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Source Identification of Florida Bay's Methylmercury Problem: Mainland Runoff Versus Atmospheric Deposition and In situ Production
Results and Discussion
This study was undertaken to better understand the sources and distribution of MeHg in Florida Bay. More specifically, the objective was to determine if Florida Bay's mercury problem was simply an extension, through the discharge of allochthonous MeHg, of the well recognized mercury problem in the upstream freshwater Everglades. Surface water sampling along the two transects did not, however, show unidirectional gradients of either THg or MeHg concentrations decreasing away from Everglades water control structures into Florida Bay (Fig. 2 and Fig. 3), as might have been expected. Instead, sampling revealed a pattern of elevated THg and MeHg concentrations within the mangrove transition zone of both Taylor Slough and C-111 Basins (Fig. 2 and Fig. 3). As the discussion below will explain, this pattern in water column concentrations together with concentrations observed in sediments and results from methylation assays indicate that direct atmospheric deposition of THg to the basins and in situ methylation might be important.
Concentrations of THg in unfiltered surface water collected along the Taylor Slough transect differed among sites (Kruskal-Wallis one-way ANOVA on ranks, H=25.1, df=5, p<0.001), with the sites within the mangrove transition zone (Argyle Henry and Taylor River) having higher concentrations than other sites (Dunn's pairwise comparisons p< 0.05, Fig. 2). More importantly, concentrations of MeHg differed also along this transect (H=28.8, df=5, p<0.001) with Argyle Henry again having higher concentrations than all other sites except Taylor River and the freshwater marsh (p>0.05, Fig. 2). Argyle Henry typically also had the highest percent of THg as MeHg (%MeHg) in both unfiltered and filtered samples, ranging between 9% and 31% (Fig. 2 and Fig. 3). Concentrations of MeHg at Taylor River were higher than at S-332D but similar to the other sites (Fig. 2). This pattern was replicated along the second transect. Concentrations of THg and MeHg differed along the C-111 Canal transect (H=24.6, df=7, p<0.001, and H=20.9, df=7, p=0.004, respectively), with concentrations of both significantly higher in the mangrove transition zone than concentrations either at the upstream control structure, S-18C, or the downstream open bay site (Nest Keys, Site N, Fig. 2). Pairwise comparisons between the reference site located in the open bay of Whipray Basin and other sites (Dunn's Method) revealed no significant difference in either average THg or MeHg concentrations (p> 0.05). Moreover, the reference site had the third highest % MeHg after Argyle Henry and Taylor River (Fig. 2).
While dissolved species generally dominated, much of the variation observed in the unfiltered concentrations resulted from the variability in the particulate or filterable fractions of THg and MeHg (i.e., calculated as the difference between average concentration of duplicate unfiltered and filtered samples). This was often observed in the mangrove transition zone where the Everglades runoff mixes with saline bay waters (Fig. 4 and Fig. 5) and may be attributable to a decrease in solubility, flocculation of negatively charged particles, or re-suspension. Surprisingly, filterable THg concentrations did not covary with water column turbidity (Pearson correlation coefficient, r=0.12, p=0.19, n =123); neither did filterable MeHg (r=0.069, p= 0.44, n =123). The open bay site on the C-111 transect, Nest Key (Site N), had the lowest filterable fraction of both THg and MeHg (Fig. 5). Occasionally, average concentrations in paired filtered samples exceeded average concentrations in paired unfiltered samples at this site (these bottles were collected sequentially, not as split samples). As previously observed in unfiltered concentrations, filtered concentrations of THg and MeHg at the open bay reference site in Whipray Basin did not significantly differ from concentrations at other sites (Dunn's Method, p>0.05).
