Flooding and groundwater dynamics in fens in eastern Poland

Abstract. Dynamics in hydrology and water chemistry in the Biebrza mires (Poland) were examined by means of a sampling survey that was repeated four times between 1987 and 1992. The dynamics in the vertical stratigraphy of water types in the peat profile are considerable from close to the mire surface to a depth of 50 cm. Water composition in the root zone correlated best with vegetation types during extremely dry or wet conditions. In the root zone of groundwater-fed rich fens with Caricetum limoso-diandrae and Calamagrostietum strictae vegetation, specific groundwater types evolve from the interaction of discharging groundwater from below the root zone and the temporal influence of precipitation and evapotranspiration. The Caricetum limoso-diandrae is fed by the continuous discharge of nutrient-poor, relatively mineral-rich water. The site conditions in the Calamagrostietum strictae are determined by occasional flooding and the presence of discharging mineral-poor groundwater in the lower part of the root zone. In the Caricetum limoso-diandrae and the Calamagrostietum strictae the maximum variations in water level were 56 and 86 cm, respectively. The composition of shallow groundwater of the Betuletum humilis/Caricetum rostratodiandrae fen is diluted most compared to other vegetation types by rainwater in wet periods. In periods of prolonged drought it has a water type that is affected by evapotranspiration and peat mineralisation. The water level varies by only 33 cm. In the Magnocaricion and Glycerietum maximae in the floodplain the water composition is determined by spring flooding of the river and the natural draw-down that occurs in the following summer. Here, maximum variations in water level were 108 and 117 cm, respectively.     

Effects of water regime on growth,yield, and nitrogen uptake of rice

Summary The effects of water regime on the performance of rice were investigated in a greenhouse experiment and two field experiments. The greenhouse experiment involved four water regimes (continuous flooding, and soil drying for 16 days — begun 2, 5, and 8 weeks after transplanting — followed by reflooding), four soils, and 0 and 100 mg N/kg. Soil drying raised the redox potentials of all soils beyond the aerobic threshold. Averaged for soils and N levels, yields from treatments in which soil drying was begun at 2 and 5 weeks after transplanting were lower than that from the continuously flooded treatment, but the simple effects of soil drying on yield and N uptake depended on the soil and the growth stage of the plant. None of the soil-drying treatments had adverse effects in the soil high in N but soil drying at 2 and 5 weeks after transplanting had adverse effects in the soil low in N. The field experiments tested the effects of three water regimes (continuous flooding, alternate drying and flooding every 2 weeks, and soil drying for 2 weeks at 6 weeks after transplanting following by reflooding), and 0, 50, 100, and 150 kg N/ha on a nearly neutral clay soil, during two seasons. None of the soil-drying treatments depressed growth, yield, or N uptake by rice at any level of N in either season. Nitrate was absent after drying, so denitrification was not possible on subsequent flooding. The adverse effects on yield of alternate flooding and drying, attributed to nitrification-denitrification, may be insignificant in wetland fields carrying an actively growing rice crop.     

Effects of temporary desiccation on the mobility of phosphorus and metals in sulphur-rich fens: differential responses of sediments and consequences for water table management

In the Netherlands, permanent damming of sulphate (SO₄ ^2–)-rich surface water, in order to rewet desiccated wetlands, has resulted in stagnation and eutrophication of surface water. Permanent damming of surface water prevents periodic drought during summer and leads to suppressed iron (Fe)-rich groundwater input and to stimulated SO₄ ^2– reduction, all likely stimulating depletion of reducible Fe in the sediment. A laboratory experiment was conducted to assess the importance of temporary desiccation, its differential effects on various sediment types and the consequences for water table management. Permanent high SO₄ ^2–-rich surface water tables above sediments that are indirectly affected by shallow groundwater flows, resulted in severe eutrophication. The effect of temporary desiccation on phosphorus (P) mobilization and immobilization strongly depended on the sediment Fe and P pools in combination with the buffering capacity of the sediment. Desiccation of sediment that is indirectly affected by shallow groundwater flows, led to a long-lasting reduction in phosphate (o-PO₄ ^3–) release from the sediment because a reduced Fe pool is present, resulting in the release of Fe upon oxidation. Formerly dry sediments that have not been influenced by groundwater for a long time do not possess such a reduced Fe pool and desiccation did not reduce P-release from these sediments resulting in considerable eutrophication of the water layer immediately after rewetting. In sediment of seepage zones that are directly and permanently influenced by deeper groundwater, reduced Fe and calcium levels are so high that o-PO₄ ^3– was effectively immobilized under oxidized as well as reduced conditions. The results indicate that restoration of desiccated wetlands can not be achieved by simply retaining water by means of constructed dams. If water retention is artificially increased, temporary drops in water level during the summer are necessary to recharge the reducible P-binding Fe pool in large zones of the wetlands in order to prevent eutrophication.     

