Perennial rivers and streams make a disproportionate contribution to global carbon (C)cycling. However, the contribution of intermittent rivers and ephemeral streams, which

Temporary streams are those water courses that undergo the recurrent cessation of flow or the complete drying of their channel. The structure and composition of biological communities in temporary stream reaches are strongly dependent on the temporal changes of the aquatic habitats determined by the hydrological conditions. Therefore, the structural and functional characteristics of aquatic fauna to assess the ecological quality of a temporary stream reach cannot be used without taking into account the controls imposed by the hydrological regime. This paper develops methods for analysing temporary streams' aquatic regimes, based on the definition of six aquatic states that summarize the transient sets of mesohabitats occurring on a given reach at a particular moment, depending on the hydrological conditions: <i>Hyperrheic, Eurheic, Oligorheic, Arheic, Hyporheic</i> and <i>Edaphic</i>. When the hydrological conditions lead to a change in the aquatic state, the structure and composition of the aquatic community changes according to the new set of available habitats. We used the water discharge records from gauging stations or simulations with rainfall-runoff models to infer the temporal patterns of occurrence of these states in the Aquatic States Frequency Graph we developed. The visual analysis of this graph is complemented by the development of two metrics which describe the permanence of flow and the seasonal predictability of zero flow periods. Finally, a classification of temporary streams in four aquatic regimes in terms of their influence over the development of aquatic life is updated from the existing classifications, with stream aquatic regimes defined as <i>Permanent, Temporary-pools, Temporary-dry</i> and <i>Episodic</i>. While aquatic regimes describe the long-term overall variability of the hydrological conditions of the river section and have been used for many years by hydrologists and ecologists, aquatic states describe the availability of mesohabitats in given periods that determine the presence of different biotic assemblages. This novel concept links hydrological and ecological conditions in a unique way. All these methods were implemented with data from eight temporary streams around the Mediterranean within the MIRAGE project. Their application was a precondition to assessing the ecological quality of these streams

Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO 2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO 2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO 2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y −1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle.

Recent studies of nutrient cycling in Sycamore Creek in Arizona, USA, suggest that a thorough understanding requires a spatially explicit, hierarchical approach. Physical configuration determines the path that water follows as it moves downstream. Water follows flowpaths through surface stream components, the hyporheic zone beneath the surface stream, and the parafluvial (sand bar) zone. Characteristic biogeochemical processes in these subsystems alter nitrogen (N) species in transport, in part as a function of available concentrations of N species. At several hierarchical levels, substrate materials are an important determinant of nitrogen dynamics in desert streams. Sand is present in bars of variable size and shape, each of which can be considered a unit, interacting with the surface stream. Groups of these stream-sandbar units form a higher level, the reach. At the next higher scale, sandy reaches (runs) alternate with riffles. Where flowpaths converge, rates of N transformation are high and, as a result, change in concentration is a non-linear function of fiowpath length. Disturbance by flash floods alters sandbar configuration. Between floods, the interaction of subsurface and surface flowpaths shapes configuration in each, thus a self-organizing element of spatial structure exists. Sandy runs are dominated by subsurface processes and are likely to be net nitrifiers while riffles are dominated by surface flow and are nitrogen fixers. Whether a stream ecosystem retains nitrogen, or transports it to downstream recipient systems, or is a net emitter of gaseous forms of N, depends upon the dynamics of a spatial mosaic of interacting elements. An understanding of the net effect of this mosaic requires a spatially e.xplicit, hierarchical, multi-scale approach.

The 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.