Mercury concentrations in lichen, deer, and puma along a coastal fog gradient

Previous research has identified that the Santa Cruz Mountains in our study area, form an effective barrier to the inland penetration of marine fog19. Lichen, deer, and puma samples were taken on both sides of the watershed divide of this range, thus providing an opportunity to test the effects of varying summertime fog and low cloud cover (FLCC) on the observed MMHg and THg concentrations in biotic samples. The ocean-facing sites from which lichen samples were taken, located to the southwest of the watershed divide (Fig. 1A), averaged 6.2 hrs/day of summertime FLCC with a high of 9.5 hrs/day (Table S1, Supporting Information). Ocean-facing samples were <1.1 km from the Pacific Ocean whereas the bay-facing group were more spread out, located >24 km from the coast, and averaged 3.7 hrs/day summertime FLCC (Table S1). However, while this FLCC frequency is only 60% less than that for the ocean-facing sites, the difference in summertime fog wet deposition is likely to be at least an order of magnitude, with ocean-facing sites being wetter21,22.

Figure 1 Geographical distributions in the Santa Cruz Mountain coastal region, California, USA, and mean (±1 SE) values by coastal sub-region of (A) MMHg concentrations in lace lichen (Ramalina menziesii) (site names correspond to data in Table S1), (B) THg in adult deer fur, and (C) THg in adult puma fur and fur-normalized whisker samples. The blue line represents the watershed boundary which was used to delineate samples as belonging to either the ocean facing (to the left of the line) or bay facing (to the right of the line) sub-regions. Asterisks indicate the p-value from one-way ANOVA test on the log-transformed concentrations, shown by the extent of the horizontal lines (**p = 0.001–0.01, ***p = <0.001). Sampling for lichen, deer and puma was done in 2017, 2008–2012, and 2006–2014, respectively. Full size image

The mean (±1 SE) MMHg concentration in lichen from ocean-facing sites was 30.2 ± 3.5 ng g−1, which is a 3.7 times enrichment (difference significant, ANOVA) over the mean from the bay-facing sites (8.2 ± 0.8 ng g−1) (Table 1, Figs. 1A and S1). This enrichment is comparable to that observed in lichen on Bathurst Island in the Canadian Arctic, where St. Pierre et al.39 observed a five times enrichment in lichen MMHg sampled on the coast nearest to ice leads exposing the open-ocean where DMHg emissions are known to occur40, compared to locations >20 km inland.

Table 1 Statistics on the subsets of concentration data used in this paper. Full size table

The mean THg concentration in central California bay-facing lichen was higher than that from ocean-facing lichen (Table 1), although the difference was not significant (ANOVA). Very high concentrations of THg were observed in lichen samples at one bay-facing site, Almaden Quicksilver, a former Hg mine (1097 ± 58.2 ng g−1), with levels that are close to an order of magnitude higher than THg in lichen from other locations (Table S1). However, in spite of the elevated THg concentrations, lichen samples from Almaden Quicksilver did not have enhanced MMHg concentrations. The mean MMHg in lichen from this site was 8.2 ± 3.3 ng g−1, the same value as the mean for all sites in the bay facing region. Overall, the lichen mean %MMHg (MMHg/THg × 100) in the ocean-facing region was 23.0% compared to 4.4% for the bay-facing region (N = 7), with the data from Almaden Quicksilver removed as an outlier (Table S1).

Samples of adult mule deer fur were collected in close proximity to the lichen sampling sites, in both ocean-facing and bay-facing sub-regions, and were analyzed for THg concentrations. These data are shown in Fig. 1B, and reveal that the mean THg concentration in deer samples from the ocean-facing region was significantly higher (41.2 ± 4.9 ng g−1, N = 18) than that from the bay-facing region (21.6 ± 3.2 ng g−1, N = 38, one outlier sample removed, see Tables 1 and S2).