Morphological changes and resource allocation of Zizania latifolia (Griseb.) Stapf in response to different submergence depth and duration

Plants around ponds, rivers and lakes are subjected to long-term partial or complete submergence. When they are flooded, water level affects the plants simultaneously with duration of submergence. Separate and interactive effects of water level and duration on the growth of the herbaceous perennial Zizania latifolia (Poaceae) were investigated by exposing the plants in greenhouse water tanks to submergence in different water depths and for different time-spans. The plants exhibited great shoot elongation upon submergence and prolonged flood duration, and the basal tiller number of the species decreased with higher water levels. Submergence treatment advanced the flowering date and increased the inflorescence number. Plant total biomass did not differ among all the treatments, while the root:shoot ratio decreased with increased water level, prolonged duration of submergence and their interaction. The high plasticity in morphology and shifts in reproductive strategy and biomass allocation enabled the Zizania plants to survive the compound effect of flooding height and duration. This may explain the occurrence of this species in habitats subjected to long-term flooding. The results obtained in this experiment will contribute to understanding the impact of flooding dynamics on plants and the ways of adaptation responses to prolonged waterlogging.     

Seasonal plasticity of Ludwigia grandiflora under light and water depth gradients: An outdoor mesocosm experiment

Abiotic factors in the introduced area can limit the establishment of exotic species. Adaptation to new abiotic parameters through plastic responses is important for their establishment and distribution pattern. Survival and growth of exotic hydrophytes are related to their plasticity and their ability to adapt to local conditions. Light and water depth are two of the predominant factors determining survival, growth, distribution and abundance of aquatic plants. Although many studies have been carried out on the effect of water level and light on plant performance, their potential interactions are poorly understood. Here, we carried out an outdoor mesocosm experiment, in order to test the role of water depth, light intensity and their potential interactions on the growth and vigour of the invasive macrophyte species Ludwigia grandiflora. Three light intensities (100%, 70% and 50% of sunlight) and three water depths (30, 50 and 80 cm) were tested over a 6-week period, in spring, summer and autumn 2011. We showed that the mortality, biomass, apical and lateral growth of L. grandiflora, as well as its biomass allocation, varied according to the season and responded to both light intensity and water depth. Interactions between water depth and light intensity can have an amplifying effect on the growth of L. grandiflora. Similarities between shade and deep water adaptation responses were observed. A higher biomass and a high morphological plasticity in response to these factors were found particularly at the beginning of the life cycle. Although the invasive amphibious L. grandiflora shows a high tolerance to different water levels and light intensities, the optimal growth conditions for L. grandiflora in our experiment seem to be under full sunlight and in 30 cm of water. Colonization of light or low shade aquatic habitats, as well as shallow environments, can occur due to the high plasticity of L. grandiflora. These results may be important relative to increasing level of water bodies under climatic changes.     

Effects of water table depth and calcium perioxide application on cowpea (Vigna unguiculata) and soybean (Glycine max)

Summary The effects of three water table (WT) depths (0, 15 and 40 cm) and calcium peroxide (Calper) on the growth and yield of cowpea (Vigna unguiculata, L.) and soybean (Glycine max) were investigated in field lysimeters for a sandy loam soil. Cowpea growth was the best at 40 cm WT depth. Leaf area, plant height, dry matter production, number of leaves and pods, grain yield and consumptive water use of cowpea increases with deeper (lower) WT depth. Application of calcium peroxide improved per cent emergence, leaf area, dry matter, number of leaves and pods, weight of 100 seeds, grain yield and water use in cowpea. The optimum WT depth for vegetative growth of soybean was 15 cm, although the highest grain yield was obtained at 40 cm WT depth. Number of pods, grain yield and water use efficiency of soybean increased with deeper water table depth. Application of calcium peroxide to soybean increased number of leaves and pods per plant, and grain yield for the 15 cm WT depth treatment.     