The concentrations reported here are within ranges reported by two previous short-term studies in Florida Bay by Kannan et al. (1998) and Lores et al. (1998). In general, water column concentrations of THg were similar to results from other coastal areas that did not have obvious point sources, whereas MeHg concentrations tended to be higher (Benoit et al. 1998; Mason et al. 1999; Lacerda et al. 2001; Guentzel and Tsukamoto 2001). The patterns observed in water column concentrations of both dissolved and filterable forms across the transects were consistent also with spatial patterns observed in other estuaries (Benoit et al. 1998; Guentzel and Tsukamoto 2001), including a study done in another mangrove-dominated estuary where THg and reactive Hg were enriched in the mangrove zone relative to open bay waters (Lacerda et al. 2001).
Mass Load Estimates
Seasonal mass loads of THg and MeHg were approximated as the sum of instantaneous fluxes (in grams per wet or dry season) and are summarized in Table 4 and Table 5. The results must be interpreted cautiously, however, due to the uncertainties introduced by: 1) estimating daily concentrations using linear interpolation between quarterly sampling events, 2) the reliance on measured deposition at a single MDN station and, 3) estimating MeHg deposition based on measurements of monthly composites from weekly rainfall samples. Despite these uncertainties, several points are evident from examining Table 4 and Table 5.Total Mercury
Direct wet atmospheric deposition of THg to the Taylor Slough Basin was two orders of magnitude greater than inputs from the canal via the S-332D pump (Table 4 and Table 5). Similarly, direct wet atmospheric loading of THg to the C- 111 Basin was 66% higher than loads delivered by the C-111 Canal. This is in agreement with previous reports (Stober et al. 1996, 1998; Fink et al. 1999) that atmospheric deposition dominates over surface water inflows as the source of inorganic Hg to the Everglades. It should be noted that, while the focus here is on wet-only deposition, dry deposition likely adds significantly (30-60% of wet deposited) to the overall atmospheric load (FDEP 2003). When inputs from S- 332D pump were combined with direct atmospheric deposition, THg loads to Taylor Slough were greater than the loads to the C-111 Basin (i.e., when inputs from the canal were combined with direct atmospheric deposition, Table 4 and Table 5). Comparison of the two basins suggests land-based runoff loads of THg at the mouth of Taylor River were similar to or slightly greater than loads in the two creeks discharging into Joe Bay (i.e., Northeast Creek and Western Creek combined, Table 4). Both Little Madeira and Joe Bay have inflows in addition to those monitored, and Western Creek likely includes flows originating in Taylor Slough. Direct atmospheric deposition of THg to these embayments was similar (Table 5) and was larger than loads in monitored rivers and creeks (Table 4 and Table 5). Because flow was not measured at the boundary of Little Madeira Bay with outer bay, loads could not be estimated in surface flows to the open bay in Eagle Key Basin. Flows were measured, however, at Trout Creek where much of Joe Bay discharges to Trout Cove and the open bay. The large THg loads that passed through Trout Creek (Table 4) reflected high water flows and modest mercury concentrations. Nevertheless, the dominant THg loads to the open bay were from direct atmospheric deposition (Table 4 and Table 5).
These findings are consistent with the conclusions of Kang et al. (2000) that runoff contributes as much as 80% of the total flux of THg to the sediments near the river sloughs but less than 20% to sediments in remote areas of Florida Bay (i.e., where the majority must be from atmospheric deposition). The present loading assessment suggests, however, that the source of the THg near river sloughs was direct atmospheric deposition to the basins immediately upstream of the embayments and not from canals routing runoff from further north. Recognition of boundary conditions of the watershed and the ratio of its land surface area to estuary water surface area must be considered when comparing loading assessments. The latter should provide an indication of the importance of indirect versus direct atmospheric deposition.
As in other deposition assessments (for review, see Hall et al. 2005), only a small percentage of the atmospheric deposition of THg was in the methyl form (less than 1% in wet season and less than 2.6% in dry season, Table 5). MeHg loads in surface water runoff were also relatively small compared to THg loads in runoff (Table 4 and Table 5). Unlike THg, MeHg loads in rivers and creeks to the two embayments were larger than atmospheric loads (Table 4 and Table 5) and differed greatly between the two basins with larger loads coming down Taylor River (Table 4). Furthermore, unlike THg, MeHg loads through Trout Creek dominated loads from direct atmospheric deposition to northeastern Florida Bay (Table 4 and Table 5). This is likely to be the case also for Eagle Key Basin downstream of Little Madeira Bay.
