved oxygen concentration was >l.O mg liter-- were considered to be epilimnetic,and those at concentrations cl.0 mg liter- were considereto be hypolimnetic. At peak stratification in 1994 this division was approximately 4.5 m above the bottom of the deehole, and in 1995 it was approximately 3 m above the botom. In 1994 and 1995, epilimnetic temperature in the summer varied between 21. and 27C, and hypolimnetic temperature varied between 10 and 13C. Directly after turnoverwater temperature was between 13 and 14C both years. Because of its location in a quartzite outcropping, the geochemistry of the lake is more similar to lakes in northernWisconsin than to southern Wisconsins calcareous lakes. Itrelatively low acid neutralizing capacity (ANC) (approximately 450 peq/liter) results in epilimnetic pH values between 7 and 8 and a hypolimnetic pH near 6 (Lathrop et a1989), values substantially lower than other southern Wisconsin lakes. An inverse correlation between ANC and walleye Hg concentration has been observed for w alleye in Wisconsin lakes (Lathrop et al. 1991). Devils Lake has consumption advisory for walleyes (Stizostedion vitreum vtreum Mitchill) (Lathrop et al. 1989), based on the fact thaDevils Lake Walleyes greater than 25 cm in length havbeen found to contain 0.5-1.0 ppm Hg (wet-weight basis(Wisconsin Department of Natural Resources 1994). Thesconcentrations are similar to those found in many northerWisconsin lakes with similar or lower ANCs (Lathrop et a1991).

Sample collection-Samples for Hg analysis were colected from June 1994 through November 1995, excludingthe winter months. In addition, whole-water MMHg anwhole-water total Hg (T-Hg) samples were collected on 3September 1993. Most sample types except for fish wercollected at approximately 6-week intervals in the spring anearly summer and on approximately 3-week intervals in thlate summer and early fall. Temperature and dissolved oxygen concentration profiles were taken more frequentlythese profiles were used to determine the date of lake destratification. Mercury samples were collected within 4 d ocomplete destratification in both years.Five types of samples were collected for total Hg anMMHg analysis: whole-water samples, particulate mattegreater than 2.2 /-Lm n diameter, particulate matter betwee2.2 and 35 pm, particulate matter between 35 and 243 pmand Daphnia. Mercury samples were collected at varioudepths from a station at the deepest location in Devils Lak(see Fig. I). All samples collected for MMHg or total mecury analysis were collected using ultraclean technique(Patterson and Settle 1976; Hurley et al. 1996) includingthorough acid cleaning of all sample containers, m inimiza


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1478 Herrin et al.tion of metal sampling equipment, and clean hands-dirty Whether all particle types in this size range were in facthands sampling protocols. consumed by this species was not determined.Whole-water and particulate samples were collected atdifferent depths to create a water-column concentration pro-file. One to three epilimnetic depths were sampled, as it wasfound that Hg concentrations varied little in the mixed zone,When the lake was stratified, however, four to five depthswere sampled in the hypolimnion to determine concentrationprofiles for T-Hg and MMHg. A lake map was used to cal-culate volumes at different depth intervals; these volumes,in conjunction with MMHg determinations, were used tocalculate masses of MMHg associated with water and par-ticles at different depths. These calculations provided ameans for assessing the movement of MMHg between par-ticulate and filterable fractions and between the hypolimnionand mixed zone.Sampl.es of unfiltered water were collected in Teflon@ bot-tles that had been cleaned in hot, concentrated nitric acid.Particulate matter was collected on Whatman Scientific QM-A quartz fiber filters rated by the manufacturer to retain par-ticles larger than 2.2 pm in diameter when liquid is the fil-tered medium. The filters are rated at 0.8 pm for retentionof dry airborne particles. These filters were housed in acid-cleaned Teflon filter cartridges. Prior to sampling, quartz fi-ber filters were ashed at 450C to volatilize any Hg associ-ated with them. Filters that capture particles less than 2.2pm in diameter (e.g., 0.45 pm) are usually used for sepa-ration of particulate and dissolved components; quartz fiberfilters, however, are the only variety that can be ashed toremove Hg. Thus, material that passes through these quartzfiber filters will be referred to as filterable and that which isretained will be called particulate.

Zooplankton were collected in a nonmetallic plankton townet that had been leached in ultrapure water. The net waspulled through the water column from a depth of 8 m (nearbut never below the thermocline), and the contents weretransferred to a clean Teflon jar filled with lake water. Jarswere double bagged, put on ice, and returned to the labo-ratory, where they were stored at 4C. Within 24 h of col-lection, individual live Daphnia were picked from the jarsusing acid-cleaned glass pipets and transferred to Teflon vi-als. The transfer process was performed in a clean roomusing clean hands-dirty hands techniques. This procedure issimilar to that used by Back and Watras (1995). Tw enty to30 of the largest possible individuals were placed in eachvial. Triplicate samples of this type were created for bothMMHg and T-Hg analyses. A drop of water from the samplejar was used as a blank. Daphnia samples were stored doublebagged and frozen. For 1994 samples, average dry mass ofDaphnia was determined by averaging the lengths of 30 in-dividuals and determining dry weight using a body weight-to-length ratio determined by Lynch et al. (1986). For 1995samples, three subsamples of 20-30 individuals each werecollected, placed on a filter, dried, and weighed to determinedry weight. The two methods gave comparable values fordry mass of individual Daphnia. Mass of individual Daphniaranged from 0.03 to 0.14 mg, with an average mass of 0.06mg-