Convergence of carbon sink magnitude and water table depth in global wetlands

Wetlands are strategic areas for carbon uptake, but accurate assessments of their sequestration ability are limited by the uncertainty and variability in their carbon balances. Based on 2385 observations of annual net ecosystem production from global wetlands, we show that the mean net carbon sinks of inland wetlands, peatlands and coastal wetlands are 0.57, 0.29 and 1.88 tons of carbon per hectare per year, respectively, with a mean value of 0.57 tons of carbon per hectare per year weighted by the distribution area of different wetland types. Carbon sinks are mainly in Asia and North America. Within and across wetland types, we find that water table depth (WTD) exerts greater control than climate- and ecosystem-related variables, and an increase in WTD results in a stronger carbon sink. Our results highlight an urgent need to sustain wetland hydrology under global change; otherwise, wetlands are at high risk of becoming carbon sources to the atmosphere.     

The relationships of vegetation to surface water chemistry and peat chemistry in fens of Alberta,Canada

The relationships between vegetation components, surface water chemistry and peat chemistry from 23 fens in boreal Alberta, Canada, substantiate important differences along the poor to rich fen gradient. Each of the three fen types have their own characteristic species. The extreme-rich fens are characterized by Calliergon trifarium, Drepanocladus revolvens, Scirpus hudsonianus, S. cespitosus, Scorpidium scorpioides, and Tofieldia glutinosa. Moderate-rich fens are characterized by Brachythecium mildeanum, Carex diandra, Drepanocladus vernicosus, D. aduncus, and D. polycarpus. Poor fens are characterized by Carex pauciflora, Drepanocladus exannulatus, Sphagnum angustifolium, S. jensenii, and S. majus. Moderate-rich fens have fewer species in common with poor fens than with extreme-rich fens, while species richness is highest in the moderate-rich fens and lowest in poor fens. Variation in vascular plant occurrence appears to be more associated with nutrient levels, while bryophytes are more affected by changes in acidity and mineral elements. Based on chemical criteria, the three fen types are clearly separated by surface water pH, calcium, magnesium, and conductivity, but are less clearly differentiated by the nitrogen and phosphorus components of the surface waters. Moderate-rich fens are chemically variable both temporally and spatially, whereas poor fens and extreme-rich fens are more stable ecosystems. Whereas components of alkalinity-acidity are the most important factors that distinguish the three fen types in western Canada, nutrient concentrations in the surface waters generally do not differ appreciably in the three fen types.     

Increased soil stable nitrogen isotopic ratio following phosphorus enrichment: historical patterns and tests of two hypotheses in a phosphorus-limited wetland

We used a P enrichment gradient in the Everglades to investigate patterns of the stable N isotopic ratio (δ¹⁵N) in peat profiles as an indicator of historic eutrophication of this wetland. We also tested two hypotheses to explain the effects of P on increased δ¹⁵N of organic matter including: (1) increased N mineralization/N loss, and (2) reduced isotopic discrimination during macrophyte N uptake. Spatial patterns of δ¹⁵N in surface litter and soil (0–10 cm) mimic those of the aboveground macrophytes (Typha domingensis Pers. and Cladium jamaicense Crantz). Peat profiles also show increased δ¹⁵N in the peat accumulated in areas near the historic P discharges since the early 1960s. The increased δ¹⁵N of bulk peat correlated well with both measured increases in soil total P and the historical beginning of nutrient discharges into this wetland. In 15-day bottle incubations of soil, added P had no effect on the δ¹⁵N of NH₄⁺ and significantly increased the δ¹⁵N of water-extractable organic N. Measurements of surface soils collected during a field mesocosm experiment also revealed no significant effect of P on δ¹⁵N even after 5 years of P addition. In contrast, δ¹⁵N of leaf and root tissues of hydroponically grown Typha and Cladium were shown to increase up to 12‰ when grown at elevated levels of P and fixed levels of N (as NH₄⁺). The magnitude of changes in δ¹⁵N resulting from altered discrimination during N uptake is significant compared with other mechanisms affecting plant δ¹⁵N, and suggests that this may be the dominant mechanism affecting δ¹⁵N of organic matter following P enrichment. The results of this study have implications for the interpretation of δ¹⁵N as an indicator of shifts in relative N limitation in wetland ecosystems, and also stress the importance of experimental validation in interpreting δ¹⁵N patterns.     

