Chronic toxicity and bioaccumulation of mercuric chloride in the fathead minnow (Pimephales promelas)

143

CHRONlC TD%lCiTY AND BIOACCUMULATIONOF MERCURIC CHLORlDE IN THE FATHEAD MINNOW WMEPHALES PROMELAS)

Fathead minnows (Rmepkzkpom&s)

were exposed IO various concentrations of mercuric chloride

in water to determine its acute and chronic (including reproduction) toxicity and to measure bioaccl.muiation. &an U&J valus of kip,Clz for juvcnik fatheads were 1(58.112,!?4, and 74 ~g Hg/l at 4,5,6, md 7 dryt. mpr(livdy. Follow=@ 41 wk of ~rposure to 0.26 to 3.69 pg/l. females were significantly smaller than controls at all con~ntrations except 0.50 #g/l and males showed reduced growth at 3.69 a/l. Over half of the fish at 2.01 and 3.69 &I were severely stunted and scoliosis was prevalent among these stunted fish. NO spawning oxurrui at and above 1.02 pg/I and the number of spawning pairs was reduced at 0.26 and 0.50 &I rcs&ng ia total egg production of 46 and 54%. respectively, of control. Thirty-day growth of ~nd-~~tion larvae (from transferred control embryos at and above 1.02 #g/l) was ~~~~y reduced at all mercury ~n~trations tested. No effecu on hatchability or larval survival wtrc detected. Chroticai&expoA fii accunndated mean whole body rorai mercury residues of 1.36 to 18.80 a/g of tissue {wet weight) from 0.26 10 3.69 @g/l, respectively. Uptake appeared proportional to water Eanrrntration; biincentration factors varied between 4380 and 5680. The influence of diet on mercury toxicity and bioaccumulattion was studied during two &I-day larval fathead exposures. CMrtdI rcciuccdgrowth and possibly enhanced toxic effects occurred in larvae on the dry trout starter diet compar&l to larvae fed on Arremiu. Mercury uptake appeared influenced by the severity of toxic effects. Key words: fathaid minnows; mercuric chloride; acme and chronic toxicity. bioaccumuidlion: dier

MC~CUW contamination of aquatic ecosystems is a serious and ecologically complex environmental problem. Mercuryoccurs in water in several organic and inorganic forms, depending on environmental variables, From a toxicological standpoint, methylmercuryis considered the most important; but from a mass balance point of view, the inorganic forms, largely sediment bound and less available to the biota, predominate(Hartung and Dinman, 1972;Jernel6v et al., 1975). Changes in chemical form can occur through bioIogica1 interconversions between organic and inorganic compounds (JernelW et al., 1975; Furutani et al., 01~5~/82/~~/SO2.75

@ Etsevier Biom~ica~ Press

144

1980; Rudd and Furutani, 1980)and through changes in physicochemical environmental variables (Beijer and Jernelbv, 1979). The loweringof pH, for example (as when a river of neutral or sli8htly alkaline pH enters an acidified lake) can cause a release of free ion from complexes and particulate matter. Under such circumstances, mercuric ion would then be more biologicallyavailable and could.become more toxicologically important. Past studies, mostly involving short-term exposures, have shown that inorganic mercury bioaccumulates rapidly and is toxic to fish (MacLeod and Pessah, 1973; Kramer and Neidhart, 1979; Wobeser, 1975; Lort, 1978). Matida (1971) studied mercury accumulation and toxicity in rainbow trout exposed to mercuric chloride for 14wk; however, no studies involving continuous exposure of fish through all life stages have been published. The present study was conducted to determine the acute and chronic (life cycle) toxicity of inorganic mercury (as mercuric chloride) to the fathead minnow (Pimcphules promekrs Rafinesque) and to measure the mercury residues accumulated in the whole body from the aqueous exposures. As part of this investigation, the influence of diet on mercury toxicity and residue accumulation also was studied.

