Wisconsin DNR, 3911 Fish Hatchery Road, Fitchburg 53711. A. K. Hanson ..... to the University of Wisconsin-Madison. ...... waters of rural Wisconsin lakes. Sci.
Limnol. Oceanogr., 38(6), 1993, 1227-1241 0 1993, by the American Society of Limnology and Oceanography, Inc.
Mercury biogeochemical cycling in a stratified estuary R. P. Mason’ and W. F. Fitzgerald Department of Marine Sciences, University of Connecticut, Avery Point, Groton 06340
J. Hurley Wisconsin DNR, 3911 Fish Hatchery Road, Fitchburg 53711
A. K. Hanson, Jr., P. L. Donaghay, and J. M. Sieburth Graduate School of Oceanography, University of Rhode Island, Narragansett 02882 Abstract Total Hg in the permanently stratified Pettaquamscutt estuary was 40% of the total Hg. Reactive Hg (Hg,) was generally 50% for three of the 26 samples and ~30% for 65% of the samples. The agreement was generally poorer for the anoxic water samples. Most of these samples contained floc particles and were difficult to subsample representatively. For these samples the calculated total concentration is likely to be more representative of the actual concentration than the measured Hg,, as both Hg,, and Hg, were based on larger sample sizes. Precision for DGHg and MMHg analyses were similar, being between 10 and 15%. MMHg,, was determined by derivatization, followed by cryogenic gas chromatography (Bloom 1989; Mason and Fitzgerald 199 1; Fig. 2). A 400-ml subsample of filtered water was acidified and extracted by hand-shaking for 5 min with 2 x 40 ml of methylene chloride. Extraction isolated MMHg from chloride ions that interfere with the ethylation procedure (Bloom 1989). The MMHg was back-extracted into the aqueous phase, derivatized to methylethylmercury with tetraethylborate, and purged from solution and trapped on a Carbotrap column. The overall procedural blank,
1231
Hg in the Pettaquamscutt Concn
0
IO
(PM) 20
30
0
Concn
(PM)
2
6
4
8
10
0
Concn
(PM)
2
6
4
Concn 8
10
0.0
(PM)
0.2
0
0.4
0
I
L-2
16
1
(4 I
I
1 2 Fluor. 10 20 sigma-t
sigma-t
F (e> 1
Fig. 4. Concentration and distribution of Hg species and hydrographic parameters (sigma-t, dissolved oxygen, electrode potential, and fluorescence) in the lower basin of the Pettaquamscutt estuary on 4 September 1990. Symbols: His, -0; Hgp-0; Hg,-•I; MMHg,--n; MMHg,-A.; Hg”-R.
determined by re-extraction of the water, was typically 5 pg for a 400-ml sample. The detection limit was 50 fM. For MMHg,, 2 ml of a 25% KOH solution was added to the 0.8~pm quartz filter to decompose the particulate matter. After 24 h of digestion at room temperature, 100 ml of Q water followed by 2 ml of glacial acetic acid was added to neutralize the solution; 30-40 ml of the solution was then Concn
0
0
10
Concn
(PM 20
30
0246810
(PM)
analyzed by the ethylation technique. This digestion method provides a quantitative recovery for fish tissue (Bloom 1989) and gave > 80% recovery for spiked samples in this study. The detection limit was estimated at 50 fM. In 199 1, samples (0.3-l .2 liters) were filtered through Whatman GF/F glass-fiber filters for phytoplankton and bacterial pigment examination. Filters were frozen and shipped
Concn 0
2
(PM) 4
Concn 6
0.0
0.2
(PM)
0
0.4
0
1 2 Fluor. 10 20 sigma-t
sig-na-t
(4 16
Fig. 5. As Fig. 4, but on 8 August 1991. (Electrode potential not determined.)
(4
1232
Mason et al.