Samples containing particulate matter in the 2.2-35pmand the 3.5-243-pm size fractions were obtained from lakewater pumped through a 243-@rn sieve and a 35-pm sievein series. Sieves were constructed from acrylic tubes andnylon mesh netting. Periodically, the water passing throughboth of these sieves was captured in a clean vessel and ameasured volume was vacuum aspirated through a quartzfiber filter. The 35-pm sieve retentate was then washed ontoa separate quartz fiber filter using lake water. Additional fil-trations of these two fractions were performed to determinemass of these particles per liter of lake water. Size-fraction-ated samples were collected either at different discretedepths between 1 and 7 m or were composites of water col-lected from 1 m and 7 m depth.

Yearling m imic shiners (Notropis volucellus) and bluegills(Lepomis macrochirus) were collected for Hg analysis dur-ing late April or early May of 1995 and 1996, and bluegillswere collected in the same time frame in 1993. Bluegillswere collected by electroshocking, and shiners were collect-ed using a shoreline seine. Whole fish from a given collec-tion date were divided into three separate samples of 22-80shiners or 9-12 bluegills each: these samples were placed inclean plastic bags and stored frozen until analysis. Shinerscollected for analysis ranged in length from 28 to 42 mm,with an average length of 34 mm. Bluegills collected foranalysis ranged in length from 45 to 80 mm, with an averagelength of 60 mm.

Collection of particles greater than 35 pm and less than35 pm was performed based on a published relationship be-tween body length of zooplankton and maximum size ofparticle ingested (Burns 1968). Daphnia pulicaria, the prin-cipal Daphnia species present in Devils Lake, can ingestparticles smaller than approximately 35 ,um but cannot in-gest significantly larger particles. Henceforward, particles inthe 2.2-35-pm range will be referred to as edible, whilethose in the 35-243~pm range will be referred to as inedible.Particles larger than 243 pm were assumed to be composedprincipally of zooplankton and were discarded. It should beemphasized that the term edible will be used to refer to par-ticles of a small enough size to be consumed by D. pulicaria.

Sample analysis-Analysis for total Hg concentration wasperformed using a two-stage gold amalgamation techniquewith a cold vapor atomic fluorescence detection system(CVAFS) (Gill and Fitzgerald 1987; Hurley et al. 1995).Organic matter was oxidized using BrCl before analysis.All samples analyzed for MMHg concentration (with theexception of fish tissue) first underwent treatment with sul-furic acid and KC1 to convert all forms of MMHg to MMHg-chloride. They were distilled to remove MMHg from asso-ciated organic matter (Horvat et al. 1994). MMHg was thenchromatographically separated from other Hg forms, pyrol-ized to Hg, and detected by CVAFS (Bloom 1989; Hurleyet al. 1995). T-Hg and MMHg samples collected in 1993were analyzed by Frontier Geosciences.Fish samples for MMHg analysis were homogenized in astainless steel blender and digested in a 25% (weight-to-volume) solution of KOH in methanol. Between 0.5 and 1g of fish tissue was placed in 20 ml of digestate solution,


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-.- Hypo. MMHg -D- Hypo. T-Hg .'. A.-- Epi. T-Hg - - - D.O.

0.5

0.0

12

2

0

1994 I 1995Fig. 2. Concentrations of T-Hg, MMHg, and dissolved 0, 1 m above the lake bottom and T-Hg concentrations in the epilimnion. MMHg sampling points that are less than detection limits arcshown to indicate that samples were taken.

sonicated for 1 h, and placed in an oven at 50C for at least24 h. A O-023-ml aliquot of this solution was neutralizedwith 2 M acetic acid and analyzed by the same procedureas described for all other MMHg samples. This procedure issimilar to that described by Bloom (1992). Fish samples forT-Hg analysis were run by the State of Wisconsin Laboratoryof Hygiene by a procedure similar to that described byBloom (1992). All fish samples were analyzed under clean-room conditions.For total Hg analysis, triplicates were run on a few rep-resentative aqueous samples and on all Daphnia samples.Particulate samples for total Hg analysis often providedhigher Hg concentration values than whole-water samplesfrom the same depth; i.e., a higher mass of T-Hg was as-sociated with the filters than would have been expected evenif they had retained all of the Hg associated with the waterpassed through them. Total Hg concentrations were all abovelimits of detection, so accurate determinations could bemade. Filters were therefore assumed to be contaminatedwith Hg and results of filter T-Hg analyses will not be pre-sented. Filters did not show evidence, however, of contam-ination with MMHg, so results of particulate MMHg anal-yses are presented. Sample collection on filters wassufficiently time intensive as to make it impossible to collectreplicates for analysis on a regular basis, so particulateMMHg concentration values in most cases represent one de-termination only. In the few cases where replicate analyseswere collected, however, samples showed fair agreementwhen analyzed for MMHg. For replicate filter samples avail-able, the absolute value of differences between measured