Patterned fens in Scotland: evidence from vegetation and water chemistry

Mires with attributes similar to the patterned fens of boreal continental regions have recently been reported from northern Scotland. This paper reports on the vegetation and water chemistry of 18 of these sites. Most of the 18 mires displayed remarkable uniformity in their vegetation, with Carex lasiocarpa being a main character species. Major variation of plant communities is associated with microtopographical niches, similar to those found on ombrotrophic mire systems and secondarily with pH and conductivity of surface waters. Nine vegetation noda are described and compared with existing phytosociological associations and National Vegetation Classification (NVC) communities. The dominant Sphagnum - Carex communities are intermediate between the Oxycocco-Sphagnetea and the Caricion lasiocarpae and are thus difficult to assign to existing associations or to NVC communities. Although there is a large variation in pH of surface waters (3.9–6.6), mean measurements for terrestrial and aquatic communities are 5.12 and 5.51 respectively. pH levels are comparable with poor fen systems elsewhere in Britain and slightly higher than patterned fens in continental regions of Europe and North America. The vegeta-tional and chemical attributes of these sites therefore confirm the existence of true patterned fen systems in the British Isles.     

The structure and function of fen lakes in relation to water table management in The Netherlands

Most relatively large (> 250 ha) and shallow (< 2 m), Dutch lakes originate from large-scale peat mining. Originally, their water tables were regulated by natural processes such as evapotransporation, precipitation, drainage and seepage (up and downwards). These lakes used to be oligo to mesotrophic, and the phytoplankton consisted mainly of diatoms and green algae. However, 30 years ago filamentous cyanobacteria were also present. Owing to the intensification of agriculture in the surroundings of the lakes, their management developed more and more towards a man-made reservoir function. This management resulted in inflow of eutrophic water from the agricultural areas in winter. In summer the lakes were increasingly used as a water supply. This supply is compensated by water imported mainly from the River Rhine. The impacts of this reservoir function on the limnology of the Dutch fen lakes are discussed with the Tjeukemeer, the Loosdrechtse Plassen and the Reeuwijkse Plassen as examples. All these lakes have become typical eutrophic waters with high turbidity, dominance of filamentous cyanobacteria and bream and complete loss of submerged macrophytes. Finally, methods and results of manipulation procedures to restore these waters are discussed.     

Community and species responses to water level fluctuations with reference to soil layers in different habitats of mid-boreal mire complexes

The present paper discusses water level fluctuations in different parts of boreal mire complexes (eleven localities), mainly aapa mire complexes, on the basis of measurements performed by means of shallow observation wells and a few deeper observation tubes (piezometers) in the coastal half of the southern aapa mire zone in Finland. The sites represented intact vegetation from 12 different habitat types (communities), which were divided a priori into habitats with a stable surface moisture status (stable habitats) and into habitats with an unstable surface moisture status (unstable habitats). In stable habitats water level fluctuations took place according to the acrotelm–catotelm model, but the unstable habitats clearly deviated from the general pattern: water level fluctuations in them were not at all concentrated to the surficial, porous peat layer. Direct gradient analysis was used for arranging the communities along the water level fluctuation gradient. Variability of the water table, using 80% amplitude of water table residence, was used for the arrangement. The gradient was split into three groups: (1) habitats with a slightly fluctuating water table, (2) habitats with a considerably fluctuating water table and (3) habitats with an extremely fluctuating water table. The last group nearly corresponded to aro wetlands, and represented a very special habitat type. Indirect gradient analysis (NMDS ordination) also revealed the water level fluctuation gradient along with the gradient of traditional water level categories. According to the results of direct and indirect gradient analysis, the water level fluctuation seems to be an independent and important vegetation gradient. In peatlands, it occurs alongside with the traditional gradient of water level categories reflecting the mean water table. The responses of species to the range of water level fluctuations seem to reflect their tolerance to disturbances and evidently to seasonal drought. Most Sphagnum species are absent or in poor condition in habitats with extremely fluctuating water table. Vascular plant species that experienced most extreme water level fluctuations (Carex nigra, Juncus filiformis) have earlier been regarded as disturbance indicators. In addition, the difference between the piezometric water level and simultaneously measured water table depth reached the highest values within the habitats of those species (i.e., within Polytrichum commune aro wetlands) showing the downward direction of water movement in sandy mineral soil.     

