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Abstract Stream drying is increasing globally, with widespread impacts on stream ecosystems. Here, we investigated how the impacts of drying on stream ecosystem connectivity might depend on stream network size and the location of drying within the stream network. Using 11 stream networks from across the United States, we simulated drying scenarios in which we varied the location and spatial extent of drying. We found that the rate of connectivity loss varied with stream network size, such that larger stream networks lost connectivity more rapidly than smaller stream networks. We also found that the rate of connectivity loss varied with the location of drying. When drying occurred in the mainstem, even small amounts of drying resulted in rapid losses in ecosystem connectivity. When drying occurred in headwater reaches, small amounts of drying had little impact on connectivity. Beyond a certain threshold, however, connectivity declined rapidly with further increases in drying. Given the increasing stream drying worldwide, our findings underscore the need for managers to be particularly vigilant about fragmentation when managing at large spatial scales and when stream drying occurs in mainstem reaches. 

Abstract Biological assemblages in streams are influenced by hydrological dynamics, particularly in non‐perennial systems. Although there has been increasing attention on how drying impacts stream organisms, few studies have investigated how specific characteristics of drying and subsequent wetting transitions influence biotic responses via resistance and resilience traits.Here, we characterized how hydrologic metrics, including those quantifying drying and wetting transitions as well as dry and wet phases, alter diversity and composition of three aquatic assemblages in non‐perennial streams in southern California: benthic macroinvertebrates, soft‐bodied algae and diatoms.We found that flow duration prior to sampling was correlated with variation in macroinvertebrate and soft‐bodied algal assemblage composition. The composition and richness of diatom assemblages, however, were predominantly influenced by the drying start date prior to sampling. Contrary to other studies, the duration of the dry phase prior to sampling did not influence the composition or richness of any assemblage. Although our study was conducted within a region in which each assemblage experienced comparable environmental conditions, we found no single hydrologic metric that influenced all assemblages in the same way.The hot‐summer Mediterranean climate of southern California likely acts as a strong environmental filter, with taxa in this region relying on resistance and resilience adaptations to survive and recolonize non‐perennial streams following wetting. The different responses of algal and diatom assemblages to hydrologic metrics suggest greater resilience to drying and wetting events, particularly for primary producers.As drying and wetting patterns continue to change, understanding biodiversity responses to hydrologic metrics could inform management actions that enhance the ecological resilience of communities in non‐perennial streams. In particular, the creation and enhancement of flow regimes in which natural timing and duration of dry and wet phases sustain refuges that support community persistence in a changing environment. 

ABSTRACT Non‐perennial streams are globally prevalent. These streams are vital components of ecosystems, yet their drying patterns and resulting impacts on hydrologic connectivity remain poorly understood at the watershed scale. Aridity is a dominant driver of stream drying, but its influences on hydrologic connectivity have not been fully explored. In this study, we investigated the role of aridity in shaping streamflow and connectivity patterns in non‐perennial stream networks that span the continental United States aridity gradient. Using hydrologic models, we simulated daily streamflow and stream network connectivity under current and future climate scenarios. Our findings support previous research showing that aridity and streamflow are strongly linked. We also found that connectivity was related to aridity, although this relationship was weaker. Under the future climate scenario, mean runoff increased in most watersheds in the future, while mean connectivity decreased in the majority of watersheds. This difference is an indicator of the complex relationship between streamflow and connectivity. Aridity was a strong predictor of changes in very high and very low connectivity periods that resulted from climate change, but aridity did not predict changes in mean connectivity. Arid watersheds tended to experience more high connectivity days due to climate change while humid networks tended to have more low connectivity days. By modelling climate impacts at the network scale and across a broad hydroclimatic gradient, we highlight the importance of considering context‐dependent changes in network connectivity in river flow management and watershed conservation plans. 

DNA‐based aquatic biomonitoring methods show promise to provide rapid, standardized, and efficient biodiversity assessment to supplement and in some cases replace current morphology‐based approaches that are often less efficient and can produce inconsistent results. Despite this potential, broad‐scale adoption of DNA‐based approaches by end‐users remains limited, and studies on how these two approaches differ in detecting aquatic biodiversity across large spatial scales are lacking. Here, we present a comparison of DNA metabarcoding and morphological identification, leveraging national‐scale, open‐source, ecological datasets from the National Ecological Observatory Network (NEON). Across 24 wadeable streams in North America with 179 paired sample comparisons, we found that DNA metabarcoding detected twice as many unique taxa than morphological identification overall. The two approaches showed poor congruence in detecting the same taxa, averaging 59%, 35%, and 23% of shared taxa detected at the order, family, and genus levels, respectively. Importantly, the two approaches detected different proportions of indicator taxa like %EPT and %Chironomidae. DNA metabarcoding detected far fewer Chironomid and Trichopteran taxa than morphological identification, but more Ephemeropteran and Plecopteran taxa, a result likely due to primer choice. Overall, our results showed that DNA metabarcoding and morphological identification detected different benthic macroinvertebrate communities. Despite these differences, we found that the same environmental variables were correlated with invertebrate community structure, suggesting that both approaches can accurately detect biodiversity patterns across environmental gradients. Further refinement of DNA metabarcoding protocols, primers, and reference libraries–as well as more standardized, large‐scale comparative studies–may improve our understanding of the taxonomic agreement and data linkages between DNA metabarcoding and morphological approaches. 