concentrations and the mean concentration was on averag18% (median = 11%) of the measured value.Analysis of variation in whole-water MMHg samples wadifficult to quantify, as many duplicates collected were below detection limits. Because determination of whole-waterconcentration does not include the potential uncertainty added by the filtration step, whole-water MMHg determinationsshould be somewhat more precise. Spikes of MMHg standard were added to some whole-water and Daphnia samplesto determine recovery from distillation. On average, 89% ospikes added to whole-water samples were recovered (standard error = 2%; n = 29), and on average 104% of spikesadded to Daphnia were recovered (standard error = 13%; n= 7). Values reported herein are not corrected for these average recoveries nor are they corrected for recoveries fromindividual analyses. Error bars shown in figures represen95% confidence intervals for multiple analyses, approximat-ed using a Z-statistic. Standard errors were calculated usinga first-order error approximation (Reckhow and Chapra1983).Fish samples collected on each date and for each speciewere analyzed in triplicate for MMHg, total Hg, and percenwater. Tissue samples spiked with the appropriate dissolvedstandard before digestion were also analyzed for total Hgand methyl-Hg analysis. Recoveries greater than 95% werecalculated for all spiked fish samples.All MMHg samples were analyzed on the basis of masHg as MMHg. MMHg concentrations are reported as thecorresponding mass of Hg throughout this text.


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1480

//

Herrin et al.

Dissolved 0, (mg L-)0.4 0.6

- o- IO/l l/94 MMHg+ 10/3/95 MMHg0/3/95 MMHg- - 1016l94 D.O.- 1016l94 D.O.. . - IO/Ed95 D.O.IO/Ed95 D.O.

0.0 0.2 0.4 0.6 0.8 1.0MMHg (ng Hg L-l)

Fig. 3. Dissolved 0, and unfiltered MMHg concentrations in hypolimnetic waters. MMHg sam-pling points that are less than detection limits are shown to indicate that samples were taken.

Results and discussionHg concentration in water-Whole-water (unfiltered) T-Hg concentrations generally varied between 0.1 and 1.0 ngliter I in the epilimnion, but hypolimnetic waters were ele-vated in Hg concentration during 1994 and 1995 (Fig. 2).MMHg concentrations in the unfiltered epilimnetic waterwere almost without exception less than the method limit ofdetection (LOD), which was 0.05 ng liter L for water sam-ples. As with total Hg, however, MMHg concentrations wereelevated in the hypolimnion (Fig. 2). In addition, the enrich-ment of Hg is concurrent with depletion of dissolved oxygenin the hypolimnion. increases in Hg concentrations (partic-ularly MMHg concentrations) in anoxic hypolimnetic lakevolumes have been observed by other researchers (Slottonet al. 1995; Watras and Bloom 1994).Dissolved oxygen concentrations 1 m above the bottomof the deep hole were lower in 1994 than in 1995. The lowerdissolved oxygen concentrations in 1994 were accompaniedby higher hypolimnetic T-Hg and MMHg concentrations for

that year. Most of the difference in T-Hg concentration be-tween years is attributable to the higher MMHg concentra-tions observed in 1994, indicating that it is concentration ofthe methylated form of Hg that is most strongly affected byconditions present in the hypolimnion. The inverse correla-tion between MMHg and dissolved oxygen concentration isspatially evident as well; as oxygen concentrations decreasedwith depth in the hypolimnion, MMHg concentrations in-creased (Fig. 3). It is not clear from these results whetherdissolved oxygen concentration itself affects hypolimneticMMHg concentration, but these results indicate that decrease

in hypolimnetic oxygen concentrations coincide spatiallyand temporally with MM Hg buildup, and that these two values may change in proportion to one another.An analysis of particulate and filterable MMHg in hypo-limnetic water shows that the large difference between concentrations in 1994 and 1995 is almost exclusively due tofilterable MMHg. For the two sampling dates in each yeadirectly before turnover, the average hypolimnetic concentration of MMHg associated with particulate matter was approximately the same in 1994 (0.07 ng liter ) as in 1995(0.05 ng liter). Average filterable MMHg concentration fothese two dates, however, was as much as 25 times higherthan concentration associated with particles in 1994 (0.46 ngliter I), while in 1995 concentration of filterable MMHg(0.02 ng liter-]) was not as high as the corresponding particulate concentration.Seasonal dynamics of Hg in biotu fractions: Edible andinedible particles--MMHg concentrations in edible and inedible size fractions of particles showed marked seasona

trends (Fig. 4). In both 1994 and 1995, MMHg concentra-tions in the two fractions more than tripled during erosionof the hypolimnion in September and October. Concentra-tions in the edible fraction peaked and then dropped to neaprestratification levels soon after, though concentrations inthe edible fraction remained elevated for a longer period in1994 than in 1995. Concentrations around turnover in thinedible fraction did not show the sharp decrease seen in theedible fraction, possibly because inedible particles are not areadily consumed by zooplankton. Particle aggregationmight also contribute to this pattern. MMHg concentrations


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Methyl-Hg in food chains 14810.0160.0140.012