Table 2. Recently measured concentrations of Hg in coastal waters. Location
Rhone River Gironde estuary and Garonne River St. Lawrence River Framvaren Fjord Saanich Inlet
Concn range (PM)
Hg, 2.9-l 5.8 Hg, 22-103, Hg ,.[: 12-38 Hg.,,l 2.4-12 Qt.1 l-8.5 Hg, 2.5-25
to the University of Wisconsin-Madison. Samples were extracted with 90% acetone, filtered, and analyzed with reverse-phase high performance liquid chromatography (Hurley and Watras 199 1). Chlorophyll a (algae), pheophytin a, bacteriochlorophyll a (purple sulfur bacteria), and bacteriochlorophyll e (brown sulfur bacteria) were found. Water (50-l 00 ml) was also filtered through 0.4~pm Nuclepore filters to determine suspended particulate matter (SPM). Salinity, temperature, pH, Eh (1990 only), sigma-t, dissolved oxygen, transmission, and fluorescence were recorded before, during, and after sample collection. Dissolved oxygen was measured by Winkler titration. A series of high resolution profiles along the axis of the basin in August 1990 showed little horizontal variability in physical, chemical, and biological parameters along density surfaces within and below the pycnocline. Because of the basin structure (Fig. l), tidal influence is restricted to the mixed layer and tidal fluctuations are typically < 20 cm (Gaines 1975). Donaghay et al. (1992) concluded that vertical processesdominate the water of this basin and that horizontal advection and diffusion could not account for the observed vertical profiles. In addition, diel variations in CO, (Hanson pers. comm.) and CH4 (Scranton et al. in press), and the lack of any diel variability in salinity and temperature demonstrate the importance of in situ processesin shaping the vertical profiles of chemical species. Results The mixed layer is typically 2-3 m thick at the sampling site (Figs. 4e, 5e). In September 1990, the density profile showed that the water was well mixed to 2.5 m and that there was a rapid change in density between 2.5 and 6.5 m (Fig. 4e). Below 6.5 m, the density increased slowly with depth, as salinity increased and temperature decreased. The mixed-layer salin:
Reference
Cossa and Martin 1991 Cossa and NoEl 1987 Cossa et al. 1988 Iverfeldt 1988 Lu et al. 1986
ity was 19.1%0,while the salinity at 10 m was 26.4o/oo.Temperature decreased from 24°C at the surface to 9°C at 10 m. Across the pycnocline, the temperature gradient was - 3.1”C m-l (z positive downward) and the salinity gradient 0.73a/oom- l. The increasing salinity and strong temperature gradient maintain a stable stratified system (6p/6z = 1.4 kg m-4 for the pycnocline). In August 199 1, the profiles were similar (Fig. 5e) but with a higher density than in September 1990, a result of both higher temperature and salinities. The salinity increased from 19.7o/ooat the surface to 28.2o/oo at 16 m. The density gradient was more pronounced as a result of the higher salinity and temperature gradients (6p/6z = 2.0 kg mm4). The Hg concentrations found in the lower basin of the Pettaquamscutt estuary are similar to those measured in Narragansett Bay (Hg, 11f 5 pM; Vandal and Fitzgerald unpubl. data). Unfiltered Hg, concentrations ranged from 2 to 21 pM in 1990 and from 4 to 24 pM in 199 1 (Figs. 4a, 5a). Hg, ranged from 0.8 to 16 pM in 199 1 and from 2.2 to 13.2 pM in 1990. These concentrations are comparable to other recent measurements of Hg in coastal waters (Table 2). For Hg,, the profiles were analogous for the two sampling periods. In 1990, Hg, concentrations decreased below 1.5 m but increased rapidly below 3.5 m (Fig. 4a). In the anoxic region the concentration remained elevated. The overall profile was comparable in 199 1, but with peaks slightly higher in the water column (Fig. 5a). The deeper waters had lower Hg, concentrations in 199 1. The differences are likely a reflection of storm-induced mixing of anoxic water into the mixed layer in October 1990 (Donaghay et al. unpubl. data). There was a partial degassing of the anoxic waters and sulfur precipitation in the surface waters. As a result, CH, concentrations were lower in August 199 1 than in September 1990 (Scran-
.