Abstract Ecosystems that are coupled by reciprocal flows of energy and nutrient subsidies can be viewed as a single “meta‐ecosystem.” Despite these connections, the reciprocal flow of subsidies is greatly asymmetrical and seasonally pulsed. Here, we synthesize existing literature on stream–riparian meta‐ecosystems to quantify global patterns of the amount of subsidy consumption by organisms, known as “allochthony.” These resource flows are important since they can comprise a large portion of consumer diets, but can be disrupted by human modification of streams and riparian zones. Despite asymmetrical subsidy flows, we found stream and riparian consumer allochthony to be equivalent. Although both fish and stream invertebrates rely on seasonally pulsed allochthonous resources, we find allochthony varies seasonally only for fish, being nearly three times greater during the summer and fall than during the winter and spring. We also find that consumer allochthony varies with feeding traits for aquatic invertebrates, fish, and terrestrial arthropods, but not for terrestrial vertebrates. Finally, we find that allochthony varies by climate for aquatic invertebrates, being nearly twice as great in arid climates than in tropical climates, but not for fish. These findings are critical to understanding the consequences of global change, as ecosystem connections are being increasingly disrupted. 

Habitat fragmentation drives biodiversity loss in rivers around the world. Although the effects of anthropogenic barriers on river connectivity are well known, there has been little research on the ways in which stream drying may alter connections among habitats and resources. Given that stream drying is increasing in many regions, there is a pressing need to understand the effects of drying on habitat fragmentation. Here, we quantify spatiotemporal drying patterns under current and future climate scenarios in the Upper Blue River Basin, Oklahoma. We used a hydrologic model to simulate daily streamflow for nine climate scenarios. For each scenario, we calculated metrics of streamflow temporal continuity (dry days, dry periods, and dry period duration) and spatial connectivity (wetted length, number of dry stream fragments, length of dry stream fragments, and dendritic connectivity index) from simulated daily streamflow. We found that stream drying is likely to increase in all future climate scenarios and that increases in stream drying reduce connectivity. However, the effects of stream drying on connectivity were highly nonlinear. Specifically, we observed a threshold around which a small increase in stream drying led to a rapid drop in connectivity. We also found that the greatest increases in stream drying were not associated with the highest emission scenarios, underscoring the complex linkages among climate, water availability, and connectivity. Given that connectivity is essential to ecosystem structure and function, we discuss water management strategies informed by impacts of stream drying. 

Abstract Studies of stream macroinvertebrates traditionally use sampling methods that target benthic habitats. These methods could underestimate biodiversity if important assemblage components exist outside of the benthic zone. To test the efficacy of different sampling methods, we collected paired reach‐wide benthic and edge samples from up to 10 study reaches in nine basins spanning an aridity gradient across the United States. Edge sampling targeted riparian‐adjacent microhabitats not typically sampled, including submerged vegetation, roots, and overhanging banks. We compared observed richness, asymptotic richness, and assemblage dissimilarity between benthic samples alone and different combinations of benthic and edge samples to determine the magnitude of increased diversity and assemblage dissimilarity values with the addition of edge sampling. We also examined how differences in richness and assemblage composition varied across an aridity gradient. The addition of edge sampling significantly increased observed richness (median increase = 29%) and asymptotic richness (median increase = 173%). Similarly, median Bray–Curtis dissimilarity values increased by as much as 0.178 when benthic and edge samples were combined. Differences in richness metrics were generally higher in arid basins, but assemblage dissimilarity either increased or decreased across the aridity gradient depending on how benthic and edge samples were combined. Our results suggest that studies that do not sample stream edges may significantly underestimate reach diversity and misrepresent assemblage compositions, with effects that can vary across climates. We urge researchers to carefully consider sampling methods in field studies spanning climatic zones and the comparability of existing data sets when conducting data synthesis studies.