Adult puma fur and whiskers sampled across the ocean- and bay-facing regions were also analyzed for THg (whisker THg concentrations are reported as fur-normalized concentrations, see Fig. S2). Mean THg concentrations between the two sub-regions were not significantly different (Fig. 1C) (ocean facing: 1346 ± 159 ng g−1, N = 53, bay facing: 1503 ± 299 ng g−1, N = 53, with one outlier removed, see Table 1 and Table S3). This lack of observed spatial pattern was expected due to the large home range size exhibited by adult pumas44.

THg in coastal vs. inland deer and puma

A broader spatial comparison was made between THg concentrations in pumas and mule deer from the coastal region (which includes both the ocean-and bay-facing sub-regions) and the inland region of California (hundreds of km distant), which includes mountainous regions of the Klamath, Sierra Nevada and Cascade ranges, as well as desert regions in the southern part of the state. For mule deer, mean THg concentrations were significantly higher in coastal regions (28.1 ± 2.9 ng g−1, N = 55, with one outlier removed) compared to inland regions (15.5 ± 1.5 ng g−1, N = 41) (Fig. 2A, Table S2). Samples of fur and fur-normalized whiskers from adult pumas also had significantly higher mean THg concentrations in coastal areas (1544 ± 151 ng g−1, N = 94) compared to inland regions (492 ± 119 ng g−1, N = 18) (Fig. 2B, Table S3).

Figure 2 (A) Deer and (B) puma sample locations in California, USA, and mean (±1 SE) concentrations of total mercury (THg) in samples from each sub-region. Asterisks indicate the p-value from one-way ANOVA test on the log-transformed concentrations, shown by the extent of the horizontal lines (**p = 0.001–0.01, ***p = < 0.001). Sampling for deer and puma was done in 2008–2012, and 2006–2014, respectively. Full size image

Different age-classes of pumas were sampled within both coastal and inland regions, and THg concentrations increased with age in both sampling areas (adult > sub-adult > kitten) (Fig. 3). Differences between adult and sub-adult were significant in the coastal region, and differences between kitten and sub-adult were significant in the inland region. Six mother-kitten pairs were also analyzed for THg from the coastal region. Kitten THg concentrations were on average 44.7% ± 9.2% of their mothers. We did not observe any significant differences in THg concentrations between sexes in coastal or inland regions. In all age classes, pumas from the coastal region had significantly higher THg concentrations compared to inland pumas and coastal kittens, on average, were, notably, 5.6 times higher than inland kittens (Fig. 3).

Figure 3 Mean (±1 SE) concentrations of THg in puma samples from California, USA, from three age classes and two geographical regions. Asterisks indicate the p-value from a one-way ANOVA test on the log-transformed concentrations, shown by the extent of the horizontal lines *p = 0.01–0.05, **p = 0.001–0.01, ***p = < 0.001. Sampling for puma was done in 2006–2014. Full size image

Archived California puma fur samples (1916–1933, Museum of Vertebrate Zoology, University of California, Berkeley, CA, USA) had a mean (±1 SE) THg concentration of 290 ± 42 ng g−1 (Fig. 2B). This value was calculated based on the measured MMHg concentration in the archived puma fur, multiplied by the factor 1.15, which was the mean THg/MMHg ratio in the nine modern-day samples analyzed for both Hg species. THg concentrations in archived puma fur were also measured, however, these values were not used due to the high probability of inorganic Hg contamination (mean concentration = 5200 ng g−1) which can be common in museum archived vertebrate samples45. The mean calculated THg concentration (from MMHg) in archived puma samples was not significantly different compared to the mean from modern-day inland samples, representing a relatively stable background THg concentration over time.