Hg in the Pettaquamscutt
1233
ton et al. in press). Gaines (1975) showed that layer to 3-5 nM in the anoxic bottom waters, the sulfide concentration of the bottom waters while organic Cu concentrations, determined by solid-phase separation techniques, dehad not returned to prestorm concentrations creased from 3-5 nM in the mixed layer to (4 mM) 15 months after a period of mixing < 15 pM at depth, even though DOC concenand overturn in November 197 1. Immediately after mixing, the sulfide concentration was 0.3 trations were similar throughout the water colmM-an order of magnitude lower than the umn. It is likely that the Cu, which is reduced prestorm concentration (Gaines 1975). It is in sulfidic waters (Dyrssen and Kremling 1990), therefore likely that the concentration of Hg is bound by sulfides, rather than DOC, in the species in the bottom waters had not returned anoxic waters. Particulate Cu (Cu,), absent in the mixed layer, increased to a maximum of to the prestorm values by August 199 1. Hg, decreased from 40% of Hg, at the sur- 9 nM in the anoxic waters. In the anoxic face to < 10% at 2.5 m (1991 data; Fig. 5a). regions, particulate Cu, was > 50% of the total Cu,, a percentage similar to that found for Below 2.5 m, Hg, constituted a large fraction of the Hg, (5 l&2 1%; n = 23). The results for Hg,. Thus, similar processes- organic com1990, based on the Hg, and Hg,, concentra- plexation in the mixed layer, sulfide complextions, were similar with Hg, being the major ation in anoxic waters-control the speciation fraction below 3.85 m (Fig. 4a). MMHg, ranged of Cu and Hg in this estuary. Hg, was generally below 3 pM, with the from the detection limit (50 fM) in the mixed layer and anoxic waters to 2.92 pM in the pyc- highest concentrations occurring in the mixed nocline in 1990 and to 6.88 pM in 1991. The layer. Concentrations decreased overall with depth of maximum concentration was similar depth (Figs. 4b, 5b); in the deeper waters they were near the detection limit of 0.4 pM. For for both sampling periods (Figs. 4c, 5~). The difference between Hg, and Hg, cannot Hg,, the general decrease in concentration in be attributed solely to Hg, which, on average, the anoxic region with increasing sulfide folaccounted for -30% of the Hg,, (Figs. 4, 5). lows thermodynamic predictions (Dyrssen 1989) and is likely a result of complexation of Estimated “dissolved strongly complexed Hg” (i.e. HgT, - Hg, - MMHg,) was highest in Hg by sulfide ligands and subsequent scavengthe mixed layer (6.6k3.9 pM for O-3 m, n = ing by FeS precipitation (Dyrssen and Krem7; 1991 data) and was relatively constant in ling 1990). Trace metals, scavenged from the the deeper waters (2.6k2.0 pM, n = 2 1). The oxic waters by Fe oxyhydroxide precipitation, are remineralized with Fe in the low-oxygen, 0.8~pm quartz-fiber filters used for filtration do not collect colloidal matter and thus a frac- low-sulfide region. At high sulfide, however, tion of the dissolved strongly complexed Hg FeS precipitates and trace metal-sulfide comis likely to be Hg associated with colloidal ma- plexes are simultaneously scavenged from soterial. There is no relationship, however, be- lution, maintaining low trace metal concentween Hg, and Hg, (r = 0.19), suggesting that trations in sulfidic waters (Dyrssen and Kremling 1990). higher Hg,, and presumably higher colloidal matter (Honeyman and Santschi 1988), does The Hg, data indicate that there are labile not necessarily coincide with higher Hg,. Thus, Hg species even in the highly sulfidic waters colloidally bound Hg is not a predominant of the estuary. Thermodynamic calculations fraction of the Hg,,. Strong complexes with indicate that the measured concentration of dissolved organic matter, derived principally dissolved Hg is below the intrinsic solubility from freshwater input (log K = 18-20 for Hg- of Hg in the presence of HgS solid (Dyrssen humic acid associations, Mantoura et al. 1978), 1989) and that all the ionic Hg is bound as also contribute to the dissolved Hg pool in the sulfide complexes. Some of these sulfide speupper reaches of the water column. In addi- cies must be “reactive” (i.e. reducible by SnCl,). tion, dissolved Hg in the anoxic waters is likely The Hg, determination used here is a suitable to be bound as sulfide species [log K > 30 for measure of the Hg substrate available for HgS, Hg(SH),, HgSZ2-; Dyrssen 19891. methylation, HgO formation, and other conA study of Cu in the upper basin of the es- version processes(Mason and Fitzgerald 1990). tuary (Mills et al. 1989) found that total dis- Therefore, there is available Hg substrate even solved Cu decreased from - 8 nM in the mixed in anoxic waters of the estuary. The presence
1234
Mason et al.