; -I 0.010Iif 0.008ps 0.0068

0.0040.0020.000

-

-+- Edible Particles Turnover :-z- Inedible Particles --y&j

I - Lp-q-o-0 : k--p--n&ysI I I I I I I I I I I I II~ASONDJFMAMJJASON1994

Fig. 4. MMHg concentrations per liter lake water associated with edible (2.2-35 pm) andinedible (35 -243 pm) particles in the epilimnion of Devils Lake .

in the edible fraction were elevated in the spring as well, per mass particles basis (Fig. 5), with some exceptions.although spring concen trations were not as high as peak con- Peaks in MMHg concentration were again evident duringcentrations near turnover. erosion of the hypolimnion. On an Hg-per-mass basis, how-MMHg concentrations of edible and inedible particles, ever, concentration in the edible fraction peaked at a slightlywhen expressed on a per liter lakewater basis (Fig. 4), had lower value in 1994 than in 1995, though the 1994 peakseasonal patterns similar to concentrations expressed on a again appeared to last longer. On a per-mass basis, concen-

1816 -

Turnover Ihj

-O- Edible Particles- 0 - Inedible Particles Turnover-

AMJJASONDJ F M A M J JA SO N1994 1995

Fig. 5. MMHg concentrations per dry mass in edible (2.2-35 pm) and inedible (35-243 m)particles in the epilimnion of Devils Lake.


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1482 Herrin et al.

A M J.J AS1994

0 N AMJ JASON1995

Fig. 6. Mass of MMHg associatedwith particles >2.2 pm in diameter.

trations associated with inedible particles again peaked nearturnover, but in this case they reached nearly the same con-centrations as edible particles and quickly fell of f in con-centration after turnover. These differences indicate that in-dividual particles in both size ranges behave in similarfashion with respect to association with MMHg, and that thedifferences exhibited in Fig. 4 are in large part the resultsof changes in the concentration of particles themselves. Suchchanges in particle concentration may result in part fromdynamics in the populations of various species of phyto-plankton. Grazing of edible particles, for example, may havecaused the sharp drop in per-liter MMHg concentrations inthis fraction, while a lack of grazing kep t inedible particlesat higher levels.The more pronounced increase in MMHg associated withparticles during fall destratification in 1994 can be seen moreclearly in whole-lake estimates of particle-bound MMHgmass for the 2 yr (Fig. 6). The degree to which particulateMMHg masses increased near turnover provides further ev-idence that much of the MMHg built up in the hypolimnionis taken up by particles at this time. In addition, calculationof changes in mass of whole-water and particle-associatedMMHg with time and depth allowed for estimation of thedegree to which hypolimnetic MMHg was transferred tomixed-zone particles after turnover. The increase in MMHgmass in the hypolimnion during the stratified period wasestimated and compared to the increase in particle-associatedMMHg masses between prestratification sampling and thesampling following lake mixing. Because whole-waterMMHg concentrations outside of the anoxic hypolimnionwere generally below detection limits, a worst-case concen-tration just below the detection limit was used to estimatethe concentration of MMHg in bottom waters before strati-fication; because this represents a maximum concentration

of MMHg in the prestratification bottom waters, it leads ta minimum calculated increase in MMHg in the hypolim-nion. In 1994, the mass of MMHg in the hypolimnion increased by approximately 220 mg during the stratified period. Oxygen profiles indicate that the hypolimnion starteto erode in late September 1994, and the thermocline was m deeper than its minimum depth by early October. Aroundthis time, concentration of MMHg in mixed-zone particulatematter increased by approximately 200 mg. The sharp increase early in the hypolimnetic erosion process may havbeen due to a plankton layer near the oxic-anoxic interfaceas detected by Watras and Bloom (1994) in a northern Wisconsin lake, or it may have been due to elevated MMHgconcentrations very close to the sediment-water interfacethroughout the hypolimnion (i.e., a continuation of the increase in MMHg concentration with depth seen in Fig. 3We did not detect such layers, but our sampling resolutionmay not have been adequate to do so. The apparent correlation was not so clear in 1995; buildup in the hypolimnionwas calculated at approximately 12 mg, and change iMMHg in mixed-zone particulate matter was calculated aapproximately 70 mg. This difference may be due to thworst-case assumption that would have had a more pronounced effect in 1995, when MMHg masses were muclower. Elevated concentrations near the sediment-water interface may also have contributed. Allowing for some errodue to smaller concentra tions in 1995, these values indicatethat MMHg stored in the hypolimnion is quickly taken uby mixed-zone particulate matter during erosion of the hypolimnion.The enrichment of MMHg in the two particle fractionsduring erosion of the hypolimnion strongly suggests that hypolimnetic MMHg is quickly taken up by particles when enters the mixed zone. Inspection of particulate matter sam


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500450400

7; 350E 300273s 25079G 200 -.5I 150-

100 -50 -


Turnov\--e- MMHg

I - q - T-Hg

0 , , / , , / , , I I I I I IA M J J A S 0 N D J F M A M J Jm1994 1995

Fig. 7. Total Hg and MMHg concentrations per dry mass n Daphnia. Error bars represent 95%confidence intervals estimated using a Z-statistic and a first-order approximation of standard error.

