Hydrology, nutrient processes and vegetation in floodplain wetlands.

Hans Estrup Andersen

PhD thesis

Summary

This thesis represents the conclusion on my PhD study carried out jointly at The Royal Veterinary and Agricultural University (KVL), Department of Agricultural Sciences, Laboratory for Agrohydrology and Bioclimatology, Copenhagen, and at The National Environmental Research Institute (DMU), Department of Freshwater Ecology, Silkeborg. My supervisors were Professor Henry E. Jensen and associate Professor Søren Hansen, both from the KVL. Senior scientist Lars Moeslund Svendsen was my internal DMU-supervisor.

The purpose of the study was to describe and quantify hydrological processes, and the implication of these for nutrient processes and vegetation in floodplain wetlands. The thesis comprises four scientific papers and a review. One of the papers is published in Aquatic Conservation, one paper is accepted for publication in Hydrological Processes, and one paper is submitted to Nordic Hydrology. The fourth paper is a manuscript prepared for Journal of Vegetation Science. Three of the papers are on work done in a floodplain wetland in the lower part of the river Gjern.

The first paper characterises the wetland and comprises an analysis of the controls on the water and nitrogen balances. Emphasis is put on unsaturated and saturated hydraulic characteristics of the wetland sediments. There is a minimal inflow of groundwater to the wetland and water exchange with the atmosphere is dominant. Large amounts of water flood the wetland, however, due to the characteristics of the sediments, infiltration is low. Denitrification amounts to 71kg nitrate per year and is limited by the supply of nitrate. Reduction of nitrate diffusing into the sediments during flooding of the wetland constitutes 75% of total denitrification.

The second paper focuses on evapotranspiration from the wetland. Evapotranspiration was estimated from continuos measurements with a Bowen ratio-set up throughout the growing season of 1999. With an average rate for the growing season of 3.6 mm day-1 evapotranspiration was higher than most published values for wetlands. The wetland evapotranspiration comprised 128% of reference-evapotranspiration calculated by the Penman-Monteith formula as prescribed by FAO. The high rates are explained partly by the capillary characteristics of the wetland sediments, which sustain near-saturated conditions in the rootzone throughout the growing season, and partly by local advection.

The third paper contains an analysis of plant species distributing factors in the wetland. Quantile regression, a new method for analysing ecological data, was tested. The method was evaluated as adequate for reducing the influence of multiple combined factors and thus to clarify the relation to single factors. A set of equations quantifying the response of 18 floodplain wetland species to six environmental factors is given in the paper. It is shown that that degree of base-saturation, exchangeable phosphate, groundwater amplitude and flooding duration are major factors in determining plant species distribution and cover in the wetland.

In the fourth paper the scale is enlarged to the subcatchment-level. The results of a monitoring study on the lower 18 km of the river Skjern, prior to the river restoration project, is described. Empirical models for riverine transport of suspended sediment and total phosphorus is developed. Assessment of the effects of the restoration, based on measured transport and estimated retention rates for suspended sediment and total phosphorus for different area types of the river system, revealed that suspended sediment and total phosphorus will be reduced by 37 and 20%, respectively.

Factors of importance for the hydrology, nutrient processes and vegetation in floodplain wetlands, and which have not been considered sufficiently in the papers, are described in the review. Emphasis is put on the differences between peat and mineral soils regarding hydraulic properties. The results presented in the papers are discussed relative to the international literature and compared to results from a wetland study also in the river Gjern system, but with a hydrology and nutrient turnover deviating from that of the floodplain wetland analysed in papers 1 - 3.

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