of available Hg substrate could explain why Hg methylation occurs, for example, after adding HgS to slightly acidic sediments (pH 6; Fagerstrom and Jernelijv 197 1) and suggests that the notion that Hg is bound as HgS and unavailable in anoxic waters is not correct. Dissolved MMHg was near the detection limit (50 fM) in the mixed layer (upper 2 m), but increased as oxygen and Eh decreased, to a maximum in the pycnocline (Figs. 4c, 5~). Concentrations remained elevated in the anoxic waters. Similar concentrations have been found in the northern Wisconsin (Bloom and Watras 1989) and Swedish lakes (Lee and Hultberg 1990). In 199 1, the MMHgD concentration of the deeper waters generally decreased from -0.9 pM (5.5-6.5 m) to ~0.2 pM between 14 and 16 m. Although no samples were collected in the deeper waters at this 20-m-deep site due to the limitations of the CTD-pumping system, the results indicate little diffusion of MMHgP from the bottom sediments into the anoxic zone. Rather, they suggest that there is diffusion of MMHgD from the pycnocline region into deeper waters. The similarity between the distribution of Hg species in stratified lakes and the Pettaquamscutt estuary indicate that similar processes control the formation and cycling of Hg in these aquatic systems.
the presence of the phototrophic bacterial community and that these bacteria could be associated with Hg methylation in the pycnocline of the Pettaquamscutt estuary. Although there is no evidence in the literature of sulfide-oxidizing bacteria methylating Hg, methylation under aerobic conditions has been demonstrated, both in marine (Topping and Davies 198 1) and freshwater environments (Winfrey and Rudd 1990), most likely by aerobic microorganisms. However, as pigmented sulfur bacteria oxidize sulfide, their photosynthetic metabolism is intimately tied to the use of reduced sulfur compounds, and thus they are often found associated with dissimilatory sulfate-reducing bacteria (Madigan 1988) at the redox interface. These sulfate-reducing bacteria, which have been shown to be the principal Hg methylators in estuarine sediments under laboratory conditions (Compeau and Bartha 1985), could be forming MMHg in the pycnocline region. The deeper maximum of MMHgr, at 5 m, both in 1990 and 199 1, below the depth of penetration of photosynthetically active radiation and in the region of increasing sulfide (Donaghay and Hanson unpubl. data) could reflect methylation of Hg by these bacteria. The maximum of MMHg, occurred 0.1-0.3 m higher in the water column than the first MMHgD peak and coincided with the rapid Sources and sinks for methylmercury change in Eh (e.g. 1990 data; Fig. 4e). In 1990, There are two probable sources of the high the peak in MMHg, was at 4.4 m with undepycnocline concentrations of MMHg: either tectable concentrations at 4.3 m. In 199 1, MMHg is produced in situ at this depth or the MMHg, peaked just above the peak in overall distribution of MMHg is controlled by redox fluorescence and bacteriochlorophylls (Fig. 6). chemistry and precipitation-dissolution is The MMHg, profile indicates that MMHg, is formed under rather specific conditions that controlling the distribution at the oxic-anoxic interface, as is found for Fe and Mn. There is exist over a small depth interval within the a significant correlation, for the 199 1 data (n redox interface. Particulate MMHg could be = 27), between MMHgD concentration and associated with bacteria if MMHg is produced both bacteriochlorophyll a (BChl a, r = 0.92) by these organisms. The region of rapid change in bacteriochloand bacteriochlorophyll e (BChl e, r = 0.85), and no correlation between MMHgD and Chl rophyll pigments is 3.5 5-4.5 5 m, similar to a (r = 0.15) or pheophytin (r = 0.37) (Fig. 6). that of MMHg,, and there is a weak correlation The principal maxima in MMHgD and in bac- between these parameters (r = 0.70 for BChl teriochlorophyll occur at the same depth (4.5 5 a; r = 0.44 for BChl e). The highest Hg, conm), and there is little MMHg, in the mixed centrations also coincide with the highest baclayer where bacteriochlorophyll concentra- teriochlorophyll concentrations (Figs. 5a, 6a). tions are below the detection limit. In the deep- MMHg, was < 10% of the Hg, in the mixed er waters (> 6 m), bacteriochlorophyll decreas- layer (< 2 m) and in the deeper waters (> 6 m), es with depth, similar to the MMHg, profile. but was the major part of Hg, at other depths. These results suggest that MMHgD forms in The highest %MMHg/Hg, (9 1%) occurred at
1235
Hg in the Pettaquamscutt Concn
0
2
4
Relotive
(PM)
6
a
10
0
10
Concn
concn
20
30
CUM)
40
t
1990 Data 1
Fig. 6. Distribution of MMHg,, (A) and MMHg, (A), pigments (Chl a --O; BChl a-m; BChl e-A), and Mn,, (0) on 8 August 1991.
