Explore the vast range of titles available.

Relatively little is known regarding the extent of intermittent streams or the general ecology of headwaters in alpine catchments with glacial influence. This study quantified the contribution of intermittent streams to the total length of the stream network along with an ecological assessment during spring-summer of headwater streams (higher than 1,900 m above sea level) in the Val Roseg, a high Alpine glacial catchment. Stream network mapping revealed that ca. 90 % (76.8 km) of the drainage network consisted of intermittent streams. Glacier-fed headwaters experienced diel surface flows in late spring and summer, most going dry during the night due to reduced glacial inputs. In contrast, groundwater-fed streams often went dry in summer with the contraction of groundwater and other subsurface inputs. A principal components analysis of physico-chemical characteristics revealed headwaters to be primarily glacial-fed (kryal), groundwater-fed (krenal), or having a mixed water source. Although quite variable, periphyton biomass reached high levels (ca. 40 mg m⁻² chl-a, 10 g m⁻² AFDM) by late spring in most headwaters. Organic matter in transport (seston) ranged from 0.03 to 0.09 mg L⁻¹ mostly consisting of fine particulate organic matter (FPOM: 33–76 %). Hyporheic sediment respiration rates varied considerably, ranging from 0.005 to 0.126 mg O₂ h⁻¹ kg⁻¹ sediment and primarily related to the amount of loosely attached organic matter. These results indicate that intermittent streams are predominant in alpine landscapes, comprising mostly 1st to 2nd order systems, and that ecosystem properties vary substantially among headwater streams likely in relation to annual/daily changes in flow and water source. Such headwaters may contribute strongly to the production, processing and transport of organic matter to downstream waters, especially in light of the expected increase in intermittent streams in alpine catchments experiencing rapid glacial recession.

Headwater streams comprise almost 90% of global river networks, and their microorganisms play critical roles in organic matter decomposition and nutrient cycling. These functions, however, are affected by recurrent drying and rewetting. This study examined spatial variation in microbial enzyme activity tied to organic carbon degradation (β-glucosidase, phenol oxidase, and peroxidase) and nitrogen (N-acetylglucosaminidase) and phosphorus (phosphatase) mineralization in water, epilithic biofilm, leaf litter, and sediment in two intermittent streams: Gibson Jack Creek (Idaho, USA) and Pendergrass Creek (Alabama, USA), representing different climactic and physiographic settings. Microbial activity was greater in Gibson Jack Creek, where the activity of leaf litter enzymes varied along the stream network, and there were strong correlations in microbial activity between different stream habitats. Microbial activity in Pendergrass Creek showed primarily within-habitat associations. Activity in water, sediment, and biofilm showed broader spatial heterogeneity in both stream networks. Ratios of microbial activity (enzyme stoichiometry) suggested that microbial communities in both systems were primarily limited by carbon and phosphorus, although there was more spatial variation in nitrogen limitation, particularly in water and sediment at Pendergrass Creek and in biofilm at Gibson Jack Creek. These findings underscore the spatial heterogeneity and environmental sensitivity of microbial processes in intermittent streams.

Mediterranean climates predispose aquatic systems to both flood and drought periods, therefore, stream sediments may be exposed to desiccation periods. Changes in oxygen concentrations and sediment water content influence the biotic processes implicated in nitrogen dynamics. The objectives of this study were to identify (1) the changes of inorganic nitrogen in stream sediments during the transition from wet to dry conditions, and (2) the underlying processes in N dynamics and its regulation. Extractable sediment NO 3 − -N and NH 4 + -N, organic matter and extractable organic carbon content were assessed during natural desiccation in microcosms with sediments from an intermittent Mediterranean stream. In agreement with our initial hypothesis, our results showed how the NO 3 − -N content of the sediment was enhanced during the first 10 days of sediment drying, whereas NH 4 + -N was lost by 14 days post-drying. During the first 10 days, sediment desiccation seemed to stimulate the net N-mineralization and net nitrification from sediments. Afterwards, the extractable NO 3 − -N concentration sharply dropped, which may be attributed to lower ammonium-oxidation rates as ammonium and organic matter are depleted, and to an increase in NO 3 − -N consumption by microbial populations. Denitrification was inhibited, with a significant decrease as % water-filled pore space lowered. We hypothesize that the sediment inorganic N content enhanced during sediment desiccation could be released as part of the N pulse observed after sediment rewetting. However, the stream N availability after rewetting dried sediments would differ depending on desiccation period duration.