On the relationship between water table depth and water vapor and carbon dioxide fluxes in a minerotrophic fen

The focus of this study is the relationship between water table depth (WTD) and water vapor [evapotranspiration (ET)] and carbon dioxide [CO₂; net ecosystem exchange (NEE)] fluxes in a fen in western Canada. We analyzed hydrological and eddy covariance measurements from four snow‐free periods (2003–2006) with contrasting meteorological conditions to establish the link between daily WTD and ET and gross ecosystem CO₂ exchange (GEE) and ecosystem respiration (R_eco; NEE=R_eco?GEE), respectively: 2003 was warm and dry, 2004 was cool and wet, and 2005 and 2006 were both wet. In 2003, the water table (WT) was below the ground surface. In 2004, the WT rose above the ground surface, and in 2005 and 2006, the WT stayed well above the ground surface. There were no significant differences in total ET (～316 mm period^?1), but total NEE was significantly different (2003: 8 g C m^?2 period^?1; 2004: ?139 g C m^?2 period^?1; 2005: ?163 g C m^?2 period^?1; 2006: ?195 g C m^?2 period^?1), mostly due to differences in total GEE (2003: 327 g C m^?2 period^?1; 2004: 513 g C m^?2 period^?1; 2005: 411 g C m^?2 period^?1; 2006: 556 g C m^?2 period^?1). Variation in ET is mostly explained by radiation (67%), and the contribution of WTD is only minor (33%). WTD controls the compensating contributions of different land surface components, resulting in similar total ET regardless of the hydrological conditions. WTD and temperature each contribute about half to the explained variation in GEE up to a threshold ponding depth, below which temperature alone is the key explanatory variable. WTD is only of minor importance for the variation in R_eco, which is mainly controlled by temperature. Our study implies that future peatland modeling efforts explicitly consider topographic and hydrogeological influences on WTD.     

Response of the microbial community to water table variation and nutrient addition and its implications for in situ preservation of organic archaeological remains in wetland soils

Wetland environments can preserve organic archaeological remains because of their anaerobic nature. The ongoing discovery of archaeological sites in wetlands is associated with a lack of funds for excavation and preservation. This situation has led to the consideration of preservation in situ the preferred option for dealing with the majority of waterlogged archaeological remains in England. To expand our understanding of the burial environment, we studied changes in environmental variables along with counts of total bacteria and microbial ¹⁴C-leucine assimilation down the soil profile at two wetlands in the North of England. Soil cores were sampled at five depth intervals between 10 and 100 cm. To test whether the addition of nutrients induces bacterial activity in the soil, inorganic phosphate and combined nitrogen were added to soil samples and the rate of ¹⁴C-leucine assimilation was recorded. Redox potential readings were positive above the water table and negative below. The total number of bacteria and the ¹⁴C-leucine assimilation rates differed among sites, but always decreased with increasing soil depth. Nutrient availability was limiting for the microbial communities in the upper soil horizons, but did not appear to limit those in the lower soil. These results allow a better understanding of the physico-chemical and microbiological conditions that potentially favour or inhibit the decomposition of organic archaeological remains at the studied wetlands.     

Vegetation death and rapid loss of surface elevation in two contrasting Mississippi delta salt marshes: The role of sedimentation, autocompaction and sea-level rise

From 1990 to 2004, we carried out a study on accretionary dynamics and wetland loss in salt marshes surrounding two small ponds in the Mississippi delta; Old Oyster Bayou (OB), a sediment-rich area near the mouth of the Atchafalaya River and Bayou Chitigue (BC), a sediment-poor area about 70 km to the east. The OB site was stable, while most of the marsh at BC disappeared within a few years. Measurements were made of short-term sedimentation, vertical accretion, change in marsh surface elevation, pond wave activity, and marsh soil characteristics. The OB marsh was about 10 cm higher than BC; the extremes of the elevation range for Spartina alterniflora in Louisiana. Vertical accretion and short-term sedimentation were about twice as high at BC than at OB, but the OB marsh captured nearly all sediments deposited, while the BC marsh captured <30%. The OB and BC sites flooded about 15% and 85% of the time, respectively. Marsh loss at BC was not due to wave erosion. The mineral content of deposited sediments was higher at OB. Exposure and desiccation of the marsh surface at OB increased the efficiency that deposited sediments were incorporated into the marsh soil, and displaced the marsh surface upward by biological processes like root growth, while also reducing shallow compaction. Once vegetation dies, there is a loss of soil volume due to loss of root turgor and oxidation of root organic matter, which leads to elevation collapse. Revegetation cannot occur because of the low elevation and weak soil strength. The changes in elevation at both marsh sites are punctuated, occurring in steps that can either increase or decrease elevation. When a marsh is low as at BC, a step down can result in an irreversible change. At this point, the option is not restoration but creating a new marsh with massive sediment input either from the river or via dredging.     