ples indicated that the majority of particulate matter collect-ed in both size fractions consisted of phytoplankton. Epilim-netic water samples were taken before and after turnover in1995 and inspected under a microscope. The majority ofparticles observed were phytoplank ton, principally Chroo-rnonas (9-16 pm in length) and Cryptomonas (25-35 pmin length). In addition, the similarity of MMHg concentra-tions in edible and inedible particle fractions on a per-massbasis (Fig. 5) suggests that MMHg diffuses into particles(e.g., it enters the cell cytoplasm of phytoplankton) as op-posed to sorbing to their surfaces. If surface sorption wasthe principal mode of MMHg association with particles, con-centration of MMHg per mass particle should be much lowerin the inedible fraction because the surface-area-to -mass ra-tio decreases as particle size increases. The relatively smalldifference between the two size fractions is a pattern moreconsistent with a mechanism in which MMHg diffusesthrough the entire particle. Mason et al. (1996) have shownthat the majority of MMHg associated with the diatom ThaE-assiosira wessjogii is found in the cell cytoplasm, ratherthan attached to the cell membrane, indicating that diffusionthrough the membrane is a more significant process thansorption to it.This evidence does not rule out the possibility that MMHgwas associated with bacteria or inorganic particles ra therthan with phytoplankton. In a California reservoir contami-nated with Hg, Slotton et al. (1995) found a positive corre-lation between peaks in zooplankton T-Hg concentrationsand concentration of Mn particles formed as a result of turn-over-related changes in redox conditions. Hurley et al.(19946) found that MMHg flux to sediment traps in a north-ern Wisconsin lake was correlated almost as strongly withFe flux as with flux of major Chl a derivatives. MMHg may

be associated with particles other than phytoplankton inDevils Lake, as well. This does not, however, rule out thpossibility that Daphnia are consuming th is particle-associ-ated MMHg. Daphnia may consume particles other thaphytoplankton if those particles are in an appropriate sizrange; for example Burns (1968) determined the range ofood sizes that Daphnia would eat by observing their consumption of plastic beads.Daphnia-Temporal trends in Daphnia MMHg concentrations on a per-gram basis (Fig. 7) were similar to trendin edible-particle MMHg concentrations and particle-asso-ciated MMHg masses throughout 1994 and 1995. Masses oindividual Daphnia varied little between sampling dates, strends in Daphnia MMHg on a per-gram basis were alssimilar to mass MMHg per individual Daphnia. An increasin Daphnia MMHg concentrations occurred near turnover i1994, concurrent with the increases in edible-particle antotal particulate matter MMHg concentrations. The increasin Daphnia MMHg concentrations that occurred near turnover in 1995 is not as large as that in 1994, a behavior thaseems to correspond with the longer-lasting period of ele

vated edible-particle MMHg concentrations and the increased whole-lake mass of particle-associated MMHg in1994.The large variability in mass MMHg per biomass Daphnia(Fig. 7) for the last three sampling dates in 1994 indicatesthat a more quantitative analysis is required to determine ithe mean concentration during the period before destratifi-cation begins is significantly different from the mean concentration after it has begun. For this analys is, the periodbefore destratification begins was defined as the first sampling in June of a given year through the first instance in


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1484 Herrin et al.Table 1. Mercury concentrations n different fractions, Devils Lake, Wisconsin.

Whole-water Hg MMHg in Hg in bluegill, ng Hg g- Hg in shiners, ng Hg g-concentration 1 m from edible MMHg in dry weight (ng Hg g-l dry weight (ng Hg g .bottom, ng Hg liter I* MMHg particles, Daphnia, ng wet weight)+ wet weight)+Hg as in all lake w Hg g- Hg g-l dry Hg as Hg asmethyl-Hg Total Hg particles, gt dry weight? weight-t methyl-Hg Total Hg methyl-Hg Total Hg1993 1.2 3.0 - - 837 814 - -

(185) (180)1994 0.52 2.0 0.23 9.6 186 509 575 483 506(115) (129) (115) (120)1995 0.07 1.2 0.10 9.1 100 265 324 287 336

(58) (70) (70) (82)*: Based on a running daily average of concentration between 7-12 September and 3-11 October. Total and MMHg data from 1993 are based on onsampling only, on 30 September 1993.-I-Based on a running daily average of Hg concentrations between 12 Septcmbcr and 15 November of the given year.$ Concentrations for fish born in the year listed and sampled the following spring.