Methylmercury toxicity and risk to coastal california puma

Bioconcentration factors (BCF) were calculated using methods described by Azad et al.46, and utilizing the following formula: log ([Hg organism ]/[Hg fog ]), where Hg was THg or MMHg (Fig. 4). BCF values indicate how much THg and MMHg can be potentially transferred to lichen, deer, and puma from fog water. The highest BCF found was between puma THg and fog MMHg (6.0). Several authors report that MMHg undergoes more efficient trophic transfer, across a wide array of species, and is more bioavailable compared with THg46,47,48, which is consistent with our hypothesis that fog water MMHg is disproportionately transferred between trophic levels compared with inorganic Hg from other sources.

Figure 4 Concentrations of THg and MMHg in rain (sampled 2014–2015), fog (sampled 2014–2015), lichen (sampled 2017), deer (sampled 2008–2012), and puma (sampled 2006–2014). All samples are restricted to locations in the coastal central California, USA region. Fog and rain data are from Weiss-Penzias et al.16. Bioconcentration factors (defined in the text) are shown for the Hg species and trophic relationship given by the horizontal line. Full size image

In the Florida Everglades elevated levels of Hg were detected in puma, however the diet of the Florida pumas included aquatic and fish-eating prey which increased the Hg exposure regime49,50. The coastal California pumas in our study, conversely, eat a less diverse diet largely of terrestrial origin. In a previous investigation, we intensively recorded coastal California puma movements and diet, and reported that black-tailed or mule deer made up greater than 90% of the edible biomass for pumas in the Santa Cruz Mountains, and our data showed that coastal pumas were not eating marine derived prey51. Studies from other parts of California have also shown deer to be their primary source of prey52,53. Since deer fur from coastal California also displayed higher THg concentrations compared to their inland counterparts, and the fact that deer are herbivores foraging on plants and lichen suggests a potential link between fog wet deposition of Hg and the accumulation of Hg in coastal, terrestrial food webs.

While mercury toxicity studies have yet to be conducted in pumas, studies on other carnivores (mink and otter) revealed that brain THg concentrations >3 μg g−1 can cause clinical Hg intoxication or subtler neurological impairments that could detrimentally affect survival54,55. Assuming that otter and mink brain concentrations of THg are 14% of those in their fur, using the predictions of Eccles et al.56, concentrations of >21 μg g−1 in fur would be considered toxic. Among pumas sampled from coastal regions in this investigation, one individual had a fur sample of >21 μg g−1, a puma found dead with no apparent cause of death known at the time.

Sub-lethal effects of Hg are known to occur at lower concentrations than those causing death, and may negatively impact predator population performance. Previous puma investigations have shown that at blood Hg concentrations >250 ng g−1, the number of offspring surviving to adulthood per female falls below 1.0, indicating significant, sub-lethal effects and risk to long-term population survival and viability50,57. Fur THg concentration in this work, converted to blood THg concentrations using a relationship for Florida panthers50, results in a mean calculated blood THg concentration of 73.9 ± 10.3 ng g−1, a factor of 3.4 lower than the threshold. However, our samples included two female pumas that had calculated blood THg concentrations of 356 and 276 ng g−1, which exceeds the threshold. Further research is needed to better understand the effects of Hg on puma reproduction and how it might interact with other factors such as low genetic diversity, fragmentation and exposure to rodenticides.

Sources of mercury to coastal food webs

Here we consider the sources of Hg in all its forms potentially contributing to the bioaccumulation of Hg in the coastal vs. inland terrestrial food webs in our study area. We hypothesize that the main source of MMHg in the coastal system is marine fog that carries emissions of organic Hg species from the ocean, as conceptually illustrated in Fig. 5. Note how the coastal range forms a barrier to fog penetration inland, and as the fog is intercepted, wet deposition of marine-derived MMHg occurs. However, our observation of higher MMHg in ocean- vs. bay-facing lichen may have alternate interpretations that should be considered. For example, we do not know the influence of the overall wetness of each habitat, and how this could affect the uptake rates of all forms of Hg to lichen. When the relative humidity is higher, lichen may be more actively taking up Hg. Humidity differences may also affect the rate at which MMHg can be produced endogenously within the lichen due to the presence of symbiotic cyanobacteria that can potentially chemically alter HgII 58. As has been pointed out by previous studies59, further research is needed to look for the hgcAB genes within the lichen biome to determine whether in situ methylation of Hg can occur.

