Sediment drying associated with large water level fluctuations is an increasingly common feature of temporary streams and lakes worldwide. Drying-induced sediment aeration and re-flooding periodically alter redox conditions, and therefore stimulate redox-sensitive processes influencing phosphorus (P) binding forms. We experimentally tested the effects of drying on P binding forms, and the P sorption potential, by drying and re-flooding lake sediments in the laboratory. Wet and dried fine sediments were re-flooded in columns, and the overlying water was continuously re-stocked to a constant P concentration. We measured changes in P forms, P uptake rates, and the pore water dynamics in each column over 36 weeks. Drying decreased the fraction of stable P, stimulated the mineralization of organic P, and increased the proportion of labile and reductant-soluble forms. Drying of sediment furthermore reduced its P sorption affinity and capacity by up to 32% in batch equilibrium experiments, and led to a fourfold increase in sediment compaction which increased P uptake rates by a factor of 1.7 in sediment column experiments. Compaction due to drying also induced the development of a sharp gradient below which P was mobilized. These results indicate that in fine sediments, a single drying event can result in the transformation of P components into more labile forms which accumulate in the uppermost sediment layer, therefore raising the potential for a pulsed P release under reducing conditions.

ABSTRACTThe assessment of the ecological status of water bodies, as requires by the European Water Framework Directive, can raise a number of problems when applied to temporary streams. These problems are because of the particular physical, chemical and biological conditions resulting from the recurrent cessation of flow or even the complete drying of the stream beds. In such non‐permanent water bodies, the reference quality standards developed for permanent streams may only be applicable under certain circumstances or may not be applicable at all. Work conducted within the collaborative EU‐funded project Mediterranean Intermittent River ManAGEment (MIRAGE) has addressed most of these difficulties and has used diverse approaches to solve them. These approaches have been brought together in the so‐called MIRAGE Toolbox. This toolbox consists of a series of methodologies that are designed to be used in a sequential manner to allow the establishment of the ecological and chemical status of temporary streams and to relate these findings to the hydrological status of the streams. The toolbox is intended to serve the following purposes: (i) the determination of the hydrological regime of the stream; (ii) the design of adequate schedules for biological and chemical sampling according to the aquatic state of the stream; (iii) the fulfillment of criteria for designing reference condition stations; (iv) the analysis of hydrological modifications of the stream regime (with the definition of the hydrological status); and (v) the development of new methods to measure the ecological status (including structural and functional methods) and chemical status when the stream's hydrological conditions are far from those in permanent streams. Copyright © 2014 John Wiley & Sons, Ltd.

Summary Seasonal flow intermittence in streams often coincides with early leaf abscission of riparian vegetation due to water stress. When accumulated on dry stream beds or in remaining pools, leaves are exposed to solar radiation or fermentation processes, respectively. However, very little information exists on how these preconditioning processes could affect leaf decomposition when stream flow has recovered. We simulated natural preconditioning of leaves by irradiation with UV‐VIS fluorescent lamps and incubation under anoxic conditions. Mass loss rates of preconditioned leaf litter from deciduous trees (Alnus glutinosa, Fraxinus excelsior, Populus tremula and Quercus petreae) were quantified in a temporary stream during base flow conditions. Coarse and fine mesh litter bags were used to study the effect of benthic macroinvertebrates and microorganisms on leaf mass loss. Preconditioning reduced the concentration of macronutrients such as P, K and Mg and increased the relative cellulose content of the leaves. Preconditioning changed the fungal community structure (analysed by DGGE) depending on leaf species and sampling date. Preconditioning in anoxic conditions also suppressed fungal decomposer biomass (measured as ergosterol) by 42% resulting in 33% lower mass loss rates in fine mesh bags. In contrast, mass loss rates were not affected by preconditioning when macroinvertebrate decomposers had access to the leaf litter. In streams exhibiting seasonal flow intermittence, preconditioning will influence organic carbon dynamics towards lower rates of microbially mediated turnover and towards poorer quality of downstream‐transported material.

Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV–VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change.

Phosphorus availability is commonly assumed to limit productivity in lowland tropical rainforests, yet there is relatively little information on the chemical forms of soil phosphorus in such ecosystems. We used the Hedley sequential fractionation scheme to assess phosphorus chemistry in five soils supporting tropical rainforest on Barro Colorado Island, Republic of Panama. The soils represented a range of orders (Inceptisols, Alfisols, and Oxisols) formed on contrasting geological substrates and topography, but under uniform climate and vegetation. Total phosphorus in surface horizons ranged between 315 and 1114mgPkg−1, being lowest on a soil derived from marine sediments and highest on soils derived from andesite. The majority of the phosphorus occurred in recalcitrant forms, although between 14% and 39% occurred as organic phosphorus. Readily-available phosphate, as extracted by anion-exchange membranes, occurred in small concentrations (4–13mgPkg−1), although labile phosphorus, defined as phosphate extracted by anion-exchange membrane plus inorganic and organic phosphorus extracted by 0.5M NaHCO3, accounted for between 4.7% and 11.4% of the total soil phosphorus. Our results indicate a strong control of geology and topography on soil phosphorus in tropical rainforests, which may have important implications for understanding the diversity and distribution of plant species in such ecosystems. Further, some of the most common soils on Barro Colorado Island, including those on the 50ha forest dynamics plot, are rich in phosphorus despite their relatively advanced stage of pedogenesis.