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Non-perennial rivers and streams - those that periodically cease flowing - are critical components of aquatic systems and comprise over half of global river and stream systems. We argue for coordinated, collaborative, standardized, and open efforts to understand their unique biogeochemical behaviour, which is becoming ever more pressing due to pronounced shifts between wet and dry as the climate changes.

Streamflow observations can be used to understand, predict, and contextualize hydrologic, ecological, and biogeochemical processes and conditions in streams. Stream gages are point measurements along rivers where streamflow is measured, and are often used to infer upstream watershed-scale processes. When stream gages read zero, this may indicate that the stream has fully dried; however, zero-flow readings can also be caused by a wide range of other factors. Our ability to identify whether or not a zero-flow gage reading indicates a dry fluvial system has far reaching environmental implications. Incorrect identification and interpretation by the data user can lead to hydrologic, ecological, and/or biogeochemical predictions from models and analyses. Here, we describe several causes of zero-flow gage readings: frozen surface water, flow reversals, instrument error, and natural or human-driven upstream source losses or bypass flow. For these examples, we discuss the implications of zero-flow interpretations. We also highlight additional methodss for determining flow presence, including direct observations, statistical methods, and hydrologic models, which can be applied to interpret causes of zero-flow gage readings and implications for reach- and watershed-scale dynamics. Such efforts are necessary to improve our ability to understand and predict surface flow activation, cessation, and connectivity across river networks. Developing this integrated understanding of the wide range of possible meanings of zero-flows will only attain greater importance in a more variable and changing hydrologic climate.

The structure, function, and dynamics of Earth's terrestrial ecosystems are profoundly influenced by the frequency and duration that they are inundated with water. A diverse array of natural and human engineered systems experience temporally variable inundation whereby they fluctuate between inundated and non-inundated states. Variable inundation spans from extreme flooding and droughts to predictable sub-daily cycles. Variably inundated ecosystems (VIEs) include hillslopes, non-perennial streams, wetlands, floodplains, temporary ponds, tidal systems, storm-impacted coastal zones, and human engineered systems. VIEs are diverse in terms of inundation regimes, water chemistry and flow velocity, soil and sediment properties, vegetation, and many other properties. The spatial and temporal scales of variable inundation are vast, ranging from sub-meter to whole landscapes and from sub-hourly to multi-decadal. The broad range of system types and scales makes it challenging to predict the hydrology, biogeochemistry, ecology, and physical evolution of VIEs. Despite all experiencing the loss and gain of an overlying water column, VIEs are rarely considered together in conceptual, theoretical, modeling, or measurement frameworks/approaches. Studying VIEs together has the potential to generate mechanistic understanding that is transferable across a much broader range of environmental conditions, relative to knowledge generated by studying any one VIE type. We postulate that enhanced transferability will be important for predicting VIE function under future, potentially non-analog, environmental conditions. Here we aim to catalyze cross-VIE science that studies drivers and impacts of variable inundation across Earth's VIEs. To this end, we complement expert mini-reviews of eight major VIE systems with overviews of VIE-relevant methods and challenges associated with scale. We conclude with perspectives on how cross-VIE science can derive transferable understanding via a 'continuum approach' in which the impacts of variable inundation are studied across multi-dimensional environmental space. [ABSTRACT FROM AUTHOR]

Soil [O.sub.2] is an important regulator of microbial processes but is rarely measured. Consequently, our understanding of temporal and spatial variation in soil [O.sub.2] is limited. This, in turn, limits our understanding of a key regulator of N loss through microbial denitrification. In this study, we explored: (i) how soil [O.sub.2] varied seasonally in wet and dry riparian areas, (ii) how this variation in [O.sub.2] exposure translated into spatially heterogeneous areas of denitrification and denitrification potential, and (iii) how [O.sub.2], N[O.sub.3.sup.-], and moisture interacted to affect denitrification rates. We collected continuous measurements of soil [O.sub.2] in 'wet' and ~dry' riparian soils and measured denitrification by removing the background [N.sub.2] headspace from intact soil cores, replacing it with a He-[O.sub.2] mixture, and measuring [N.sub.2] production with time. We found that soil [O.sub.2] varied considerably in the wet site, ranging from anoxic conditions when the water table was high in late spring to completely oxic conditions (20% [O.sub.2)] during summer when the water table dropped. In contrast, the dry site remained at 20% nearly year round Bulk soil [O.sub.2] strongly controlled denitrification rates in the wet site but not in the dry site, which only denitrified when N[O.sub.3.sup.-] was added. Denitrification enzyme activity was approximately twice as high in the wet site compared with the dry site, both of which responded predictably to 02 exposure. Experimental manipulation of [O.sub.2], N[O.sub.3.sup.-], and C may help to identify real hot and cold spots for denitrification in landscapes. doi: 10.2136/sssaj2009.0463