Litter breakdown is a pivotal ecosystem function in headwater streams, where it fuels food webs and controls C flux. Breakdown rates depend on environmental characteristics and can display strong seasonal variation, particularly in intermittent streams. To identify the environmental factors driving seasonality of litter breakdown, we ran 5 breakdown experiments with poplar leaves during the wet phase (November–August) in a 3rd-order intermittent Mediterranean stream. We assessed the contribution of decomposers and detritivores to total breakdown seasonality by measuring total (coarse-bag) and microbial (fine-bag) breakdown and estimating invertebrate-mediated breakdown rates (difference between coarse and fine mesh). Breakdown rates (k) increased from autumn to early summer when expressed as k/d and decreased during the drying phase. However, when expressed k/degree-day (dd), rates peaked in early spring and subsequently decreased. The high fine-mesh/coarse-mesh ratio (0.70) indicated that microbes drove total breakdown. Hierarchical partitioning (HP) analyses in k/d showed that temperature was the most important environmental factor for microbial breakdown and affected invertebrate breakdown. Temperature presented strong synergistic effects with other variables. Following removal of the temperature effect, total breakdown was related mainly to current and conductivity, microbial breakdown was related to water quality (conductivity, pH, and O2), and invertebrate-mediated breakdown was related only to current. The relation of invertebrate breakdown with current variation might be explained by strong seasonality in total invertebrate and shredder densities, which seemed to be linked to seasonality in discharge. Climate change can profoundly affect stream ecosystems, but the responses of invertebrates and microbes in terms of litter processing and C fluxes can be difficult to predict in intermittent Mediterranean streams.

Watershed resilience is the ability of a watershed to maintain its characteristic system state while concurrently resisting, adapting to, and reorganizing after hydrological (for example, drought, flooding) or biogeochemical (for example, excessive nutrient) disturbances. Vulnerable waters include non-floodplain wetlands and headwater streams, abundant watershed components representing the most distal extent of the freshwater aquatic network. Vulnerable waters are hydrologically dynamic and biogeochemically reactive aquatic systems, storing, processing, and releasing water and entrained (that is, dissolved and particulate) materials along expanding and contracting aquatic networks. The hydrological and biogeochemical functions emerging from these processes affect the magnitude, frequency, timing, duration, storage, and rate of change of material and energy fluxes among watershed components and to downstream waters, thereby maintaining watershed states and imparting watershed resilience. We present here a conceptual framework for understanding how vulnerable waters confer watershed resilience. We demonstrate how individual and cumulative vulnerable-water modifications (for example, reduced extent, altered connectivity) affect watershed-scale hydrological and biogeochemical disturbance response and recovery, which decreases watershed resilience and can trigger transitions across thresholds to alternative watershed states (for example, states conducive to increased flood frequency or nutrient concentrations). We subsequently describe how resilient watersheds require spatial heterogeneity and temporal variability in hydrological and biogeochemical interactions between terrestrial systems and down-gradient waters, which necessitates attention to the conservation and restoration of vulnerable waters and their downstream connectivity gradients. To conclude, we provide actionable principles for resilient watersheds and articulate research needs to further watershed resilience science and vulnerable-water management.

We aimed to establish the effect of seasonal drought on leaf litter breakdown and invertebrate communities. Differences in breakdown rates of Nothofagus pumilio were experimentally compared using the litter bag method (coarse and fine mesh size bags) in two first-order streams, one intermittent and one perennial, during two different hydrological periods. Colonizing fauna found in coarse mesh bags was quantified, identified and compared with benthic biota from the same streams. Leaf litter decay rates in low flow conditions revealed that breakdown was principally a consequence of microbial action in the intermittent stream. In contrast, breakdown in high flow conditions was caused by invertebrate feeding in both streams. Collector–gatherers constituted most of the abundance and biomass in bags from the intermittent stream, due to their rapid benthic recolonization. Shredders peaked at approximately 50% remaining leaf litter mass in both streams only during high flow, which coincided with general models of detritus breakdown in streams. Considering global warming scenarios, with drought and water temperature increases expected for many regions of the world, these studies on the consequences for the biota and ecological processes of small streams will allow the prediction of negative effects on such vulnerable ecosystems.

Intermittent streams drain much of the world’s tropical savannas, yet these vitally important stream ecosystems are among the most poorly understood components of tropical savannas. Fire is a widely used management tool in tropical savannas, but the effects of prescribed burning on savanna streams or riparian zones have not been investigated experimentally. This study was undertaken within a catchment-scale, replicated experiment to determine the effect of prescribed burning on savanna riparian zones in northern Australia. Regardless of position along the stream, burning had dramatic effects on the composition and structure of the riparian vegetation. Woody species richness and total abundance, the abundance of small- and medium-sized trees, total basal area, canopy cover, and the richness and cover of vines were reduced by burning, whereas grass cover was much higher in burned areas. Fire also reduced seed production of the dominant riparian eucalypt. This study adds greatly to our understanding of the effects of fire management on savanna riparian zones and demonstrates that they are far more fire sensitive than the surrounding savanna.