MATERIALS ANI) METHODS

Exposure system

The flow-through exposure system consisted of a proportional diluter (Mount and Brungs, 1967) thar delivered five mercury concentrations and a lake water control to duplicate 90 cm x 30.5 cm x 30.5 cm glass tanks, each receiving 500 ml/5 min cycle. During the spawning period, two 30.5 cm X 15.2 cm x 30.5 cm glass chambers with stainless steel mesh ends were p!aced in the rear third of each tank for iarval exposure studies. Concurrently, the water supply to each tank was divided between these chambers and then flowed through the screen ends into the main tank. The 90% volume replacement times for the main tank and the larval chamber were approximately 14 h and 5.5 h, respectively (Sprague, 1969). Unfiltered Lake Superior water was heated in a stainless steel headbox to 25f 2 “C before entering the diluter. Water temperatures were checked daily in the test tanks and ranged from 23-24’C during the acute toxicity test and from 23-26 “C during the life-cycle tests. Mean ( f SD ) chemical characteristics of the test water, determined weekly according to standard methods (American Public Health Association et al., 1971) were total hardness, alkalinity, and acidity o! 45.7 * 1.0, 42.8 i 2. I, and 3.1 f 0.4 me/l as CaCO3, respectively; and pH of 7.2-7.5. Mean dissolved oxygen levels were 86 * Solosaturation with a range of 62-98% over the entire study period.

146

minnows. At the start of each exposure, fifty 4- to 6dayold fathead larvae were randomlydistributedto each experimental tank. Curing the first experiment, the bae werefed to satiation Glenco Mills trout chow starter 2 or 3 times daily. After 30 days of exposure, survival and growth (total !ength) were determined by the photographic method used by McKim and Benoit (1971). McKone et al. (1971) obw~ed that a large portion of the mercury accumulated by Boldfish from water wasconcentrated in the external mucus. Therefore, after 60 days, mercury-exposed fiih were put into clean Lake Superior water for 1 h to remove surfacemercury.All fish were killed, their w&hts and total lengths measured,and the number of deformed fish determined. Fish were homogenized and frozen for later residue analysis. Two weeks later, a second experiment was begun in the same exposure system. These fish were fed live newly-hatched Arremiu nauplii 4 times daily for 30 days then live nauplii and frozen adult Artemiu 4 times daily for the next 30 days. Survival and growth data were collected as in the first experiment except at 60 days I5 fish were randomly selected front each tank to continue through a reproductive cycle. The remaining fish were killed, measured, and frozen for residue analysesas previously described. Throughout the remainder of the chronic exposure, fish were fed frozen Artemia 3 times daily supplemented with live laboratory-cultured mixed zooplankton (mostly cladocerans and copepods) 4 times weekly. On day 143 of exposure the male and female fish in the control and 3 lowest mercury concentrations wei paired to obtain information on individual reproductive performance. With the larval chambers in the rear third of each tank, stainless steel mesh was used to divide the remaining area into 4 compartments and one pair of minnows was placed into each with one spawning tiie. The fish in the two highest mercury concentrations were not thinned to 8 fish/tank because they were considerably smaller, extremely variable in size, and showed no visible sexual dimorphism. Test procedures during the spawning period were those recommended b:r U.S. EPA (1971) with spawning activity recorded daily and hat
AI1 date werz a~yz~ by Student’s f-test for $iff~renee§ between rephcates. No significant differaxg w@e detected (P = WI!+);therefore, replicate data were combined, 1.C~ values were computed from the acutie toxicity data by the trimmed Spearman-Karber method (Hamilton et al., 1977). Survival, growth, deformity, spawning, and ~at~~abilit~ data were analyzed by one-way analysis of variance (with arcsin t~~~~~~~t~u~ of proportion data) and ~nnett~s procedure (Steel and Turrie, LW).

Mean LCRI values (9% Cf) of HgCh for juvenile fathead minnows were 168 WO-282). 112 (Y&-164),I34(68-104). and ‘74(Q-90) fig/l after 4,5,6, and 7 days, respectively. Results of thcl4Rkiay

