The effect of forest vegetation on the formation of mercury accumulation signals in lake sediments
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Mercury (Hg) accumulation in aquatic systems and its enrichment in aquatic food chains is still a matter of concern despite anthropogenic emission have strongly decreased in the last decade with the exception of emissions from smalll scale gold mining. Most Hg in lake catchments is derived by atmospheric deposition and enters the lake directly or through transport from the catchment. Although a large number of lake studies on Hg accumulation in sediments exist, the formation of Hg signals in the lake and the role of catchment vegetation, esp. forest type are still poorly understood. In this project, we investigated the effect of Holocene changes in forest type on the accumulation of Hg in lake sediments by means of sediment core analyses at three oligotrophic brown water lake sites in the Black Forest, Germany. In addition, we analysed the present day Hg signal formation in those brown water lakes by means of soil, lake water phase and sediment trap analyses and compared the results to those obtained from three meso- to eutrophic lakes/reservoirs in the Harz mountains with catchments characterized by decidious forest and higher nutrient fluxes. The combination of vegetation reconstruction based on pollen data and a large geochemical data set reveals that forest type had an influence on sediment Hg concentration during preanthropogenic times, where coniferous forest caused higher Hg concentrations in sediments but did not increase Hg accumulation rates. However, sediment Hg accumulation rates appear to increase during warmer periods as a results of Hg scaveging by algae when phyoplankton productivity in the lakes increased. With the upcoming human influence Hg accumulation rates began to increase significantly attributed to forest clearing induced enhanced erosion and emissions from metal mining and processing. Principal component analysis further demonstrates that these human induced processes largely overwrite natural variation in Hg accumulation in these lakes. The comparison of the present day Hg signal formation at the two lake sites shows that soil Hg concentrations are in a similar range in the coniferous and the deciduous lake catchments sites while water phase Hg concentrations were higher in the oligothrophic brown water lakes attributed to higher concentrations of dissolved organic matter (DOM). The most striking difference between the two sites were found in the sediment trap material and Hg accumulation rates. Although Hg concentrations were on an average about fourfold lower, Hg accumulation rates in the Harz lakes were up to 14-fold higher than those derived from the traps of the brown water lakes. The chemical composition and the FTIR spectra of the trap material from the Harz lakes clearly show that the material consists to a large extent of algae material and that the higher Hg accumulation rates can not be explained by higher Hg erosion fluxes (higher precipitation rates, larger catchments) or higher atmospheric Hg deposition. We explained the lower Hg concentrations and high Hg accumulation rates in the sediment traps from the Harz lakes by intense and repeated Hg scavenging by algae derived organic matter from the water phase, which causes dilution of Hg concentrations in the trap material and Hg depletion in the water phase. To obtain Hg mass balance, between 13 and 55 scavenging events (where all Hg is remove from the water phase) in the productive Harz lakes are necessary, whereas only between 0.6 to 5 events are necessary in the Black Forest lakes. Our results clearly indicate for the first time that Hg scavening by algae organic matter is a major control on the extent of Hg accumulation in lakes and that productive lakes are by far larger Hg sinks than oligotrophic brown water lakes.
Publications
- Do changes in forest vegetation change mercury accumulation in lake sediments? - A case study from the black forest (Germany). International Conference on Mercury as a Global Pollutant (ICMGP) 2017
Schütze, M. Rösch, M., Biester, H
- (2018) Prediction of Holocene Mercury Accumulation Trends by Combining Palynological and Geochemical Records of Lake Sediments (Black Forest, Germany). Geosciences, 8, 358
Schütze, M. Tserendorj, G., Pérez-Rodríguez, M., Rösch, M., Biester, H.
(See online at https://doi.org/10.3390/geosciences8100358) - Does coniferous forest lead to a larger export of Hg to aquatic systems? A Holocene vegetation study of the northern Black Forest (Germany). EGU General Assembly 2018
Schütze, M. Tserendorj, G., Biester, H
- Distribution of Hg in lake systems ̶ coniferous versus deciduous forest and the important role of Hg scavenging by algae. International Conference on Mercury as a Global Pollutant (ICMGP) 2019
Schütz, M., Gatz, P., Gilfedder, B-S. Biester, H.
- (2020) Why productive lakes are larger mercury sedimentary sinks than oligotrophic brown water lakes. Limnology and Oceanography, 9999, 1-17
Schütze, M., Gatz, P., Gilfedder, B-S. Biester, H.
(See online at https://doi.org/10.1002/lno.11684)