Not surprisingly, inputs from both direct atmospheric deposition and from runoff were much reduced during the dry season (Table 4 and Table 5). Estimates of atmospheric deposition may be underestimated, however, because they do not include dry deposition, which may be less seasonal. Concentrations of both THg and MeHg were highest during the summer wet season with heavy rainfall and peak discharges (Fig. 4). Quarterly sampling, however, prevented identification of any time lag from the onset of rains (i.e., direct deposition of "new" inorganic Hg) or peak discharges (i.e., indirect deposition and runoff) and concentration pulses in THg or MeHg. Routine mercury monitoring for more than 8 years at water control structures in the freshwater Everglades shows flows and, more importantly, concentrations of THg and MeHg peak during the late summer months of July-September, corresponding to the rainy season (Rumbold et al. 2007a).
Differences Between Terrestrial Watersheds
Taylor Slough Basin had smaller inputs from water management canals but larger inputs from atmospheric deposition, owing to larger surface area, than the C-111 Basin (Table 4 and Table 5). More importantly, median and seasonal maximal concentrations of THg and MeHg were much greater in surface water within the mangrove transitional zone of Taylor Slough than C-111 Basin (Fig. 2, Fig. 3, and Fig. 4). These greater concentrations when combined with slightly larger flows yielded larger loads of both THg and MeHg delivered to Little Madeira Bay by Taylor River than delivered to Joe Bay by the Northeastern and Western Creeks (Table 4). The differences in loads of allochthonous MeHg (produced in the upstream marshes and the mangrove transition zones) to the two embayments appear to be significant. This may explain the greater MeHg biomagnification observed in the biota of Little Madeira Bay compared to Joe Bay (Cantillo et al. 1997; Goodman et al. 1999; D. Evans, NOAA).
As Hg species enter the estuaries and travel through the numerous gradients that occur there (e.g., gradients in salinity, pH, nutrients, organic matter, etc.), they are subject to numerous competitive physico-chemical processes such as ion-ligand exchange, adsorption-desorption, colloid aggregation, and degradation of particulate organic matter (for reviews, see Horvat et al. 1999; Conaway et al. 2003 and references therein). Clearly, differences in quality of stormwater runoff, especially as it relates to carrier phases, such as dissolved organic matter (DOM), will cause Hg species to behave differently among estuaries and within the same estuary. Although it was beyond the scope of the present study, a more thorough investigation of these processes may explain the differences between these two watersheds and improve our understanding of Hg distribution in Florida Bay.
Concentrations in sediments collected from the bay and upstream marshes and canals ranged from 5.8 to 145.6 ng THg/g dry weight and from 0.05 to 5.4 ng MeHg/g dry weight (Fig. 6 and Fig. 7). The percent of THg that occurred as MeHg in sediments ranged from 0.1% to 88.9% (Fig. 6 and Fig. 7). Although the latter seems implausibly high, there was no evidence of analytical error. When data were pooled over time, both sediment-THg and sediment-MeHg differed significantly among sites on the Taylor Slough transect (H= 19.8, df=4, p<0.001 and H=16.8, df=4, p=0.002, respectively). Pairwise comparisons along this transect revealed that sediments from the open bay site (Site B in Eagle Key Basin) differed significantly from both Argyle Henry and Taylor River in THg and from the freshwater marsh and Taylor River sediments in MeHg; no other pairwise comparisons were significant. Both sediment-THg and sediment-MeHg differed also along the C-111 Canal transect (H=24.3, df=6, p<0.001 and H=13.7, df=6, p= 0.03, respectively). Here, sediments from Stump Pass differed in THg only from sediments from S-178 and Joe Bay creek (p<0.05); no other pairwise comparisons in either THg or MeHg (suspected spurious MeHg value from the marsh site was excluded) were statistically significant (p>0.05). When compared to all other sites, sediments from the reference site differed only from the Taylor Slough marsh site in MeHg content (Dunn's post-hoc, p<0.05); no other pairwise comparisons in MeHg or THg were significant. Notice also that sediments collected from the open bay near the Nest Keys (i.e., saline end-member of the C-111 transect), contained up to 1.8 ng MeHg/g, which constituted almost 8% of the THg present. Median %MeHg in sediments at this open bay site was 3.9%, which was similar to the percentages seen in sediments from the freshwater marsh sites. When pooled across sites, %MeHg in sediments was not correlated with the salinity of bottom water (Pearson Coefficient=-0.16, df=60, p=0.214; excluded the likely spurious %MeHg).