which the dissolved oxygen concentration at the top of thehypolimnion was determined to .have increased to a valuegreater than 1 mg liter (i.e., the first evidence that the hy-polimnion was eroding). In both 1994 and 1995, this oc-curred in early October. All samplings later than this date ina given year were included in the postdestratification period.Mann-Whitney tests were performed on individual Hganalyses of Daphnia (two to four per sampling trip) dividedby the mean dry biomass per Daphnia determined for thatdate. Mean dry biomass was used because more variabilitywas observed in the MMHg determinations than in the bio-mass determinations. The Mann-Whitney test was used be-cause the group of samples from each period was small (n= 8 to n = 13), so it was difficult to determine if the sampleswere from a normal distribution. In 1994, MMHg concen-tration in Daphnia during the period before erosion beganwas significantly different (P = 0.0003) from the concentra-tion in the period after it had begun. In 1995, though, thedifference between the two groups was not significant (P =0.365). This test indicates that, in spite of the large vari-ability in 1994 analyses, the mean MMHg concentration inDaphnia was in fact higher after destratification commenced.The same cannot be said for 1995.The concentrations of MMHg in Daphnia collected beforedestratification were similar in 1994 and 1995 (see Fig. 7).During destratification, however, concentrations were higherin 1994 than in 1995, similar to the differences in hypolim-netic MMHg mass (see Fig. 6). MMHg concentrations inDaphnia, like those in particles, indicate that buildup ofMMHg in the hypolimnion manifests itself as an importantsource of MMHg to the lake food chain during destratifi-cation. Thus, there is strong evidence that introduction ofMMHg to the oxic zone during destratification has importanteffects on the lower food chain of Devils Lake, and that theextent to which lake destratification affects Daphnia MMHgconcentration is dependent upon the degree to which MMHgbuilds up in the hypolimnion. Slotton et al. (1995) and Reg-nell (1995) have also seen increases in, respectively, T-Hgand MMHg concentrations in zooplankton during destrati-fication. Furthermore, Regnell (1995) determined that not allof the MMHg stored in hypolimnetic waters was accountedfor in postturnover surficial sediments, indicating that only

a fraction of the MMHg mixed into the water column duringdestratification was quickly incorporated into the sedimentThus, much of this large quantity of MMHg has a differentfate. Although demethylation is another potential fate for thisMMHg, Daphnia MMHg concentration dynamics indicatethat a large fraction is incorporated into lower-food-chainorganisms.T-Hg concentrations in Daphnia exhibit a less consistenbehavior in relation to lake turnover (Fig. 7). In 1994 T-Hgconcentration in Daphnia increased as turnover approachedand decreased sharply thereafter, but in 1995 Daphnia T-Hgconcentrations decreased as turnover approached. Reasonfor these temporal responses of T-Hg concentrations inDaphnia are not clear.Planktivorous Jish-Hg concentrations measured in yearling bluegill and shiners represent an annual index of Hg

concentrat ions in lower food-chain organisms. In their firsyear of life, the diet of both fish species consists primarilyof zooplankton (Becker 1983). Our own analyses of thstomach contents of Devils Lake shiners show that at almosall times, their stomach contents consist of at least 15% zooplankton. In May and October at least 85% of their stomachcontents consisted of Daphnia (Gorski et al. in prep.). Concentrations of MMHg in fish were remarkably similar between species in a given year. As is the case for particles,Daphnia, and hypolimnetic water, fish MMHg concentrations declined from 1993 to 1995 (Table 1). Running averagconcentrations in Table 1 were calculated by determining tharea under the appropriate concentration plot (see Fig. 2between the dates listed and dividing this value by the number of days between sampling dates. This provides averagwhole-water concentrations I m above the bottom of thdeep hole for the period of peak stratification and averagparticle and Daphnia concentrations for the period of hypolimnetic erosion and turnover.Analysis results indicate that, within 1 standard error, thMMHg and T-Hg concentrations are not significantly differ-ent in the 1993 bluegill and 1994 shiners. For all sampleanalyzed, the Hg in fish was almost exclusively in the formof MMHg (Table 1). Bloom (1992) saw a similar abundancof MMHg in several saltwater and freshwater fish and shell


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A4ethyl-Hg in food chainsTable 2. Biomagnification factors between different compart-ments of the food web. Edible particlc and Daphnia concentrationsused are running daily averages for dates between 12 Septemberand 15 November of the given year. Fish concentrations used arefrom fish born during the given year and sampled in spring of thefollowing year.

19941995

Edible particles Duphnia to Daphnia toto Daphnia mimic shiners bluegill19 2.6 2.711 2.9 2.1

fish, though only muscle tissue of the various species wasanalyzed in that study.Bioconcentration factors, which compare biota Hg con-centration on a wet-weight basis to Hg concentration in wa-ter were not calculated for MMHg concentrations due to thefacts that epilimnetic MMHg concentrations in whole-watersamples were below detection limits and that all particle andDaphnia mass determinations were performed on a dry-weight basis. Assuming, however, that the operationally de-fined edible particles adequately represent the diet of Daph-nia, and that adult Daphnia adequately represent the diet ofthe two species of fish, biomagnifica tion factors (BMF) fromedible particles to Daphnia and from Daphnia to fish can becalculated using Eq. 1:

BMF = mass H&dry mass predatormass Hg/dry mass prey (1)Defined in this manner, BMF comprises a method for deter-mining the number of times Hg concentration increases asit moves up the food chain. BMF values for MMHg. fromedible particles to Daphnia and from Daphnia to the twospecies of fish studied a re based on the concentration datain Table 1 and are shown in Table 2. Values for edible par-ticles and Daphnia represent concentrations during erosionof the hypolimnion. Average BMFs from edible particles toDaphnia are over 7.5 times those for Daphnia to fish.The particle-to-Daphnia BMF values in Table 2 are withinan order of magnitude of the highest particle-to -zooplanktonvalues determined by Back and Watras (1995). They arethree to six times higher than those determined from theresults of Watras and Bloom (1992). These differences maybe in part due to the fact that in both of these studies thepresumed zooplankton food source was separated for anal-ysis by a different technique than the edible-inedible sepa-ration used here.