and Fe,, (0)
4.25 m, coincident with the maximum able for the Pettaquamscutt, but dissolved Fe MMHg, but slightly higher than the bacterio- and Mn have been measured on the 1990 samchlorophyll maxima at 4.55 m and the Hg, ples (Fig. 6). The concentration of dissolved maximum at 4.7 m. Bloom et al. (1991) found Fe and Mn increases rapidly below 3.9 m, a similar peak in %MMHg,/Hg, at the redox reaching a maximum around 5 m. The peak interface (8 m) in Little Rock Lake, Wisconsin. in Fe, should occur in the low-oxygen, lowHurley et al. (199 1) showed that the highest sulfide region, above the peak in Fe, (i.e. -4 concentrations of particulate and dissolved Hg m). FeD precipitates as it diffuses up out of the in this lake during summer stratification oc- anoxic region into the low-oxygen upper pycnocline waters and it is possible that the precurred at the oxic-anoxic interface, coincident with the maximum for chlorophyllous pig- cipitation of Fe is scavenging Hg and MMHg ments and total unfiltered Fe. Below the redox from solution, resulting in the MMHg, peak interface, MMHg, concentrations decreased higher in the water column than the MMHgD while Hg, concentrations remained elevated, peak. The rapid change in MMHg, concensimilar to the Hg, speciation in the Petta- tration in the zone of high MMHg, and the quamscutt anoxic zone. It is likely that MMHg, relatively low mixed-layer MMHgD suggestthat is formed where the relative concentration is MMHg is being removed by particulate scavmaximal, in the region of rapid change in ox- enging and sinking at this depth. ygen and Eh. There should also be a peak in Hg,, in this It is possible that Fe and Mn cycling at the region. There is an increase in Hg, concentraredox boundary is influencing the distribution tion from 2.0 pM at 3.55 m to 9.2 pM at 4.0 of Hg and MMHg. Erel and Morgan (1991) m, and Hg, concentrations remain elevated demonstrated the importance of adsorptionthroughout the pycnocline zone. The profile desorption from particle surfacesfor trace metal for Hg, is less readily interpreted than that of cycling, and showed that, in seawater, Hg has MMHg, as there is a significant flux of Hg, a strong affinity for oxide surfaces. Haraldsson from the mixed layer into the pycnocline (flux and Westerlund (1988) suggested that Fe cy- > 125 pmol rnw2 d-l for 5 pM Hg,; sinking cling influences the distribution of Cu, Cd, Pb, particulate-suspended matter = 0.025 and Co, and Zn in Framvaren Fjord and the Black sinking at > 1 m d- l). Hg, is likely being formed Sea. These data support the notion of Fe con- by Fe and Mn precipitation, and Hg, will also trol over Hg cycling at the redox interface. At be associated with living organisms present in present, no particulate Fe or Mn data are avail- this region. Overall, the profiles for the Hg,
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Mason et al.
