Water table fluctuations and groundwater supply are important in preventing phosphate-eutrophication in sulphate-rich fens: Consequences for wetland restoration

Nitrate leaching from agricultural land leads to oxidiation of FeSx in FeSx-containing subsoils resulting in $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ mobilisation. Pollution of the groundwater with $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ causes a higher availability of $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$, eutrophication and loss in biodiversity in groundwater fed fens with stagnating surface water. Under natural conditions, fens along the river Meuse are continuously fed by groundwater that besides $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ mostly also contains high concentrations of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ and bivalent cations (Ca2+ and mg2+). During summer groundwater input is restricted resulting in periodic drought. Under these conditions no $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ induced $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$ eutrophication occurs. Periodic drought and a high discharge of $\mathrm{N}{\mathrm{O}}_{3}^{-}$, have a strong effect on S and P biogeochemistry in sulphate-rich fens. $\mathrm{N}{\mathrm{O}}_{3}^{-}$ inhibits $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ reduction and concomitant $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$ mobilisation in fen sediments by being an energetically more favourable electron acceptor. In addition, $\mathrm{N}{\mathrm{O}}_{3}^{-}$ is capable of oxidising reduced Fe compounds, including FeSx, increasing the amount of oxidised Fe in the sediment capable of binding $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$. Periodic drought is important in reincreasing the concentration of oxidised Fe in the top layer of S-rich sediments preventing $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$ mobilisation and an undesirable vegetation development. Damming of surface water, in order te restore desiccated sulphate-rich fens, prevents periodic drought and decreases groundwater input. This leads to $\mathrm{N}{\mathrm{O}}_{3}^{-}$ depletion, stimulation of $\mathrm{S}{\mathrm{O}}_{4}^{2-}$ reduction, Fe depletion, $\mathrm{o}-\mathrm{P}{\mathrm{O}}_{4}^{3-}$ mobilisation and, in contrast to what was hoped for, in massive growth of algae, lemnids and fast growing wetland grasses. Therefore discharge of $\mathrm{N}{\mathrm{O}}_{3}^{-}$ – rich groundwater and the fluctuation of the water table are vital for succesful restoration of desiccated sulphate-rich fens. Successful rewetting of these type of fens, without causing stagnation of surface water and without preventing periodic drought, can be achieved by raising the water table to level