Effects of root mat buoyancy and heterogeneity on floating fen hydrology

Abstract

Floating fen ecosystems are home to several protected habitats and species. Their development and conservation require special attention regarding water management. Although they are known to be heterogeneous and partially buoyant, their root mats are simulated in hydrological models as homogeneous, static systems. The objective of this study is to quantify root mat heterogeneity and buoyancy and to assess their effects on groundwater flow and transport, and to determine if these factors need to be taken into account in modelling. We conducted field measurements of root mat heterogeneity and buoyancy in the 'Nieuwkoopse Plassen', the Netherlands. We found that hydraulic conductivity varied over four orders of magnitude and negatively correlated with degree of decomposition, resulting in a zonation of high conductivity near the surface and low conductivity in the deeper layers. Also, we found that the root mat moved vertically with the surface water. It became more buoyant with higher temperatures, but less buoyant with increasing groundwater levels relative to the surface. We implemented the findings in a semi-steady state hydrological model of a floating fen to compare the effects with other parameters. The profound heterogeneity had a limited effect on the water budget, but a clear effect on the flow lines and thus should be taken into account when modelling transport processes in floating fens. Although buoyancy affected the relative groundwater level near the root mat edge, it did not affect the water budget or the flow lines and may therefore be neglected in water budget modelling.

Salinity-induced increase of the hydraulic conductivity in the hyporheic zone of coastal wetlands

“…Literature reports large decreases in the hydraulic conductivity of peat soils as a consequence of increased gas bubble content; Baird and Waldron () report a decrease by a factor two, while Reynolds et al () found a decrease by a factor four. Although the hydraulic conductivity in peat soils can vary greatly due to large variations in peat quality, bulk density, and rate of decomposition (e.g., Boelter, ; Chason & Siegel, ; Stofberg et al, ), gas bubbles may affect the hydraulic conductivity in a wide range of peat soils.…”

“…Most studies, however, deal with mineral soils, and little knowledge exists about the effects of salinity on the interaction between biogeochemical processes and the hydraulic conductivity of peat soils. Water flow through wetland sediments is affected by the hydraulic conductivity, which can vary greatly among different types of peat with varying degrees of decomposition (e.g., Boelter, ; Chason & Siegel, ; Stofberg, Van Engelen, Witte, & Van der Zee, ). Laboratory experiments and models (e.g., Comas & Slater, ; Kettridge & Binley, ; Ours, Siegel, & Glaser, ) indicate that increased salinity can increase flocculation of particles such as humic acids in peat soils, causing pore dilation which potentially influences the hydraulic conductivity of peat soils.…”

Hydrological controls on salinity exposure and the effects on plants in lowland polders

“…We developed a hydrological model to simulate groundwater and soil water dynamics of a floating fen, using hydrological field measurements of a study area in the Netherlands, in combination with data of a previous study (Stofberg et al, 2016). The model was calibrated and validated using groundwater level data that were collected in the field.…”

“…At the newly created edge, the vegetation has so far remained ombrotrophic. The root mat material consists of peat in various stages of humification, as described in Stofberg et al (2016). The topography was determined in three transects perpendicular to surface water edge, using a level and rod, and the root mat thickness was estimated using augers and a device that can be unfolded after it is pushed into the soft soil, making it possible to feel where the root mat begins when it is pulled upwards.…”

“…Considering the important role of hydrology with regard to direct effects (wetness, drought) and indirect effects (transport processes affecting water biogeochemical processes), the hydrological system needs to be well understood. Previous research focused on steady state analyses (Stofberg et al, 2016;van Wirdum, 1991) or such long term budgets, that variations in time became underemphasized (Koerselman, 1989). However, for shallow systems as floating fens, dynamics are likely to occur at relatively small timescales, which are not well understood.…”