species (MMHg, and Hg,) are consistent with the premise of scavenging of Hg and MMHgD by Fe and Mn precipitation. In addition, the lower MMHg, peak (at 4.95 m in 1990) corresponds to the broader maximum in dissolved Fe and Mn (4.75-5.3 m; Fig. 6) suggesting that MMHgr, is released as particulate matter dissolves in the anoxic regions in a similar manner to Fe. There should also be a peak in Hg, in this region if Fe and Mn dissolution is releasing significant amounts of Hg into solution. No distinct peak was found (Figs. 4b and 5b). However, complexation of Hg by dissolved organic complexes could mask any increase due to particulate dissolution. There is an increase in “dissolved nonreactive Hg” (I.e. Hg,, - H&t - MMHg,) slightly deeper in the water (between 5.5 and 6 m) indicating that Hg, is released into solution at deeper depths than MMHg,. In addition, Hg,, remains elevated in the deeper anoxic waters, suggestive of strong complexation of Hg by sulfide ligands, similarly for MMHg (log K = 2 1 for CH,HgS; Dyrssen 1989). The relatively high MMHg, concentrations in this region could reflect strong complexation by sulfide or, alternatively, MMHg, production throughout the anoxic zone, or in the sediments, by sulfate-reducing bacteria. These observations and correlations provide a consistent explanation for the overall distributions of MMHg, and MMHg,. MMHg is formed in situ in the presence of the bacterial community in the lower pycnocline region, resulting in the elevated concentrations of MMHg in this zone. Although this investigation has provided further evidence that MMHg is formed in the water column by the bacteria associated with sulfur cycling at the redox interface, it has not identified the microbial community responsible for MMHg production. Even though the internal cycling and distributions of MMHg in the pycnocline are complex, it is possible to estimate the net rate of formation of MMHg required to balance the losses of MMHg from this region (2.5-6 m). The MMHg, concentrations are low both in the mixed layer and anoxic waters (199 1 data), and thus the flux of MMHg, into the pycnocline region is likely to be similar to the flux out, assuming a constant sinking velocity for particulate matter. In addition, the concentration of MMHg, is low in the mixed layer and
there is little gradient in MMHg, concentration above - 3.5 m. Thus diffusion of MMHgD into the mixed layer from the pycnocline region is not a significant flux of MMHg,, as MMHg, is scavenged by precipitation of Fe and Mn in the upper reaches of the pycnocline (-4 m). The predominant diffusive flux of MMHg, is out of the thermocline region into the deeper anoxic waters. The flux is estimated at 100 pmol m-2 d- I, assuming a diffusion coefficient of 5 X lo+ m2 s-l and the concentration change between 5.5 and 8 m (240 pmol m-4; Fig. 4~). The diffusion coefficient tias estimated with the relationship between the buoyancy gradients [W = g( l/p)@p/8z) = 10e2 sm2here] and diffusion coefficient (Sarmiento et al. 1976). Demethylation is not an important sink for MMHg below the pycnocline as the Hg” concentration decreases to the detection limit below 4 m, suggesting there is little Hg” formation and consequently little demethylation. Thus the diffusive flux of MMHg into the anoxic waters is the predominant flux of MMHg from the pycnocline region and can be used as an estimate of the overall net formation of MMHg in this zone. The net MMHg formation rate (expressed as the rate of conversion of the available substrate) is estimated to be 1.7% d- l (2 X 10- 7s- I), assuming formation occurs over the whole region (3-m-thick pycnocline) and assuming an average pycnocline Hg, concentration of 2 pM. This rate is substantially higher than the specific methylation rate (i.e. the amount of added Hg converted) measured in laboratory culture experiments with lake water and high concentrations of Hg (O.Ol-0.3% d-l; Gilmour and Henry 199 1) and is also higher than the estimated formation rate for methylated Hg species in the low-oxygen region of the equatorial Pacific Ocean (0.01% d-l; Mason 1991). Although the estimated methylation rate in the Pettaquamscutt estuary (1.7% d- ‘) is higher than the rates estimated in the laboratory, the actual rate of formation of MMHg (-0.07 ng liter-’ d-l) is considerably slower than that measured during the laboratory exposure experiments (- 1 ng liter-’ d-l; Gilmour and Henry 199 1). Thus, water-column methylation could easily sustain the relatively high specific methylation rate of 1.7% d- I, even if actual methylation rates in the Pettaquamscutt