Determining critical light and hydrologic conditions for macrophyte presence in a large shallow lake: The ratio of euphotic depth to water depth

Light determines macrophyte distribution, community composition and biomass in shallow lakes. Therefore, it is vital to determine the critical underwater light climate thresholds for macrophyte degradation and recovery. In this study, we first proposed a novel index, defined as the ratio of euphotic depth (Z_eu) to water depth (WD), as a measure of the underwater light supply for macrophytes. The underwater light environment in Lake Taihu (a large, shallow, eutrophic lake) was then characterized based on this index (Z_eu/WD) using field measurements collected from 2006 to 2013 (8 years × 4 seasons × 32 sites). The distribution of the macrophyte presence frequency (MPF, the number of investigations that identified macrophytes divided by the total number of investigations) was greater than 0.70 in Xukou Bay and East Lake Taihu over the 32 investigations, followed by the other sites distributed in East Lake Taihu. The proportion of macrophyte coverage increased with the increase in Z_eu/WD. A significant relationship was observed between Z_eu/WD and MPF for the 19 sites with macrophytes (r² = 0.48, p < 0.001, n = 19). In the region with high nutrient concentrations and serious water pollution, better underwater light conditions are required for the growth of macrophytes. A Z_eu/WD value of 0.80 can be regarded as the critical underwater light threshold for the growth of macrophytes in Lake Taihu. The region with Z_eu/WD ranging between 0.57 and 0.80 was usually covered by sparse macrophytes; this region should be vital for macrophyte recovery and environmental management in Lake Taihu. The distribution of Z_eu/WD was further obtained using MODIS satellite-derived Z_eu from June to October in 2003 and 2013. Xukou Bay and Guangfu Bay in the southern part of Lake Taihu could be regarded as potentially crucial regions for the recovery of macrophytes from the perspective of underwater light and nutrient levels.     

Relationship between water regime and hummock-building by Melaleuca ericifolia and Phragmites australis in a brackish wetland

We investigated the relationship between hummock height and depth of inundation in a permanently inundated wetland in south-eastern Australia. Our survey of 318 hummocks, in water ranging from 0 to 70 cm depth, revealed a significant positive linear relationship and strong correlation between hummock height and water depth (r² = 0.53 and 0.79 for Melaleuca ericifolia and Phragmites australis hummocks respectively). We also investigated whether water regime affects the decomposition rate of litter on hummocks; specifically, whether constant inundation slows decomposition to an extent that would promote accumulation of litter and hummock-building. On the contrary, we found that constantly submerged M. ericifolia litter decomposed faster than dry litter, but at a similar rate to litter that experienced intermittent inundation (decay rates (k) 0.0015 d⁻¹, 0.0010 d⁻¹ and 0.0008 d⁻¹ for submerged, intermittent and dry treatments respectively). Submerged P. australis litter also decomposed faster (k = 0.0024 d⁻¹) than dry litter (k = 0.0011 d⁻¹). We discuss the interaction of water regime and decomposition of organic material and implications for the maintenance of hummock and hollow topography.     

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 FeS _x in FeS _x -containing subsoils resulting in SO ₄ ²⁻ mobilisation. Pollution of the groundwater with SO ₄ ²⁻ causes a higher availability of o-PO 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 SO ₄ ²⁻ mostly also contains high concentrations of NO ₃ ⁻ and bivalent cations (Ca²⁺ and mg²⁺). During summer groundwater input is restricted resulting in periodic drought. Under these conditions no SO ₄ ²⁻ induced o-PO ₄ ³⁻ eutrophication occurs. Periodic drought and a high discharge of NO ₃ ⁻ , have a strong effect on S and P biogeochemistry in sulphate-rich fens. NO ₃ ⁻ inhibits SO ₄ ²⁻ reduction and concomitant o-PO ₄ ³⁻ mobilisation in fen sediments by being an energetically more favourable electron acceptor. In addition, NO ₃ ⁻ is capable of oxidising reduced Fe compounds, including FeS _x , increasing the amount of oxidised Fe in the sediment capable of binding o-PO ₄ ³⁻ . Periodic drought is important in reincreasing the concentration of oxidised Fe in the top layer of S-rich sediments preventing o-PO 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 NO ₃ ⁻ depletion, stimulation of SO ₄ ²⁻ reduction, Fe depletion, o-PO 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 NO 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 levels below the potential groundwater table using a controllable dam.