merazrk chloride exposure of fathmds fed dry trout starter are presented in Table t. Survival was signi~c~t~~ reduced compared to the control MI& at tfte M&eSt ~a~trat~on~ 4.51 @gII(P < tU.3). Most of the mortality occurred within the first 30 days of ~~~~~~. After 30 days of exposure s~g~~~~a~~ growth retardation was seersat and above f .2T pgfi. After 6@days fish at 0.58 pgii were &so sjg~~~~~t~y smakr (kss in total length and weight) than control fW {P < &OS], & ~~~~s~ng incidence of spinal curvature was seen with ~o~rea~~~~ mercury catteentration among survivors at 60 days; however, this teratogenic effect was statistically different from controls only at the highest exposure level, 3.51 iugil. Results of the 4Wday exposure of ArWrria-fed fish are presented in Table II. Mean mercury corr&%ntrations during this exposure were W-20% lower than during the dry truut fbad ~~rirnent. No significant difference in survival was seen at any mercury ~~~t~~~o~ [P ) &OS).After 30 days exposure, fish at and above Wu p@ were s~~~c~t~y shorter than the controls (P of O&Q. However, after 60 days, m&a t&se at 2.01 a& 3.69 pg,4 were ~~~~~~~~t~~ shorter an9 Eghter is ~ve~~~~ t&an eontrrrfs (P < &OS),No deformed fish were ~bs~ve~ at 50 days, C~~~~ @fthe two @-day exposures fTables I and 11)rev&s tit&zdifference in survival not ~t?~bu?able to mercury expas~re” control survival at 60 days ws 94% and 96% for dry trout starter- and Arre&r;t-fed fish, respectively. Howeve&. growth was natio&ly improved on the Artmicr diet among all experimental groups. ArrPmicr~fedfish were also more uniform in size than those on the trout starter, ~rtj~~~r~y in controt and the lower mercury concentrations. For exampb, the relative standard hviatiuns @SD= standard deviation expressed as a percem of the mean) for @-day lengths and weights were t2.8 and 41.6% respecrively, for

30 Days % survival Toti length (mmj *:D(N)

94 16.5 & 3.5 @If

0.I.W * 0.062 (82) l/%3 8.4

27.5 * 4.0

0.x2 * 0,103 (76) 3/8% 3.4

7/73 9.6

0.143b * 0.082 470)

83 25.Ob f: 3.5 (83)

a8 WJt

73 24.Ob i 4.5 (73)

86 16.0 t 3.0 (86)

89 17.5 i 2,s (89)

76 15.Ob zt 3.5 (73)

131505 26.0

O.r31b* 0.090 (47)

0.131b f 0.072 (71) lOf72 13.9

50s 23.0b 2 5.0 (JOI

52s 12-W i 2.5 02)

72 22.W f. 4.0 (72)

73 14.53 * 2.5 (73)

Thirty- and 6&zo_dny survival and growth of dry trout starter-fed fathead minnows exposed to various water concentrations of H&Iz. Rcplicata combin&. _--_-_--” H&I, concentration (,? f SE as pg H&/I) .__I 2.43 f 0.34 4.51 f 0.62 0.56 * 0.09 1.27 f: 0.16 0.31 zt 0.05 Control (< 0.01)

TABLE f

f

&&+&u-fed controls and 18.0 and 65.6870,respectively, for starter-fed control fish. Growth (weight only) and reproduction data from the chronically-exposed Wsh and survival and growth data for the second generation fish are presented in Table 111.No statistical differences in survival were detected at any mercury concentration during the 41.wk chronic exposure. Significant growth retardation was detected at 3.69 fig/l among males and at all mercury concentrations except 0.50 cg/l among females after 41 wk of continuous exposure (P < 0.05). At 2,Ol and 3.69 fig/l gonads of many of the smallest fish were undeveloped and sexes could not be accurately distinguished by gross autopsy. These fish have been designated as ‘undeveloped’ in Table III. Among these ‘undeveloped’ fish, scoliosis (lateral spinal curvature) occurred in 2 of 11 (18%) and 5 of 13 (38~) at 2.01 and 3.69 fig/l, respectively. Spawning data show significant reproductive impairment of chronically-exposed fathead minnows. No spawning occurred in the 3 highest mercury concentrations; in fact, no femaies were mature in either 1.02 or 3.69 pg/l at termination. However, 3 of 5 females at 2.01 lg/l had at least partially developed ovaries. During the 4.mth spawning period, all 8 control females spawned but only 3 at 0.26 @g/land 6 at 0.50 pg/l spawned. Total egg production at 0.26 and 0.50 pg/l was 46 and $4%, respectively, of control. The number of eggs spawned per female viuied considerably at each concentration (ranges given) and no significant differences were seen in either mean eggs per spawning female or mean eggs per spawn (P > 0.05). Hatchability and larval survival at 30 days post-hatch was not signi~c~~tly affected at any mercury concentration either from embryos incubated at parental exposure concentrations or from embryos transferred from control and incubated at concentrations where no spawning occurred. However, growth of larval fish was reduced. After 30 days of exposure, second generation larvae at all mercury concentrations were significantly lighter and shorter than controls (P c 0.01). Residue accumulation Results of the residue analyses are summarized in Table IV. After 60 days of exposure fathead minnows fed dry trout starter accumulate mean whole body mercury concentrations ranging from 0.80 pgfg at the lowest water concentration (Cl.31fig/l) to 4.18 gg!g at the highest concentration (4.51 &g/l). Although the residues increased with increasing exposure concentration, the relationship was nonlinear: bioconcentration factors (BCFs) decreased with increasing water concentrations from 2580 to 930 for the lowest to the highest water concentrations, respectively. Exposed fish maintained on the Artemiu diet accumulated mean whole body residues after 60 days ranging from 0.62 Fg/g at the lowest concentration (0.26 &g/l) to 7.M)&g/g at 3.69 &g/l. In contrast with the dry-food experiment, uptake appeared linear over the exposure range; BCFs varied between 2060 (at 3.69 fig/l)

