Overview
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Key numbers
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- similar in burned and unburned watersheds, but filter-passing MeHg was 51% greater in burned watersheds, and suspended particles
- invertebrates, MeHg in grazers, filter-feeders, and collectors was 33, 48, and 251% greater in burned watersheds, respectively, but did
- have exacerbated these effects.9 As a result, larger, longer-lasting, contributes to 81% of all fish consumption advisories within the
- Depending on burn intensity and other factors, up to 97% of Hg laboratory studies have shown that although wildfire ash can
- To address some of the knowledge gaps related to the effects watershed) followed by shrub/scrub (median 13.2%; Table
- the understanding of medium-term wildfire effects on Hg water and sediment samples are summarized in Table S2. Six
- Oregon, Washington, and Idaho, U.S.A. (Figure 1A; Table S1). or D-nets. Soil samples were collected from areas with different
- watershed burned at high severity; Figure 1D; Table S1). processing and analysis.
- Watershed areas ranged 1.19−235 km2 (median 11.0 km2). Water samples were analyzed for filter-passing (<0.7 μm)
- cover dominated by evergreen forest (median 80.4% of (p.THg and p.MeHg), and suspended particulate material
- to this work can be accessed online.59 partial residuals that have been adjusted using the median value
- Statistical significance was assessed using an α value of 0.05. have been adjusted using the median value of each parameter
- tions below the laboratory reporting limit (42% of f.MeHg and resource type as fixed-effects, a burn status X resource type
- 59% of p.MeHg values) were used as reported (i.e., values that interaction, DOC, a burn status X DOC interaction, and a
- Figure 2. Least-squares mean (±SE) dissolved organic carbon (DOC; A), filter-passing total mercury (THg; C), and filter-passing methylmercury
- catchments. The horizontal green lines and shaded bands in parts B, D, and F represent the least-squares mean ± SE concentrations in the 21 reference
- least-squares mean MeHg concentration of the guild as a watersheds when accounting for differences in DOC (p = 0.23;
- 19% lower in burned watersheds compared to associated on filter-passing fractions. Burned watersheds had higher
- reference watersheds (p = 0.10, Figure 2A). Although DOC concentrations of SPM (99% greater, p = 0.067; Figure 3A),
- concentrations tended to decline with increasing watershed volumetric p.THg (89%, p < 0.001; Figure 3C), and volumetric
- burn severity, this relationship was weak and not significant (p = p.MeHg (178%, p < 0.0001; Figure 3E) compared to reference
- (p < 0.001−0.034; Table S3), with the exception of THg 3B), volumetric p.THg (p = 0.028; Figure 3D), and volumetric
- status X DOC interaction p = 0.12−0.95; Table S3), except that high burn severity (p = 0.026; Figure 3H). In contrast, burned
- the slope between DOC and filter-passing THg concentrations watersheds had 40% greater gravimetric p.MeHg concentrations
- = 0.022; Table S3). Given the apparent role of DOC in The percentage of p.THg as p.MeHg (%p.MeHg) was also 38%
- Figure 3. Least-squares mean (±SE) suspended particulate material (SPM; A) and particulate total and methylmercury (THg and MeHg, in ng/L and
- The horizontal green lines and shaded bands in parts B−J represent the least-squares mean ± SE concentrations in the 21 reference watersheds. Least-
- concentrations did not increase with an increasing percentage of Table S3). The relationships between DOC and biological
- in burned and reference watersheds (p = 0.82). As noted in the 0.86; Table S3), except among grazers and collectors (burn
- in Figure 4A−C, p ≤ 0.017; Table S4). However, interaction terrestrial leaf litter and periphyton, respectively) had similar
- watersheds (Figure 4A−C; regression p ≥ 0.25, Table S4). reference watersheds when accounting for differences in DOC
- relationships between stream volumetric p.THg and soil THg concentrations were elevated in grazers (33%, p = 0.096),
- individual relationships were weak (p = 0.042−0.47; Table S4). burned watersheds compared to reference watersheds (Figure
- between soil THg and stream sediment THg (p < 0.001−0.22), any of the basal resources or invertebrate guilds (Table S5).
- terms ≥0.19; Figure 4G−L; Table S4). relationships between Hg concentrations in water and
- Figure 4. Relationships between area-weighted geometric mean THg concentrations in watershed soil layers (duff, organic soils, and mineral soils) and
- (Figure S3; Table S7). Similarly, relationships between sediment southeastern U.S. However, when DOC was not accounted for,
- Figure 5. Least-squares mean (±SE) MeHg concentrations in (A) in-stream terrestrial leaf litter, (B) periphyton, and (C) aquatic invertebrate guilds
- collected from 21 reference and 36 burned watersheds in the northwestern U.S. Least-squares means account for variation among individual fires and
- burned watersheds than in reference watersheds (25%; p = watersheds to headwater streams,71−74 the temporal variability
- wildfire-affected streams typically increase in the short term (i.e., p.MeHg were 89−178% greater in burned compared to
- ment.14,15,33,85,86 Although our results suggested that the on the suspended particles from burned watersheds (40%
Methods (brief)
- sampled soils, water, sediment, in-stream leaf litter, periphyton, and
- Figure 1. (A) Map of sampled wildfires from 2020 to 2021 in the northwestern U.S. (B) Example of sampled burned and reference watersheds
- sediment, and biological samples in Bauers Creek and Cox Creek. (D) Summary of wildfire severity in each of the 57 sampled watersheds based on the
- recovery one-year post burn. These conditions fill an important Sample Collection and Analysis. Sample collection and
- in the period after the first flush/snowmelt post fire (i.e., short- in-stream leaf litter, and aquatic invertebrates were collected in
- of wildfire occurrence and severity on (1) in-stream and soil samples (duff, organic soil, and mineral soil) were
- concentrations of aqueous filter-passing and particulate THg collected at upgradient locations (Figure 1C). All samples were
- and MeHg, and sediment total Hg (THg); (2) Methylmercury collected using trace metal clean procedures51 during baseflow
- with wildfire effects and enabled us to identify broad patterns Water samples were collected directly into 2 L polyethylene
- number of fire-affected watersheds. To date, this is the first of flow. Sediment samples consisted of composites of the top
- Hg cycling targeting watershed (soils) and instream (water, sampled reach of each stream and were collected by using a clean
- the understanding of medium-term wildfire effects on Hg water and sediment samples are summarized in Table S2. Six
- cycling and bioaccumulation in headwater streams and highlight water sample field blanks were also collected and are
- and increasing pressures on quality water supplies worldwide. terrestrial leaf litter was collected from submerged leaf-dams and
- Site Selection. Between 2021 and 2022, we sampled 57
- not included in the samples. Aquatic invertebrates representing
- complexes (henceforth collectively referred to as wildfires) in predators. Invertebrates were primarily collected using kicknets
- Oregon, Washington, and Idaho, U.S.A. (Figure 1A; Table S1). or D-nets. Soil samples were collected from areas with different
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