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Freie Universität Berlin (2013)

PHOSPHORUS AND LEAF LITTER TURNOVER IN TEMPORARY AQUATIC SYSTEMS

DIETER DANIELA

Titre : PHOSPHORUS AND LEAF LITTER TURNOVER IN TEMPORARY AQUATIC SYSTEMS

Auteur : DIETER DANIELA

Université de soutenance : Freie Universität Berlin

Grade : Doctor rerum naturalium (Dr. rer. nat.) 2013

Résumé
Water level fluctuations are a global phenomenon creating temporary aquatic systems. Recent trends in climate and land use changes have led to the spatio-temporal expansion, meaning that temporary streams and lakes show longer periods of low water level or that currently permanent systems switch to a temporary regime. Ecological effects of droughts have been widely studied, but the underlying ecological and physicochemical processes at the transition between dry and wet stages are poorly understood. Drying and re-flooding often greatly affect redox-sensitive processes. The decomposition of organic material and nutrient dynamics are generally regarded as key ecosystem processes and are both sensitive to changing redox conditions making them valuable indicators for functional ecosystem health. The input of organic material, such as leaf litter, may be the most important carbon source for stream communities. In temporary streams, peak leaf fall often coincides with cessation of flow so that leaves accumulate at the surface of the dry streambed or in residual stagnant pools, where they are subject to physicochemical preconditioning before subsequent decomposition in flowing water. The experiments described in this thesis showed that photodegradation as occurring by solar radiation on dry streambeds and anaerobic fermentation as occurring in anoxic ponds enhanced the leaching of nutrients and labile carbon compounds. This reduced the leaf quality considering it a substrate for decomposer communities. Indeed, living fungal biomass was repressed and a change in the fungal community structure was indicated. As a result, leaf decomposition rates in flowing water were reduced for preconditioned leaves, which held true for a range of streams and leaf species differing in chemical characteristics and quality. The results suggest that in streams developing seasonal flow intermittence, preconditioning will influence leaf litter processing towards lower rates of microbially-mediated turnover and towards poorer quality of downstream-transported material. The observed leaching of nutrients during preconditioning of leaves was particularly pronounced for phosphorus, which plays a key role in determining the trophic state of an aquatic system. The availability of phosphorus in the water column is controlled by the capability of the sediment to retain additional phosphorus input, which is therefore regarded as one of the most important ecosystem functions. Because phosphorus cycling is highly redox-sensitive, changes due to drying and re-flooding of the sediments may involve shifts in phosphorus uptake and mobilization. The experiments described in this thesis revealed that drying mobilized more stable phosphorus fractions, stimulated the mineralization of organic phosphorus compounds, and increased the proportion of labile and reductant-soluble fractions. Drying reduced the phosphorus sorption affinity and sorption capacity of the sediment, but also led to a sediment compaction, which in contrast enhanced initial phosphorus uptake rates. Following re-flooding, the compaction due to drying also induced the development of a sharp redoxcline below which P was mobilized. The results indicated that even a single drying event can result in the transformation of phosphorus components into more labile forms, which are accumulated in the near-surface sediment layer, and therefore raise the potential of pulsed P release under reducing conditions.

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Page publiée le 29 novembre 2013, mise à jour le 28 décembre 2018