Multiyear Hg dynamics-Notable changes in Devils LakeHg concentrations occurred between the years covered inthis study as well as within them. Mercury concentrationsfor each fraction shown in Table 1 without exception de-creased as the study progressed from 1993 to 1995. The rateof decrease in Hg concentrations was greatest in whole-wa-ter, hypolimnetic MMHg concentrations. The fact that thisdecrease extends to planktivorous fish suggests that the rel-ative extent of hypolimnetic MMHg buildup in a given yeardirectly affects the food chain at least up to the level o fplanktivorous fish.

A question remains regarding the reason for the steadydecrease in MMHg concentrations from 1993 to 1995. Onepossible explanation relates to the extremely heavy rain-storms that occurred in 1993. It is likely that runoff intro-duced large amounts of inorganic Hg to the lake, and thatadditional Hg was introduced from newly flooded shorelineareas. Studies of newly impounded reservoirs indicate thatflooding formerly dry land adds inorganic Hg and organicnutrients to the associated water body (Johnston et al. 1991).In addition, measurable levels of MMHg have been foundin precipitation and in runoff (Rudd 1995), so rain eventscan be a source of MMHg as well as inorganic Hg to waterbodies. Increased inorganic Hg concentrations add to thepool of Hg available for methylation, and increased organicnutrient concentration can increase rates of primary produc-tion that can in turn lead to increased strat ified periods.The decline in hypolimnetic MMHg between 1994 and1995 corresponds to an increase in hypolimnetic dissolvedoxygen concentrations between those years, though furtherresearch is needed to determine if hypolimnetic dissolvedoxygen concentration and MMHg buildup are indeed related.If this linkage exists, then more traditional lake managementtechniques that improve hypolimnetic oxygen concentrations(i.e., that reduce the extent of stratification) may have somebenefit for reducing Hg levels a t all levels of the lake foodchain.Conclusions

Results from 2 yr of intensive sampling of lake water andbiota indicate that MMHg stored in the anoxic hypolimnionduring the summer stratified period is likely to represent animportant source of MMHg to the food chains of lakes withanoxic hypolimnia. This stored MMHg begins to becomeavailable to lake biota during erosion of the hypolimnion inthe fall and remains available until a short time after com-plete turnover. The concentration of MMHg in both zoo-plankton and algae can increase several times between peakstratification and complete mixing ; two- to fourfold increaseswere observed in this study. Results from planktivorous fish,zooplankton , and particles of different sizes indicate that themass of MMHg stored in the hypolimnion is directly relatedto the degree to which particle and biota concent rationschange. It appears that the body burden of MMHg in zoo-plankton and planktivorous fish is a result primarily of trans-fer from ingestion of lower-food-cha in organisms and par-ticles. This biomagnification occurs to the greatest degreebetween edible-sized particles and zooplankton .ReferencesBACK, R. C., AND C. J. WA~RAS.1995. Mercury in zooplankton ofnorthern Wisconsin lakes: Taxonomic and site-specific trendsp. 931-938. In D. B. Porcella, J. W. Huckabec, and B. Wheatley [cds.], Mercury as a global pollutant. Kluwer.BECKER,G. C. 1983. Fishes of Wisconsin. Univ. of WisconsinPress.BLOOM, N. S. 1989. Determination of picogram levels of methyl-mercury by aqueous phase ethylation, followed by cryogenicgas chromatography with cold vapor atomic fluorescence detection. Can. J. Fish. Aquat. Sci. 46: 1131-1140.


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1486 Herrin et al.-. 1992. On the chemical form of mercury in edible fish and LYNCH, M., L. J. WEIDER, AND W. LAMPERT. 1986. Measurementmarine invertebrate tissue. Can. J. Fish. Aquat. Sci. 49: lOlO- of the carbon balance in Daphniu. Limnol. Oceanogr. 31: 17-1017. 33.BLOOM, N. S., C. J. WATRAS, AND J. I? HURLEY. 1991. Impact ofacidification on the methylmercury cycle of remote seepagelakes. Water Air Soil Pollut. 56: 477-491.BURNS, C. W. 1968. The relationship between body size of filter-feeding c ladocera and the maximum size of particle ingested.Limnol. Oceanogr. 13: 675-678.GILL, G. A., AND W. E FITZGERALD. 1987. Picomolar mercury mea-surements in seawater and other materials using stannous chlo-ride reduction and two-stage gold amalgamation with gas phasedetection. Mar. Chem. 20: 227-243.CILMOUR, C. C., E. A. HENRY, AND R. MITCHELL. 1992. Sulfatestimulation of mercury methylation in freshwater sediments.Environ. Sci. Technol. 26: 2281-2287.HORVAT, M., V. MANDIC, L. LIANG, N . S. BLOOM, S. PADBERG, Y.-H. LEE, H. HINTELMANN, AND J. BENOIT. 1994. Certificationof methylmercury compounds concentration in marine sedi-ment reference material, IAEA-356. Appl. OrganometallicChem. 8: 533-540.