Figure 5 Conceptual diagram showing the hypothesized sources and mechanisms of transfer of organic Hg species from the ocean to the coastal terrestrial food web in central California, USA. Bar charts indicate the mean concentrations of MMHg observed in this work plus fog and rainwater concentration from16. Full size image

Our observation that THg concentrations were more elevated in bay-facing lichen samples, whereas the opposite trend was observed for MMHg concentrations, can also have multiple interpretations. One of these is that there is an additional atmospheric deposition source of inorganic Hg being taken up by lichen at the bay-facing sites. We suggest that gaseous oxidized inorganic Hg from the free troposphere may undergo dry deposition to lichen and cause greater accumulation of inorganic Hg at the bay-facing sites. Wright et al.60 found that Hg dry deposition was a factor of 10 times higher at Lick Observatory in the Diablo Range (Blue Oak Ranch in Fig. 1A), likely due to the prevalence of high pressure subsidence atmospheric conditions61, compared with Hg dry deposition at Elkhorn Slough, a site on the Pacific coast within 30 km of our ocean-facing lichen sampling sites. This suggests that the inorganic Hg source from the free troposphere is important for bay-facing sites, whereas ocean-facing sites may be more influenced by Hg° emissions from the ocean and have lower inorganic Hg burdens as a result. We must also consider the potential for the transformation of inorganic to organic Hg within lichen, which may result in higher MMHg but not higher THg, if MMHg in lichen is more volatile and can be lost to the atmosphere over time as Hg°. More laboratory and field studies are needed to understand the dynamics of Hg species in lichen59.

We must also consider the potential for dry deposition of gaseous and particulate forms of MMHg, of which there have been no published reports. Throughfall fog water, which drips off of tree foliage, may become enhanced in MMHg due to the washing off of dry deposited MMHg that potentially accumulates during the long rainless season in California, similar to that observed in a mountain-top cloud water study of THg62. Additionally, marine fog water contains 90% of its MMHg in the particulate phase16 suggesting that either gaseous methylated Hg compounds are taken up by fog droplets and quickly partitioned to particles within the droplet, or MMHg in aerosols are scavenged by the droplet. Because there are no MMHg measurements in aerosols in coastal areas, we cannot determine whether wet or dry deposition of MMHg is a more source to coastal terrestrial ecosystems.

Geogenic sources of Hg from legacy mining activities in the Santa Cruz Mountains must also be considered as a potential source of Hg to the food web of this area63, as this impact has been observed near the Idria mine in Slovenia64. Results from our lichen samples from the Almaden Quicksilver mining site suggest that mining sources are a large contributor to inorganic Hg, but not MMHg in lichen. However, neither deer fur nor puma fur and whisker THg concentrations were abnormally high in the immediate vicinity of the Almaden Quicksilver mining site (Figs. 1B,C), suggesting that mining Hg may not be particularly mobile in the terrestrial food web compared to atmospheric Hg. The spatial pattern of enhanced puma THg concentrations spanned the entire central California coastal mountain area, with many of these locations are >100 km from Hg mining sites, suggesting that mining sources are of limited importance to the bioaccumulation of Hg in puma in this region.

Lastly, food web variation between coastal and inland puma populations may also potentially explain the differences in concentrations of MMHg between the two geographical areas. However, this is not likely since the diet of pumas in this region is well documented to be fairly simplified especially compared to other puma populations (i.e. limited use of aquatic prey). Moreover, we observed higher MMHg concentrations in coastal areas, compared to inland sites, across all measured trophic levels and this provides multiple lines of evidence that the marine fog input, to the base of the food web, is an important driver of the overall THg bioaccumulation regime in these terrestrial food webs in central coastal California.