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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