Previously unaccounted atmospheric mercury deposition in a midlatitude deciduous forest

Abstract

Mercury is toxic to wildlife and humans, and forests are thought to be a globally important sink for gaseous elemental mercury (GEM) deposition from the atmosphere. Yet there are currently no annual GEM deposition measurements over rural forests. Here we present measurements of ecosystem-atmosphere GEM exchange using tower-based micrometeorological methods in a midlatitude hardwood forest. We measured an annual GEM deposition of 25.1 µg ⋅ m^-2 (95% CI: 23.2 to 26.7 1 µg ⋅ m^-2), which is five times larger than wet deposition of mercury from the atmosphere. Our observed annual GEM deposition accounts for 76% of total atmospheric mercury deposition and also is three times greater than litterfall mercury deposition, which has previously been used as a proxy measure for GEM deposition in forests. Plant GEM uptake is the dominant driver for ecosystem GEM deposition based on seasonal and diel dynamics that show the forest GEM sink to be largest during active vegetation growing periods and middays, analogous to photosynthetic carbon dioxide assimilation. Soils and litter on the forest floor are additional GEM sinks throughout the year. Our study suggests that mercury loading to this forest was underestimated by a factor of about two and that global forests may constitute a much larger global GEM sink than currently proposed. The larger than anticipated forest GEM sink may explain the high mercury loads observed in soils across rural forests, which impair water quality and aquatic biota via watershed Hg export.

Keywords: dry deposition; mass balance; mercury cycling.

Fig. 1.

(A) 30-min resolution GEM exchange fluxes measured over Harvard Forest. Negative fluxes denote deposition, and positive fluxes represent emissions. (B) Daily mean GEM fluxes and median monthly GEM fluxes (green lines). (C) Cumulative sums of daily fluxes of GEM and CO₂ starting on May 1, 2019. Missing flux values were interpolated using median monthly values. Shaded lines represent 95% CIs based on random error analysis as described in SI Appendix. Corresponding patterns of LAI are shown together with the CO₂ fluxes on the secondary y-axis.

Fig. 2.

(A) Diel patterns of ecosystem-level GEM exchange fluxes shown as monthly hourly median fluxes and upper and lower quartile ranges. (B) Corresponding diel patterns of monthly hourly median fluxes of CO₂.

Fig. 3.

Cumulative sum of ecosystem-level and forest-floor GEM exchanges. Forest-floor GEM fluxes are quantitatively uncertain due to application of the flux-gradient method below canopies and should be viewed as a measure of underlying flux direction. Shaded lines represent 95% CIs based on random error analysis as described in SI Appendix. Corresponding patterns of LAI (in meters² leaf area and meters⁻² ground surface area) and daily air temperatures (in °Celsius; line represents smoothed data) are shown on the secondary y-axis.