---

50s 27.5b i 2.0 (20) 0.192b f 0.041 (20)

85s 25.Ob zt 2.5 (51) 0.153” f 0.045 (51)

70s 21.5b + 2.5 (28) 0.09Ob + 0.024 (28)

0 35s (5)

f Replicate = group of SOembryos incu~~i~ from one spawn. BTransferred from control as embryos < 24 h after spawning.

85 29,ob i 2.0 (51) 0.233” f 0,055 (51)

0 88s (4)

0.96s * 0.42 (f3)

3.01. f 0.66 (41

3.69

CFive of 13 exhibited scoliosis.

50 28.Sb + 1.0 (30) 0.192b zt.0.026 -(30)

0 91s (4)

861 (180-1483) 198 5165 78 (16)

4.41 f 2.29 (8) 1.56. f 0.40 (5) 0.84d f 0.29 011 O/S

2.01

bSignificant)y fess than control, P < 0.01. ‘Sex could not be distinguished by gross autopsy. dTwo of Ii exhibited scoliosis.

0.26f f 6058 (87) -_----

037)

R7 30.0 It 2.5

O/8

6.23 f 0.71 (7) I .7Ou f 0.37 (81

1.02

618

5.52 f 1.00 (7) 2.08 f 0.20 t8)

0.50

aSignifkantly less !han control, P < 0.05.

Weight (gf R &SD(N)

*SD(N)

Length [mm) x

Progeny after 30 days of exposure Vo survival

(N of replicate@

1485 (365-2590) 223 4455 60 (18)

1204 @o-2204) 214 %35 83 (32)

Mean eggs per spawning ferntie (range) Mean eggs per spawn Total eggs spawned Mean % hatch

5.75 z.t 1.18 (8) I .!w f 0.36 (7)

318

5.38 i 0.97 (81 2.32 f 0.26 (61

818

SD (N)

0.26

Proportion of females tha1 spawned

Undevelopedc

Females

Mali3

weight fp) x +

Chronically exposed fish

Control (< 0.01)

H&l2 concentration (as ~g Hg/l)

of fathead minnows after 41 wk of exposure to mercuric chloride and survival and growth of their progeny after 30 days of exposure. Replicates combined.

HI

Growth and reproduction

TABLE

and 2590 (at 1.02 pg/l). After 41 wk of mercuric chloride exposure, mean whoie body residues were approximately double those at 60 days, ranging from 1.36 @g/g to 18.80 fig/g from the lowest to the highest exposure ievels. BCFs suggest that uptake was directIy proportional to water concentr,ation.