Comparisons of Hg levels in these sediments with levels in freshwater marl sediments from the lower Everglades, muck sediments from the northern Everglades, or sediments from other estuaries must be done cautiously because concentrations should first be normalized to total organic carbon or bulk density (note, bulk density was not measured in the present study, and %LOI was measured only in later cores). With this caveat in mind, sediment- THg concentrations appeared similar to values for other sediments collected from Florida Bay (Kannan et al. 1998; Kang et al. 2000) but lower than concentrations reported for sediments collected from the northern Everglades (Scheidt and Kalla 2007; Cohen et al. 2009) and from other estuaries, especially estuaries known to be contaminated by localized industrial activity (Bloom et al. 1999; Mason et al. 1999; Mason and Lawrence 1999; Hammerschmidt et al. 2004). Alternatively, sediment-MeHg concentrations were occasionally elevated compared to values previously reported for Florida Bay (Kannan et al. 1998) and other coastal marine sediments (Benoit et al. 1998; Bloom et al. 1999; Mason and Lawrence 1999; Hammerschmidt et al. 2004). The %MeHg in the bay sediments, especially from the Nest Keys, was high as compared to sediments from the freshwater Everglades collected as part of the present study and other studies (Gilmour et al. 1998; Rumbold et al. 2007b). The unusually high %MeHg was due, in part, to the low THg concentrations in these sediments compared to other areas, especially compared to contaminated sediment from highly urbanized estuaries or those near heavy industry (Bloom et al. 1999; Mason et al. 1999; Mason and Lawrence 1999; Hammerschmidt et al. 2004).
Although sulfide was not determined in the sediments from the present study, sediments of much of Florida Bay are known to be highly sulfidic. Carlson et al. (1994) reported sulfide often >1 mM but as high as 13 mM in sediment pore water of Florida Bay. They concluded that these values resulted from a combination of rapid microbial sulfate reduction and the low Fe content and limited capacity of carbonate sediments to precipitate sulfide. Florida Bay is also known to frequently become hypersaline. Both hypersalinity and high sulfides have been implicated in seagrass die-offs over large areas of Florida Bay (Robblee et al. 1991; Carlson et al. 1994). Yet, as shown here, the percentage of the THg as MeHg in bay sediments was similar to the percentages seen in sediments from the freshwater marsh sites. This was unexpected based on information available at that time.
