Influence of catchment vegetation on mercury accumulation in lake sediments from a long-term perspective

- Vegetation affect both Hg concentrations and accumulation rates in Herrenwieser See.
- Coniferous vegetation give higher Hg concentration but lower Hg accumulation rate.
- Lake sediments give valuable insights into the effect of slow and gradual processes.

Organic matter (OM) cycling has a large impact on the cycling of mercury (Hg) in the environment. Hence, it is important to have a thorough understanding on how changes in, e.g., catchment vegetation - through its effect on OM cycling - affect the behavior of Hg. To test whether shifts in vegetation had an effect on Hg-transport to lakes we investigated a sediment record from Herrenwieser See (Southern Germany). This lake has a well-defined Holocene vegetation history: at ~ 8700 years BP Corylus avellana (hazel) was replaced by Quercus robur (oak), which was replaced by Abies alba (fir) and Fagus sylvatica (beech) ~ 5700 years BP). We were particularly interested in testing if coniferous vegetation leads to a larger export of Hg to aquatic systems than deciduous vegetation. When hazel was replaced by oak, reduced soil erosion and increased transport of DOM-bound mercury from the catchment resulted in increases in both Hg-concentrations and accumulation rates (61 ng g− 1 and 5.5 ng cm− 2 yr.−1 to 118 ng g− 1 and 8.5 ng cm− 2 yr.−1). However, even if Hg-concentrations increased also in association with the introduction of fir and beech (173 ng g− 1), as a result of higher Hg:C, there was no increase in Hg-accumulation rates (7.6 ng cm− 2 yr.−1), because of a decreased input of OM. At around 2500 years BP Hg-accumulation rates and Hg-concentration indicated an additional input of Hg to the sediment (316 ng g− 1 and 10.3 ng cm− 2 yr.−1), which might be due to increased human activities in the area, e.g., forest burning or mining. Our results contrast those of several paired-catchment studies that suggest a higher release of Hg from coniferous than deciduous forest, and there is a need for studies with a long-term perspective to increase our understanding of the effects of slow and gradual processes on mercury cycling.

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