DISCUSSlON

Inorganic mercury, added as HgCl2, is highly toxic to the fathead minnow with low pg/I water concentrations resulting in decreased survival, spinal deformities, reduced growth and impaired repr~uction. The Cday (96-h) LCSOvalue for juvenile fathead minnows of 168pg/l in this study is similar to acute toxicity values of mercuric chloride reported for other freshwater fish. MacLeod and Pessah (1973) reported 96-h LCSOvalues for juvenile rainbow trout (S&no gairdnri) of 400, 280, and 220 @g/l at 5, 10, and 20°C. Also with HgC12and rainbow trout, Matida et al. (1971) obtained a 96-h LCso of 210 pg/l at 16.7”C and Wobeser (1975) reported a 24-h LC50of 903 &g/l at 1O’C. Lorz (1978) reported a 96-h L&t of 240 @g/lfor juvenile coho salmon (O~co~~y~c~~~kifu&h). Hanumante and Kuikarni (1979) reported a 96-h LCJOof lXl0 @g/I for the Indian food fish, Chaana guchua. The present study showed that chemical exposure to mercuric chloride reduces survival well below these acute values with only 52% of larval fatheads survivin~~30days of exposure to 4.51 fig/l. In this study, a sjgnificantly higher incidence of spinal curvatures occurred among the dry trout starter-fed fathead minnows at 4.51 pg/l. This deformity also was noted among the severely stunted fish chronically exposed to 2.01 and 3.69 ~8/1. Teratogenic and embryotoxic effects of HgCIz to tdeosts have been previously reported. Heisinger and Green (1975) reported reduced hatch, inability to uncurl, and other abnormalit.ies in Japanese medaka, O~yziaslatipes, tollowing embryonic exposure to 15 pg Hgll. Weis and Weis (1977) observed developmental anomalies, including reduced axis formations and forebrain defects, among Fundulus heteroclitusembryos exposed to 10 to 1000cg Hg/l as HgC12and reported retarded growth among embryos that did not exhibit malformations. Recently, studying this same marine k~l~ifish,Sharp and Neff f 1980)found reduced hatching success at and above 20 pg Hg/i and increased incidence of lateral spinal curvature at and above 30 pg Hgi’l among eleutheroembryos emerging following chronic embryonic exposure (32 days). Growth and reproduction were even more sensitive indicators of the direct toxbcity of inorganic mercury. Matida et al, (1971) observed retarded growth of juvenik rainbow trout exposed to HgClz in water at 21 &I and reported a ~rmissible con~ntration range, based on growth, between 2.1 and 21 fig/I. In the present study, mercury exposure at all concentrations tested, 0.26 to 3.69 &g/l, reduced growth of second generation larvae. Among the first generation fish that

IV

2.43

I.27

0.01 konIrol) 9.31 &SK

..__,

Dry trout starter Arremiu

1.31 1.98 2.75 4.18 0.22

Dry trout starter Dry trout starter Dry trout starter

_ ..,.

2.64 f. 0.08 4.76 2. 0.17 7.60 + 0.36

0.62 i 0.03 I.24 + 0.06

* 0.04 2 0.10 -c 0.20 + 0.15 f 0.01

0.12 c 0.01 0.80 + 0.04

x i St

60-Day exposure

(fish per sample)

N samples

2370 2tJiIO ,Ix___

24AO 2590

2390

I560 1130 930

2580 2260

--

BCF

2*x4 ” 0.13 4.47 f 0.19 9.41 t 0.67 1X.80? 2.34

649) 8 (99 6(l) 4 (19

5 (19

8 /II

N sampits (fish per tample)

I .36$20.04

SE

0.32 + OS91

-1

Xk

41-wk exposure

_. .._._.“...^ ._-

Mercury residues, pg/g of tissue (wet weight)

Dry trout starter Dry trout starter

Ar1t?m& Arremru Artemiu 2.01 Arremiu 3.69 Arfemiu I._. --.. --_- -- -.. - .“, .__

4.51 c: 0.01 bXlnIrol) 0.26 0.50 i&t

CL

frf5 Hg&

Diet

body c~n~n~ra~jons of total mercury in farhead minnows fed different diets during exposure 10 WgC12in water.

Mean exposure ~nn~fra~in~

W&e

fABLE

5680 43RO 46M9 5100

5230

BCF

wcy 4-6 days old at the initiation of exposure, growth retardation was ~monstmt~ as low as 0.50 ~11 during the first 60 days of exposure,Followi~41 wk of ~ntlnuous exposure, females were ~~n~~cantly smaller at all ~n~ntrations ertwpt 0.50 &I (the next to lowest concentration) while males were si8niRcantly smaller only at 3.69 &I, Fart of this apparent diffcremial sensitivity to mercury by sex could reflect the nonrandom ptcspawn@ thinniv when mnlcs showing obvious

sccondwy

sexual characteristics, probably the largest and fastest growing individual were selected, Reproductive inhibition of chronically-exposed fish was observed at all mercury

soucentrations tested. This effect on reproduction rcsultcd mainly from the failure

of some mercury-exposed fish to mature and spawn as no differences were detected in either the size of the spawns or the mean eggs per female that spawned.