Hg in the Pettaquamscutt basin are lower than those measured in laboratory experiments. Air-water exchange of mercury DGHg measurements were made in 1990 and 1991. There was no evidence of DMHg in these waters, and therefore the DGHg consists primarily of HgO. Mason and Fitzgerald (1990) reported evidence of DMHg in the lowoxygen waters of the equatorial Pacific, but there have been no reports of DMHg based on positive identification in freshwater systems (Vandal et al. 199 1; Bloom pers. comm.). Laboratory experiments indicate that the stability of DMHg decreases with increasing temperature and decreasing pH (Mason 199 1) but that DMHg is still unstable even in the low-oxygen, cold, high-pH environment of the subthermocline equatorial Pacific Ocean. Thus, the lack of DMHg in the Pettaquamscutt estuary suggeststhat, if DMHg is formed, the rates of formation and decomposition must be similar. At present, there is insufficient evidence to determine whether DMHg is formed in all aquatic systems and to assesswhich conditions enhance the stability of DMHg so that it can accumulate to detectable levels. In 1990, concentrations of Hg” were at the detection limit (~25 fM) for the mixed layer, while concentrations increased in the pycnocline region (Fig. 4d). The lack of Hg” in the mixed layer can be attributed to a degassing of the surface waters due to enhanced mixing of the upper layers during a strong northeasterly storm the day before measurements were made (3 September 1990). The wind, blowing along the axis of the river, deepened the mixed layer and caused the intrusion of oxygenated water into the low-oxygen pycnocline region (Donaghay et al. unpubl. data) as indicated by the in situ fluorescence and Eh spectrum obtained (Fig. 4e). The bimodal peak in fluorescence in 1990 is not evident in the August 199 1 profile (Fig. 5e) which shows a distinct maximum in fluorescence at -4.5 m. In addition, the area of elevated fluorescence is somewhat broader in 1990, but the intensity of the maximum is greater in 199 1. If excessive mixing of the water column caused a degassingof all the Hg” from the mixed layer, it would take at least 1 d before Hg” concentrations built up to detectable levels (25 fM). This estimation was made with an av-
1237
erage Hg, concentration of 3 pM, a conversion rate of 5% d- 1 (Mason 199 1; Vandal et al. 199 l), and an assumed negligible loss of Hg” from the surface waters. The presence of Hg” below 2.5 m and the similarity between the 1990 and 199 1 profiles below 2.5 m (Figs. 4d, 5d) indicate that mixing caused little loss of Hg” from below the mixed layer. Concentrations decreased rapidly in the anoxic region during both years. There was measurable Hg” in the mixed layer in 1991 (190&45 fM, n = 5) and the concentration was uniform in the upper 2 m. The flux of Hg” upward from the subsurface maximum at 2.8 m is estimated at 130 pmol m2 d-l with a d’ff i usion coefficient of 5 X lop6 m2 s-l (W = 0.8 x 1O-2 ss2). The flux to depth is similar (100 pmol m-2 d-l). The flux at the surface due to gas exchange is estimated at 100-200 pmol m2 d-l, assuming a gas exchange coefficient of 0.5-1.0 m d-l (i.e. for windspeeds of 12-l 9 km h-l). This calculation suggeststhat if the system was at or near steady state during the 199 1 sampling and gas evasion is the only sink for Hg” in the estuary, then there is little formation of Hg” within the upper 2 m of this basin. In addition, the shape of the profile in 199 1 and the elevated concentrations in the pycnocline region relative to the deeper waters in 1990 and 199 1 indicate that Hg” is formed primarily at the bottom of the mixed layer and in the upper reaches of the pycnocline. This finding suggeststhat Hg” formation is not a surface layer, photochemically induced reaction. In 199 1, the production rate necessary to maintain the Hg” profile can be estimated at 230 pmol m-2 d-l or 3% d- 1 for a 2 pM Hg, concentration and assuming production in the upper 4 m of the water column, If Hg” production is restricted to the 2-4-m region, as suggested above, the formation rate would be double this estimate. Examination of the fluxes of MMHg showed that demethylation is unlikely to be an important source of Hg” throughout the water column, and thus Hg” is formed principally by direct reduction of ionic Hg. There is some correlation between Hg” concentration and Chl a (r = 0.6, n = 16) suggestive of a phytoplankton role in Hg” production. However, this correlation could also be a reflection of the fact that concentrations of both species are highest in the mixed layer
1238
Mason et al. Atmospheric
Gas
evasion
T
150
deposItIon
I
170 130
Wet Dry Surface
ProductIon of Hg” Productlon of
MMHg
All fluxes in pmol m2d-’
Fig. 7. Model of Hg cycling in the lower basin of the Pettaquamscutt estuary.