Because MeHg concentrations and %MeHg were unexpectedly high in bay sediments, methylation assays were added to the study. The median rate of 202Hg methylation was 2.7% day-1 in the 0- to 4-cm horizon of cores taken in June 2001, with rates ranging as high as 11.2% day-1. The maximum 202Hg methylation rate occurred in a core taken at the outlet of Little Madeira Bay, Site L. Regrettably, there was a problem during the analysis of the top horizon from the duplicate core, and there was no confirmation of this unusually high conversion rate. The lower horizon (4 to 10 cm) from both cores exhibited variable but still relatively high rates of conversion (1.8% day-1 to 9.1% day-1). More importantly, 202Hg methylation rates in cores from the open bay both from the Nest Keys and Whipray Basin were as high or higher (with good agreement between duplicate cores) than rates in cores from freshwater canal sites (note, marshes were inaccessible in June 2001, Fig. 8). Median rates of 202Hg methylation was only slightly lower (at 2.3% day-1) in lower horizons (4 to 10 cm) of cores taken in June 2001. In September 2002, incubated cores were cut into three sections with a median rate of 202Hg methylation of 2.8% day-1 in the top 1-cm horizon, 2.7% day-1 in 1- to 4-cm horizon and 2.3% day-1 in the lower 4- to 10-cm horizon. The highest conversion during the September 2002 sampling event occurred in sediments from the Nest Keys (15% day-1); however, this time, the duplicate core exhibited a much lower methylation rate (2% day-1). Nonetheless, cores from several other sites, including Little Madeira Bay and Whipray Basin, again exhibited conversion rates as high or higher than cores from the freshwater marshes, with good agreement between duplicates (Fig. 8). 202Hg methylation rates in the present study were similar to or higher than values reported in incubation studies of other estuarine sediments (Hammerschmidt and Fitzgerald 2004). Although the methylation assays provide evidence that sediments from Florida Bay have the potential to methylate Hg, caution must be exercised when interpreting these results. First, there are a number of factors, such as incubation duration, pre-equilibration, and spike concentration that can influence measurement of true methylation potential (for review, see Heyes et al. 2006). Furthermore, because demethylation was not measured in the present study, we cannot gauge the importance of net in situ methylation as a source of MeHg to biota.
Since initiation of the present study, a growing number of studies have reported mercury methylation in marine sediments, even where sulfides are in the millimolar range (King et al. 2000; Langer et al. 2001; for reviews, see Sunderland et al. 2006). Several non-exclusive explanations have been offered for in situ methylation in high sulfide environments including: 1) vertical migration of the oxycline and redox transition zone induced by tidal pumping or by daytime peaks in photosynthesis (Langer et al. 2001), 2) wind induced or current driven mixing of the sediments (Sunderland et al. 2004) or via bioturbation (Hammerschmidt et al. 2004), 3) dissolved Fe scavenging of sulfides (Hammerschmidt and Fitzgerald 2004), and 4) creation of oxidized microenvironments by roots (Marvin-DiPasquale et al. 2003).
Many of these same processes, individually or in combination, could explain the methylation rates observed in the present study. For instance, Langer et al. (2001) observed methylation in sulfidic sediments (>28 mM) and posited that it occurred in the upper zone of the oxycline in surface sediments, particularly after inundation of the salt marsh with oxygenated water during flood stage. Net MeHg production would be enhanced under these conditions. Later, water with high MeHg concentrations would drain from the salt marsh as the tide ebbed. An analogous scenario was offered as an explanation for unprecedented MeHg concentrations draining from a (sulfate contaminated) freshwater stormwater treatment area where frequent use of large pumps inadvertently mimicked tidal pumping and caused water levels to fluctuate greatly, which in turn likely resulted in vertical migration of the redox transition zone (Rumbold and Fink 2006). Further evidence that tidal pumping can be significant in estuarine systems is provided by the data of Lacerda et al. (2001), who showed surface and pore water rich in THg and reactive Hg draining from mangrove transition zone of Sepetiba Bay, Brazil during the ebb tide. Such a scenario would easily explain the high MeHg and high THg in the mangrove transition zone of Florida Bay, which owing to its gentle slope, has extensive areas of coastal wetlands that can be inundated during wind-driven surges, seasonal sea-level fluctuations and the relatively minor tidal variations that occur in northeastern Florida Bay.
