Wetland eco-engineering with fine sediment

Abstract

Fine sediment – available in large quantities in many river deltas – may be used as an alternative to coarse sand for land reclamation. In this thesis, the Marker Wadden project functioned as a case for optimizing ecological engineering designs that use fine sediment as building material. The research has highlighted that (bio)geochemical, hydrological, and wetland ecological processes interact and enhance ecosystem development on wetlands constructed from fine sediments. In greenhouse experiments, biogeochemical processes were identified that influence pore water chemistry. Key processes were found to be pyrite oxidation and associated calcite dissolution. Reed (Phragmites australis) enhanced the oxidation process significantly by aerating its environment via radial oxygen loss. Several feedback mechanisms between the sediment and reed plants were identified. One is a negative feedback loop that arises because plant roots induce aeration, which promotes iron toxicity, which in turn decreases plant growth and results in plant death. There is also a positive feedback loop, as iron toxicity might induce reduction processes after root death. Reduction of the sediment leads to P mobilization and hence enhances plant growth and regeneration. In a follow-up experiment, the effect of iron toxicity on biomass production was studied. Reed produced substantial amounts of iron plaque. Interestingly, the iron-bound phosphorus content of the sediment determined the composition of the iron plaque on the plant roots: phosphorus co-precipitated with Fe in high Fe-P sediment. In this thesis we also show how the hydrological regime and the sediment type interfere with ecosystem development, especially in terms of nutrient availability. The results show that P availability depends on the hydrological regime only, whereas N availability is determined by both the hydrological regime and the sediment type. In column experiments, it was studied how reed can act as an ecological engineer on the Marker Wadden by draining the soft, cohesive sediment. This plant effectively altered the pore pressure gradient via water extraction. This finding highlights the feature of this plant to act as an eco-engineer to fasten drainage, in turn promoting consolidation and ripening. Lastly, the potential effect of bioturbating Tubificidae on the nutrient availability in the sediment was identified. Both the measured and modelled results showed that bioturbation by Tubificidae effectively aerated the upper layer of the sediment. The model predicts an optimum of 12,000 worms m-2 and showed that the effect of bioturbation on oxidation first increases with increasing densities and then, after the optimum density is exceeded, the aeration effect of Tubificidae diminishes as burrows start overlapping geometrically, while the respiration rate per worm stays the same. Furthermore, the worm activity led to enhanced nitrogen availability because of increased rates of ammonification and nitrification, whereas phosphate concentrations in the water diminished because of P immobilization. The aforementioned results are translated into several recommendations for the Marker Wadden project. These recommendations may solve some of the issues that surfaced during the wetland construction phase and give relevant directions for correctly implementing the initial plan to use fine sediment as building material.