and upper pycnocline and low in the anoxic waters. Studies in the Wisconsin lakes have also shown that Hg” concentrations are generally lower in the anoxic regions compared to oxic waters (Vandal et al. 1991), as found in the Pettaquamscutt estuary. In addition, in lakes with significant thermocline productivity in summer (and a resultant middepth oxygen maximum), the maximum Hg” concentration was at middepth, suggestive of a biological role in Hg” production (Vandal et al. 199 1; Vandal unpubl. data). The average Hg” % saturation (-600%) for the mixed layer of the Pettaquamscutt is similar to the lower range of % saturation found in the Wisconsin lakes in summer (Vandal et al. 1991) and in the equatorial Pacific Ocean (Mason 199 1). The differences in the fluxes from the various bodies of water depend strongly on the wind regime, with highest fluxes estimated for the equatorial Pacific where winds are typically stronger. The estimated flux from the Pettaquamscutt estuary (100-200 pmol me2 d- ‘) is similar to fluxes from the lakes in Wisconsin (Mason 199 1). Mercury cycling in the Pettaquamscutt estuary Atmospheric wet deposition would supply - 170 pmol m-2 d- 1 Hg, to the estuary, assuming a rain Hg, concentration of 50 pM and 10 cm of rain per month. Dry deposition would contribute a comparable flux (130 pmol rnd2 d- l; 0.5 cm s- l depositional velocity, 60 pg m-3 atmospheric particulate Hg, Fitzgerald et
al. 199 1). Atmospheric flux is the net source of Hg to the system, assuming that stream fluxes into and out of the lower basin are similar. The sinks for Hg are gas exchange (100-200 pmol m-2 d-l) and particulate deposition to the sediment (Fig. 7). Net sedimentation is estimated, by difference, at 100-200 pmol me2 d-l. Partic u 1at e scavenging and sinking removes Hg from the bottom of the mixed layer at a rate similar to the net sedimentation flux. Thus, Hg” formation and evasion and particulate scavenging remove Hg, from the mixed layer at similar rates and are the predominant removal mechanisms for mixed-layer Hg,. There is little formation of MMHg in this region and Hg” formation is the predominant reaction consuming Hg, in the oxic region of the estuary (Fig. 7). These processes also predominate in the epilimnion of the Wisconsin lakes and in the mixed layer of equatorial Pacific Ocean (Mason 199 1). Concentrations of MMHg, and MMHg, are low in the mixed layer (upper 2 m), but some methylation could occur in this region. The principal external source of MMHg is atmospheric deposition, which would contribute - l-2 pmol m-2 d- 1 MMHg, assuming concentrations in wet and dry deposition of the same order as found for the Pettaquamscutt estuary and in northern Wisconsin (Fitzgerald et al. 199 1; Mason and Vandal unpubl. data). Any MMHgD that diffuses into the mixed layer from the pycnocline would be removed by particulate uptake or by demethylation. Demethylation ,could be important in the upper reaches of the pycnocline and in the mixed layer. However, there is no strong MMHg,, gradient between 2 and 4 m, indicating that the diffusive flux of MMHg, out of the pycnocline region is small. In addition, the magnitude and position of the peak in MMHg, suggests that most of the MMHgD formed in the pycnocline region is scavenged from solution in the thermocline. Thus, the flux of MMHgr, out of the pycnocline is relatively small and, as a result, demethylation of MMHgD in the mixed layer is not an important process. Hg is transported downward by diffusion and particulate sinking to the oxic-anoxic transition zone where Hg released as particulate is remineralized. Methylation occurs in this region (Fig. 8) and is the principal reaction of Hg, there. In the anoxic region there is little
Hg in the Pettaquamscutt
Atml>spheric
deposi
1239
ion
Mixed layer < Pycnocline
Diffusion
Anoxic zone
Diffueion
’ Hgtll)
