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For the first time in history, humans have exceeded the sustaining capacity of Earth's global ecosystems. Our expanding footprint has tremendous momentum, and the insidious explosion of human impact creates a shockwave that threatens ecosystems worldwide for decades-possibly centuries.Walter K. Dodds depicts in clear, nontechnical terms the root causes and global environmental effects of human behavior. He describes trends in population growth, resource use, and global environmental impacts of the past two centuries, such as greenhouse effects, ozone depletion, water pollution, and species ext

Streams and rivers are important conduits of terrestrially derived carbon (C) to atmospheric and marine reservoirs. Leaf litter breakdown rates are expected to increase as water temperatures rise in response to climate change. The magnitude of increase in breakdown rates is uncertain, given differences in litter quality and microbial and detritivore community responses to temperature, factors that can influence the apparent temperature sensitivity of breakdown and the relative proportion of C lost to the atmosphere vs. stored or transported downstream. Here, we synthesized 1025 records of litter breakdown in streams and rivers to quantify its temperature sensitivity, as measured by the activation energy (E-a, in eV). Temperature sensitivity of litter breakdown varied among twelve plant genera for which E-a could be calculated. Higher values of E-a were correlated with lower-quality litter, but these correlations were influenced by a single, N-fixing genus (Alnus). E-a values converged when genera were classified into three breakdown rate categories, potentially due to continual water availability in streams and rivers modulating the influence of leaf chemistry on breakdown. Across all data representing 85 plant genera, the E-a was 0.34 +/- 0.04 eV, or approximately half the value (0.65 eV) predicted by metabolic theory. Our results indicate that average breakdown rates may increase by 5-21% with a 1-4 C rise in water temperature, rather than a 10-45% increase expected, according to metabolic theory. Differential warming of tropical and temperate biomes could result in a similar proportional increase in breakdown rates, despite variation in E-a values for these regions (0.75 +/- 0.13 eV and 0.27 +/- 0.05 eV, respectively). The relative proportions of gaseous C loss and organic matter transport downstream should not change with rising temperature given that E-a values for breakdown mediated by microbes alone and microbes plus detritivores were similar at the global scale. ; US Long Term Ecological Research (LTER) Network through award DEB from National Science Foundation (NSF)National Science Foundation (NSF) [0936498]; NSF EFNational Science Foundation (NSF) [1064998]; NSF DBINational Science Foundation (NSF) [1216512]; Department of Energy's Office of Science, Biological and Environmental Research; US DOEUnited States Department of Energy (DOE) [DE-AC05-00OR22725] ; We thank many authors who graciously provided requested information that was not included in published literature and three anonymous reviewers who provided suggestions that improved the clarity of the manuscript. The US Long Term Ecological Research (LTER) Network provided financial support for this project, through an award (DEB#0936498) from the National Science Foundation (NSF). JSK was supported by NSF EF#1064998. MA was supported by NSF DBI#1216512. NAG was supported by the Department of Energy's Office of Science, Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US DOE under contract DE-AC05-00OR22725. ; Public domain authored by a U.S. government employee

Over half of global rivers and streams lack perennial flow, and understanding the distribution and drivers of their flow regimes is critical for understanding their hydrologic, biogeochemical, and ecological functions. We analyzed nonperennial flow regimes using 540 U.S. Geological Survey watersheds across the contiguous United States from 1979 to 2018. Multivariate analyses revealed regional differences in no-flow fraction, date of first no flow, and duration of the dry-down period, with further divergence between natural and human-altered watersheds. Aridity was a primary driver of no-flow metrics at the continental scale, while unique combinations of climatic, physiographic and anthropogenic drivers emerged at regional scales. Dry-down duration showed stronger associations with nonclimate drivers compared to no-flow fraction and timing. Although the sparse distribution of nonperennial gages limits our understanding of such streams, the watersheds examined here suggest the important role of aridity and land cover change in modulating future stream drying. Plain Language Summary A majority of global streams are nonperennial, flowing only part of the year, and are critical for sustaining flow downstream, providing habitat for many organisms, and regulating chemical and biological processes. Using long-term U.S. Geological Survey measurements for 540 watersheds across the contiguous United States, we mapped patterns and examined the causes of no-flow fraction, the fraction of each climate year with no flow, no-flow timing, the date of the climate year on which the first recorded no flow takes place, and length of the dry-down period, the average number of days from a local peak in daily flow to the first occurrence of no flow. We found differences in patterns of no-flow characteristics between regions, with higher no-flow fraction, earlier timing, and shorter dry-down duration in the western United States. No-flow fractions were greater and less variable in natural watersheds, while no-flow timing was earlier and dry-down duration was shorter in human-modified watersheds. Aridity had the greatest effect on intermittence across the United States, but unique combinations of climate, biophysical, and human impacts were important in different regions. The number of gages measuring streamflow in nonperennial streams is small compared to perennial streams, and increased monitoring is needed to better understand drying behavior. Key Points . Three metrics reveal regional and human-driven patterns of nonperennial flow: no-flow fraction, day of first no flow, and dry-down duration Streams with human modifications generally dry more quickly than unmodified streams, especially in California and the Southern Great Plains Climate strongly influences no-flow fraction and timing, but physiographic variables are more important for the duration of dry down ; National Science FoundationNational Science Foundation (NSF) [DEB-1754389]; Kansas Water Resources Institute ; Published version ; This manuscript is a product of the Dry Rivers Research Coordination Network, supported by funding from the National Science Foundation (DEB-1754389). Although this work was approved for publication by the EPA, it may not reflect official Agency policy. S.C.Z. was supported by the Kansas Water Resources Institute. We thank David Wolock and two anonymous reviewers for their suggestions and edits. ; Public domain authored by a U.S. government employee