MASON, R. P., W. E FITZGERALD, ANDRE M. M. MOREL. 1994. Thebiogeochemical cycling of elemental mercury: Anthropogenicinfluences. Geochim. Cosmochim. Acta 58: 3191-3198.- J. R. RHINF&DER, ANU E M. M. MOREI.. 1996. Uptake,toiicity and trophic transfer of mercury in a coastal diatom.Environ. Sci. Technol. 30: 1835-1845.MATILAINEN, T. 1994. Involvement of bacteria in methylmercuryformation in anaerobic lake walers, p. 757-764. In C. J. Watrasand J. W. Huckabee reds.], Mercury pollution: Integration andsynthesis. Lewis.PATTEI~SON,C. C., AND D. M. SETTLE. 1976. Accuracy in traceanalysis: Sampling, sample handling, and analysis. SpeciaPublication 422. U.S. Bureau of Standards, p. 321-351.RECKHOW, K. H., AND S. C. CHAPRA. 1983. Engineering approachesfor lake management, Vol. 1: Data analysis and empirical mod-eling. Butterworth.HURL~~Y, . I?, J. M. BENOIT, C. L. BABIARZ, M. M. SHAFER, A. W.

ANDREN, J. R. SULLIVAN, R. HAMMOND, AND D. A. WEBB.1995. Influences of watershed characteristics on mercury levelsin Wisconsin rivers. Environ. Sci. Technol. 29: 1867-1875.-, D. l? KRABBI~NHOFT,C. L. BABIARZ, AND A. W. ANDREN.1994~. Cycling of mercury across the sediment-water interfacein seepage lakes, p. 425-449. In L. A. Baker [ed.], Environ-mental chemistry of lakes and reservoirs. ACS Advances inChemistry Series 237.

REGNELL, 0. 1995. Methyl mercury in lakes: Factors affecting itsproduction and partitioning between water and sediment. Ph.D.Thesis, Lund Univ.

-, M. M. SHAFER, S. E. COWELL,, J. T. OVERDIER, P E.HUGHES, AND D. E. ARMSTRONC. 1996. Trace metal assessmentof Lake Michigan tributaries using low level techniques. En-viron. Sci. Technol. 30: 2093-2098.-, C. J. WATRAS, AND N. S BLOOM. 1994b. Distribution andflux of particulate mercury in four stratified seepage lakes, p.69-81. In C. J. Watras and J. W. Huckabee [eds.], Mercurypollution: Integration and synthesis. Lewis.JOHNSTON,T. A., R. A. BODALY, AND J. A. MATHIAS. 1991. Pre-dicting fish mercury levels from physical characteristics in bo-real reservoirs. Can. J. Fish. Aquat. Sci. 48: 1468 -1475.LATHROP, R. C., K. C. NOONAN, P M. GUENTIIER, T. L. BRASINO,AND P W. RASMUSSEN. 1989. Mercury levels in walleyes fromWisconsin lakes of different water and sediment chemistrycharacteristics. Tech. Bull. 163. Wis. Dept. Nat. Resour.-, I? W. RASMUSSEN, AND D. R. KNAUER. 1991. Mercuryconcentrations in walleyes from Wisconsin (USA) lakes. WaterAir Soil Pollut. 56: 295-307.

RUDD, J. W. M. 1995. Sources of methyl mercury to freshwaterecosystems: A review. Water Air Soil Pollut. 80: 697-713.SLOTTON, D. G., J. E. REUTER, AND C. R. GOI.DMAN. 1995. Mercuryuptake patterns of biota in a seasonally anoxic northern Cali-fornia reservoir. Water Air Soil Pollut. 80: 841-850.VERTA, M., T. MATILAINEN, P. PORVARI, M. NIEMI, A. UUSI-RAUVAANU)N. S. BI-OOM. 1994. Mcthylmercury sources in boreal lakeecosystems, p. 119-136. In C. J. Watras and J. W. Huckabec[eds.], Mercury pollution: Integration and synthesis. Lewis.WATRAS, C. J., AND N. S. BLOOM. 1992. Mercury and methylmer-cury in individual zooplankton: Implications for bioaccumu-lation. Limnol. Oceanogr. 37: 1313-1318.-, AND N. S. BLOOM. 1994. The vertical distribution of mercury species in Wisconsin lakes: Accumulations in planktonlayers, p. 137-152. In C. J. Watras and J. W. Huckabee [eds.]Mercury pollution: Integration and synthesis. Lewis.-, AND OTHERS.1994. Sources and fates of mercury and meth-ylmercury in Wisconsin lakes, p. 153-177. In C. J. Watras andJ. W. Huckabee reds.], Mercury pollution: Integration and syn-thesis. Lewis.WISCONSIN DEPARTMENT OF NATURAL RIZSOURCES. 994. Healthguide for people who eat sport fish from Wisconsin watersPUBL-IE-019 4194REV.

Received: II August 1997Accepted: 5 February 1998

hypolimnetic methyl mercury and its uptake

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