Roos Loeb

Biogeochemical constraints on the ecological rehabilitation of wetland vegetation in river floodplains

The European policy for river management during peak discharge periods is currently changing from exclusion strategies (reinforcement of dykes) to allowing a more natural situation by creating more floodplain space to reduce water levels during peak discharges. In addition, water retention and water storage areas have been created. The new measures are generally being combined with nature development strategies. Up till now, however, ecological targets of broadscale floodplain wetland restoration including sedge marshes, species-rich floodplain forests and carrs, riparian mesotrophic grasslands and other biodiverse riverine ecosystems, have hardly developed in these areas. Most studies on the conditions needed for sustainable ecological development of floodplains have focused on hydrological and geomorphological rather than biogeochemical issues (including nutrient availability and limitation). There are, however, large differences in the composition of river water and groundwater and in sediment quality between rivers in densely populated areas and those in more pristine areas, which serve as a reference. It is very likely that these factors, in combination with heavily altered hydrological regimes and the narrow areas confined between the dykes on both sides of the rivers, impose major constraints on sustainable ecological development of riverine areas. Another issue is that existing wetlands are generally considered to be very appropriate for water retention and conservation, although recent research has shown that this may pose a serious threat to their biodiversity. The present paper reviews the biogeochemical constraints on the combination of floodplain rehabilitation, water conservation and the conservation and development of wetlands. It is concluded that biogeochemical problems (mainly related to eutrophication) predominantly arise in less dynamic parts of the river system, to which the flood-pulse concept applies less. Sound knowledge of the biogeochemical processes involved will contribute to greater efficiency and a better prediction of the opportunities for restoration and development of riverine wetlands. This information can be directly applied in nature management, water management, policy-making and consultancy.

Prediction of phosphorus mobilisation in inundated floodplain soils.

After flooding, iron reduction in riverine wetlands may cause the release of large quantities of phosphorus. As phosphorus is an important nutrient causing eutrophication in aquatic systems, it is important to have a tool to predict this potential release. In this study we examined the P release to the soil pore water in soil cores from floodplains in the Netherlands and from less anthropogenically influenced floodplains from Poland. During the inundation experiment, concentrations of P in the pore water rose to 2-90 times the initial concentrations. P release was not directly related to the geographic origin of the soils. An important predictor variable of P release was found in the ratio between the concentration of iron-bound P and amorphous iron. This ratio may provide a practical tool for the selection of new areas for wetland creation, and for impact assessment of plans for riverine wetland restoration and floodwater storage.

Biogeography of sulfate-reducing prokaryotes in river floodplains

In this study, a large-scale field survey was conducted to describe the biogeography of sulfate-reducing prokaryotes (SRPs) in river floodplains. Fingerprints obtained with three methods, i.e. 16S rRNA gene-based oligonucleotide microarray, dsrB-based denaturing gradient gel electrophoresis (DGGE) and polar lipid-derived fatty acid (PLFA) analyses, were used as a proxy to describe the SRPs community diversity. Each set of profiles was subjected to a combined multivariate/correlation analysis in order to compare SRP community profiles and to highlight the environmental variables influencing the SRPs distribution along environmental gradients. Floodplain soils harbored distinct SRP communities displaying biogeographic patterns. Nearly all profiles from the tidal sites consistently separated from the nontidal sites, independently from the screening method and the multivariate statistics used. The distribution of the microarray/DGGE/PLFA-based fingerprints in the principal component plots could be correlated to eight soil variables, i.e. soil organic matter, total nitrogen, total phosphorous and total potassium, and extractable ammonium, nitrate, phosphate and sulfate, as well as seven pore water variables, i.e. phosphate, sulfate, sulfide, chloride, sodium, potassium and magnesium ions. Indication of a salinity- and plant nutrient-dependent distribution of SRPs related to Desulfosarcina, Desulfomonile and Desulfobacter was suggested by microarray, DGGE and PLFA analyses.