Mercury in demo areas

In the project WAMBAF we collected biota, water and sediment samples in four beaver dam demo areas in Poland, Latvia, Lithuania and Sweden, to investigate if removal of beaver dams could mitigate the risk of elevated methylmercury (MeHg) concentrations downstream. Furthermore, we have evaluated if ditch cleaning activities increase Hg in biota in demo areas in Sweden and Finland.


Motivation for studying beaver dam removal

The Eurasian beaver (Castor fiber L.) was successfully re-introduced in many European countries during early 20th century, after extirpation in these areas during mainly the 19th century. Beavers as true ecosystem engineers build dams and transform streams into a series of wetlands and/or ponds with locally stagnant water. This may influence the accumulation, distribution and availability of elements by altering hydrological regimes that can change biogeochemical pathways. Beavers may also mobilize elements from the terrestrial vegetation to sediments during flooding. This may affect the cycling of mercury, and specifically the transformation of Hg to its more bioavailable form, methylmercury. Elevated mercury concentration in biota and water downstream of beaver dams have thereby been detected in former studies. Here we test the hypothesis that removal of beaver dams could mitigate elevated mercury concentrations in downstream biota.

Motivation for studying effects of ditch cleaning

A high share of productive forest land in Sweden, Finland and Latvia among other countries, consists of artificially drained peatlands. As these ditches age, ditch cleaning is commonly needed to allow the next generation of trees to establish and grow. There is, however, currently very limited empirical data about environmental consequences of ditch cleaning activities. Ditch cleaning can cause erosion of sediment/peat into the runoff column.

Furthermore, ditch cleaning may influence the hydrological pathways, both by changing the ground water level and by causing compaction and rutting in the riparian zone when using heavy vehicles. This could all together influence the release of both total mercury and methylmercury to downstream ecosystems.

Sampling design and sampling methods

We sampled stream biota upstream and downstream of beaver ponds and in ditch cleaning networks during six occasions in a three years period in demo areas in Finland, Sweden, Latvia, Lithuania and Poland. To evaluate the effect of beaver dam removal and ditch cleaning, sampling was conducted three times before and three times after the dam was removed and the ditches were cleaned, respectively. Different taxa of stream organisms covering a wide range of trophic levels were sampled. We divided these animals in six groups: blackfly larvae, crustacean, mid-size mayfly larvae, small mussels, freshwater snails and fishes. The relation of biota mercury concentrations in downstream compared to upstream site were compared for each location, sampling occasion and animal group, before and after treatment (dam removal and ditch cleaning, respectively), using a mixed model approach.

Sediment samples were collected in beaver dams from upstream, within the beaver pond and downstream sites, to evaluate if the beaver pond sediment acted as a source of methylmercury. Sediments were collected during two occasions; around one to six months before and around one to three months after beaver dam removal. Sediments were analyzed for total mercury, methylmercury, nitrogen, carbon and sulfur concentrations.

Water samples for analyses of mercury were sampled during two occasions in the upstream and downstream sites in all demo areas. The aim of these measures was mainly to identify the approximate levels of mercury and methylmercury in the water phase in these systems, not to evaluate any management practices.


High fish mercury concentrations are of concern in Scandinavia, and many monitoring activities have thereby been carried out in Sweden, Norway and Finland. In several of the countries in the Baltic Sea Region, for example Latvia, Lithuania and Poland, there is a lack of data on mercury in biota. The WAMBAF project have thereby greatly increased the data availability in these countries. The results show that the concentrations of Hg in biota in these countries are in the same range as in Sweden and Finland, indicating that high Hg in biota is not only a Scandinavian issue, but may be a concern in the other Baltic Sea Region countries as well.

Results from beaver dam removals

Our results indicate that the sediments in the beaver ponds acted as a source of MeHg. After removing the beaver dam and re-transforming the system to a stream, the former sediments, which were now transformed to terrestrial soil, were still high in MeHg.

Analysis of biota samples indicate that after removing the beaver dam concentrations of Hg in biota decreased to the level of the reference (upstream) values. In a Hg-laden environment, beaver dam removal could therefore be a tool to mitigate high mercury downstream. However, this should be balanced against beaver activities often having other positive environmental effects such as buffering of stream flow, increasing habitat and landscape heterogeneity, and functioning as sediment trap with beneficial effects on biota diversity.

Earlier research found young pioneer beaver ponds to increase Hg concentrations more compared to old recolonized beaver ponds. Therefore, we suggest that the removal of beaver dams would be more efficient in decreasing Hg burden downstream in young pioneer systems compared to old recolonized systems.

Results from ditch cleaning activities

Concentrations of mercury in biota did not change in response to ditch cleaning in demo areas in Sweden and Finland. The lack of change in Hg concentration could be due to ditch cleaning having limited effect on the release of bioavailable methylmercury. However, these results could also be caused by the quite limited relative area that was affected by ditch cleaning in the downstream site, both in Sweden and Finland.

  • Last Updated: 12/8/2020