Environmental DNA (eDNA), or DNA that is shed into the environment by an organism, can be used to detect the presence of cryptic species. However, eDNA methodology requires validation of an assay in both laboratory and field environments. Here, we describe the development of a quantitative PCR (qPCR) assay and field protocol for detecting a secretive amphibian, the Streamside Salamander (Ambystoma barbouri). This fossorial species is rarely encountered because adults are only active for several months during the winter when they breed and deposit eggs underneath rocks within intermittent streams. We designed and validated a qPCR assay for A. barbouri against five ambystomatid congeners and the Southern Two‐lined Salamander (Eurycea cirrigera), which occur within the range of A. barbouri in the focal study area. We detected DNA of A. barbouri in the laboratory to 0.0004 ng/µl, while all other congeners were rarely detected below 40 ng/µl. We collected 1‐L water samples from 45 streams from December 2016 to May 2017 and during April 2018 to validate our methods in the field. Our assay was effective at detecting A. barbouri in 24 of 45 streams. We confirmed physical presence of A. barbouri (adults, larvae, and/or eggs) at 21 out of 24 of these positive sites. The detection probability was 0.85 ± 0.05; CI: 0.75, 0.95 within a single sampling event that incorporated 5 water samples from each of 17 repeat‐visit sites. The per water sample sensitivity was 68% (163/238) and specificity was 100% (22/22). The per site visit sensitivity was 85% (46/54) and specificity was 100% (6/6). Lastly, the per site sensitivity was 95% (21/22) and specificity was 100% (3/3). Collectively, this protocol outlines an efficient and cost‐effective method to detect A. barbouri and provides a technique to rapidly identify sites where breeding is occurring and to determine the distribution of this species. Here, we describe the development of a quantitative PCR (qPCR) assay and field protocol for detecting a secretive amphibian, the Streamside Salamander (Ambystoma barbouri) in the intermittent streams in which the species breeds in middle Tennessee. We successfully designed and validated a qPCR assay that was both sensitive and specific to A. barbouri in the laboratory and in a field setting. We detected A. barbouri presence at 24/45 of tested sites and confirmed physical presence at 21/24 of these positive sites, providing an effective technique to rapidly identify breeding streams and to determine the distribution of this species.

Headwater streams represent the key sites of nutrient retention, but little is known about temporal variation in this important process. We used monthly measurements over 2 years to examine variation in retention of soluble reactive phosphorus (SRP) and ammonium (NH ₄ ⁺ ) in two Mediterranean headwater streams with contrasting hydrological regimes (that is, perennial versus intermittent). Differences in retention between streams were more evident for NH ₄ ⁺ , likely due to strong differences in the potential for nitrogen limitation. In both streams, nutrient-retention efficiency was negatively influenced by abrupt discharge changes, whereas gradual seasonal changes in SRP demand were partially controlled by riparian vegetation dynamics through changes in organic matter and light availability. Nutrient concentrations were below saturation in the two streams; however, SRP demand increased relative to NH ₄ ⁺ demand in the intermittent stream as the potential for phosphorus limitation increased (that is, higher dissolved inorganic nitrogen:SRP ratio). Unexpectedly, variability in nutrient retention was not greater in the intermittent stream, suggesting high resilience of biological communities responsible for nutrient uptake. Within-stream variability of all retention metrics, however, increased with increasing time scale. A review of studies addressing temporal variation of nutrient retention at different time scales supports this finding, indicating increasing variability of nutrient retention with concomitant increases in the variability of environmental factors from the diurnal to the inter-annual scale. Overall, this study emphasizes the significance of local climate conditions in regulating nutrient retention and points to potential effects of changes in land use and climate regimes on the functioning of stream ecosystems.

Seasonal variations of dissolved inorganic nitrogen (DIN) (NO₃-N and NH₄-N) and dissolved organic nitrogen (DON) were determined in Fuirosos, an intermittent stream draining an unpolluted Mediterranean forested catchment (10.5 km⁲) in Catalonia (Spain). The influence of flow on streamwater concentrations and seasonal differences in quality and origin of dissolved organic matter, inferred from dissolved organic carbon to nitrogen ratios (DOC:DON ratios), were examined. During baseflow conditions, nitrate and ammonium had opposite behaviour, probably controlled by biological processes such as vegetation uptake and mineralization activity. DON concentrations did not have a seasonal trend. During storms, nitrate and DON increased by several times but discharge was not a good predictor of nutrient concentrations. DOC:DON ratios in streamwater were around 26, except during the months following drought when DOC:DON ratios ranged between 42 and 20 during baseflow and stormflow conditions, respectively. Annual N export during 2000-2001 was 70 kg$\\text{km}^{-1}$ $\\text{year}^{-1}$, of which 75% was delivered during stormflow. The relative contribution of nitrogen forms to the total annual export was 57, 35 and 8% as NO₃-N, DON and NH₄-N, respectively.