Langer et al. (2001) raise the possibility that the oxycline might also be affected during daytime peaks in photosynthesis by benthic algae producing near super-saturated oxygen concentrations at the sediment surface. This could be important also in Florida Bay because benthic microalgae are a major component of autotrophic biomass and have been shown to have high photosynthetic rates (Cornwell et al. 2003; Armitage et al. 2006); sea grasses and other submerged aquatic vegetation are also likely to play a significant role in the oxycline and biogeochemistry. Others (Gagnon et al. 1996; Hammerschmidt and Fitzgerald 2004) have noted the importance of Fe in scavenging sulfides allowing Hg methylation by reduction of sulfide inhibition. Florida Bay sediments are generally low in Fe, and this process may not be as important here. Chambers et al. (2001) reports relatively high levels of reduced sulfur compounds but generally low levels of Fe in bulk sediments. Chambers et al. (2001) did conclude, however, that Fe availability, for sulfide sequestration, was higher in northeastern Florida Bay.
In the present study, organic matter content, estimated by loss on ignition (LOI), ranged from 4% to 84% (median was 9%). The LOI was generally highest at Argyle Henry pond, Site A, and lowest in the open bay of Eagle Key Basin (i.e., saline end-member of Taylor Slough Transect, Site B). We found a positive correlation between organic matter and ambient THg concentration (r=0.57, p<0.001, n =74) and ambient MeHg concentration (r=0.69, p<0.001, n =74). This is in concurrence with the results of Mason and Lawrence (1999) who reported that THg and MeHg covaried with sediment organic matter. Alternatively, we found 202Hg methylation only correlated poorly with organic matter (r=-0.12, p=0.28; top horizons only, Fig. 9). Visual inspection of the data in Fig. 9 hints at a possible non-linear relationship between 202Hg methylation and organic matter, with an inhibitory effect in sediments with both low and high organic content (i.e., %LOI). Similar to the results reported here, Hammerschmidt and Fitzgerald (2004) also observed a reduction of methylation in sediments with relatively higher organic matter. Interestingly, in addition to stimulating the bacteria, several investigators (Le Roux et al. 2001; Hammerschmidt et al. 2004; Sunderland et al. 2006) suggest that the quantity and quality of DOM in pore waters may also influence the bioavailability of Hg(II) for methylation.
In addition to the remaining uncertainties regarding methylation potential, we lack also quantitative information on the amount of autochthonous MeHg entering the food web, which is the defining question. MeHg in sediments may enter the food web through efflux from sediment pore waters (both diffusive and advective), by movement of benthic organisms into the water column, or by direct grazing on surface sediments and benthic organisms. Rates of these processes are affected by many factors (Mason et al. 2006). Consequently, the importance of in situ MeHg production in Florida Bay will remain unclear until benthic flux measurements and benthic-pelagic trophic transfer studies are undertaken.
Management Relevant Conclusions
This study provides evidence that mercury methylation can occur in sediments of Florida Bay. Therefore, we need a more complete understanding of the fluxes and bioavailability of inorganic mercury as a factor, among many, that control in situ methylation rates. Concerns have been raised regarding restoration efforts inadvertently worsening existing mercury problems in other estuaries (Mason and Lawrence 1999; Hammerschmidt and Fitzgerald 2004). If, as it appears, THg and MeHg loads in canals delivering water from the Everglades and northern urban areas are not as significant as atmospheric loading and in situ methylation, then increasing freshwater flows in canals to Florida Bay in an effort to hydrologically restore the Everglades and Florida Bay ecosystems is unlikely to have a significant impact on inputs. Based on current data, we cannot meaningfully predict, however, whether restoration-induced changes (including increased deliveries of allochthonous DOM) will enhance or diminish methylation rates or subsequent biomagnification. At a minimum, continued monitoring of mercury levels in bay fish seems prudent. Because Hg levels are already high, small increases could result in even more restrictive fish consumption advisories (i.e., upgrading "limited consumption" to "no consumption"). Thus, monitoring Hg biomagnification in gamefish may not alert us to changes in time to take corrective action. Moreover, simply monitoring Hg levels in fish would not provide the information to discriminate which influential factor was altered and led to increased methylation or biomagnification. Information needed to make sound management actions would require detailed process level monitoring. One area that is clearly deserving of more investigation is the underlying differences in sediment or water quality that results in lower exports from the C-111 Basin.
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 15 January, 2013 @ 12:43 PM (KP)