Reproductive impairment was seen in a previous HgClz-fathead minnow chronic exposure (Snarski and Olson, unpubl. data) where spawning activity occurred sporadically only among the three lowest concentrations, ranging from 0.75 to 2.75 fig/l. At 0.75 pg/l the total eggs spawned was only 40% of the control; at 1.50 fig/I no spawning occurred; and at 2.75 pig/l spawning occurred in only one duplicate tank, total eggs spawned was only 5% of the oontro!, and the mean spawn size was reducedto only 30% of the control (69 eggs vs. 209 eggs). While inorganic mercury generally has been considered less toxic than methylmercury, the chronic toxicity of HgCl2 to fathead minnows reported here compares to the chronic toxicity of methylmercuric chloride (MMC) to brook trout (salvelinus~ontinalis) reported by McKim el‘al, (19%). Their maximum acceptable toxicant concentration (MATC) of MMC for brook trout was between 0.29 and 0.93 fig/l. At a MMC concentration of 0.93 rg/I, s~ond-generation trout developed deformities and all but one female died during spawning. Survival, growth, and reproduction were not affected at 0.29 cg/I over the 3-generation study. In our study, the MATC of HgCl2for fathead minnows is < 0.26 Pg/l, our lowest mercury concentration. Diet appears to indirtzrtly influence residue levels by in~uencing mercur}’toxicity. Bioconceutration factors appeared inversely related to the severity of the toxic effects. Similar BCFs at the lowest concentrations in both 60-day experiments showed that uptake was proportional to water concentration even though Artemiufed fish weighed 4 to 6 times more than trout starter-fed fish. In the dry trout starter experiment, BCFs decreased with increased water concentration end growth was progressively reduced beginning at 0.58 &g/l, thu lowest observed effect concentration (EOEC), up to 4.51 fig/l, where both growth and survival were affected. In the Artemia experiment, toxic growth reduction at 60 days was first detected at 2.01 pgcg/l aud BCFs were generally similar with only the BCF at the high concentration slightly iswer (2060 w. 2370-2590). Possibly the dry starter diet was nutritionally deficient and enhanced the mercury toxicity. Or, alternatively, the richer Arremia cilietmay have allowed the fish to compensate for the toxic effects as

the fish reached a less sensitivetife stage while the smaEterfish, on the more marginal trout starterdiet, were unable to overcomethe initial growth suppression. Limited data tend to support the second interpretation. An apparent drop in

sensitivity of SfrfeM-fed fish ocxurred between 30 and 64)days with the LOEC changitu from 0.50 p&4 to 2.01 MA. Wobeser (1975) previously reported an increase in relative resistance to mercury compoundswith increasingageand size of rainbow trout. Nutritional inadequacyof the dry starterdiet for larval fatheadsmay also have contributed to the developmentof terataat 60 daysthat werenot observed at 60 days among Artomia-fed larvae exposedto mercury, Mean whok Body mercuryresiduesaQer 41 wk of e..po%treof 1.36 to 5.80 ggb‘g werea~~~imat~y doublethoseafter 60 days of e;\rposure.BcFs from 4380 to 523tI vacate that &due kveis were ~e~r~~~ pro~rtjoRa1 to water ~ncentration. Olson et al. fW?YtS) reportedwhole body totat mercuryresiduesin fathead minnows of 1.47 to 10.90 a/g following 48 wk of exposure to methylmercury water ~~n~ntrations of 0.018 to 0.247 &l, reqxztively. BCFs decreased from 8.2 x i@ to 4.4 x tOJ for ttu lowest to highest water concetttration,respectively. These two

studies showed the uptake of mercury to be from 8.5 to 16 times higher from exposure.,Others have also reported slower and Iesseroverall uptake of mercuryfrom inorganic exposure compared to ~h~~merc~r~ exposure (Matida et al., 197i; Kramer and N&&art, 197%. The present study shows mercuric chioride to be toxic and substantially bioaccumtdatedin fish exposed to water concentrationsas tow as 0.26 pgfl overan extended period. These fittdiugs suggest the potential danger to fish populations if ionic mercury is released from ~~t~inat~ sediments by changing envjro~mentai variablessuch as lowered pH associated with acid p~ipitatjon.

methytme~ury exposure than from H$+

We thank Ms. C. Lindberg for valuable daily assistance with biological procedures and routine chemical analysis and staff of the Environmental Research ~aboratury-~iuth for advice and review of the manusxipt. REFERENCES

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