A comprehensive cross-biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N-rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0-1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean. ; National Science Foundation (NSF) through the Long-Term Ecological Research Network Office (LNO), National Center for Ecological Analysis and Synthesis (NCEAS), University of California-Santa Barbara [1545288]; NSFNational Science Foundation (NSF) [1556603]; New Hampshire Agricultural Experiment Station; USDA National Institute of Food and Agriculture McIntire-Stennis Project [1006760, 1016163]; Natural Environment Research Council, UK Large Grant [NE/K010689/1] ; Published version ; This work was conducted as a part of the Stream Elemental Cycling Synthesis Group funded by the National Science Foundation (NSF) under grant DEB#1545288, through the Long-Term Ecological Research Network Office (LNO), National Center for Ecological Analysis and Synthesis (NCEAS), University of California-Santa Barbara. The authors acknowledge the efforts of Julien Brun for assistance with data synthesis and the efforts of multiple individuals who collected and analyzed samples. Partial support for ASW during data synthesis and manuscript preparation was provided by NSF grant DEB#1556603 (Deciphering Dissolved Organic Nitrogen). Partial funding was provided by the New Hampshire Agricultural Experiment Station. This is Scientific Contribution 2880. This work was supported by the USDA National Institute of Food and Agriculture McIntire-Stennis Project 1006760. Support for AA was provided by the USDA National Institute of Food and Agriculture McIntire-Stennis Project 1016163. Partial support for PJJ and CAY was provided by Natural Environment Research Council, UK Large Grant NE/K010689/1 (DOMAINE: Characterizing the Nature, Origins and Ecological Significance of DOM in Freshwater Ecosystems). The authors are also grateful for feedback from two anonymous reviewers whose comments significantly improved this manuscript. This paper is dedicated to the memory of Dr. John Schade, a friend, colleague, and mentor to many of us. John studied ecological stoichiometry in freshwater ecosystems and led the Long-Term Ecological Research (LTER) group at the US National Science Foundation. ; Public domain authored by a U.S. government employee

Non-perennial streams are widespread, critical to ecosystems and society, and the subject of ongoing policy debate. Prior large-scale research on stream intermittency has been based on long-term averages, generally using annually aggregated data to characterize a highly variable process. As a result, it is not well understood if, how, or why the hydrology of non-perennial streams is changing. Here, we investigate trends and drivers of three intermittency signatures that describe the duration, timing, and dry-down period of stream intermittency across the continental United States (CONUS). Half of gages exhibited a significant trend through time in at least one of the three intermittency signatures, and changes in no-flow duration were most pervasive (41% of gages). Changes in intermittency were substantial for many streams, and 7% of gages exhibited changes in annual no-flow duration exceeding 100 days during the study period. Distinct regional patterns of change were evident, with widespread drying in southern CONUS and wetting in northern CONUS. These patterns are correlated with changes in aridity, though drivers of spatiotemporal variability were diverse across the three intermittency signatures. While the no-flow timing and duration were strongly related to climate, dry-down period was most strongly related to watershed land use and physiography. Our results indicate that non-perennial conditions are increasing in prevalence over much of CONUS and binary classifications of 'perennial' and 'non-perennial' are not an accurate reflection of this change. Water management and policy should reflect the changing nature and diverse drivers of changing intermittency both today and in the future. ; US National Science FoundationNational Science Foundation (NSF) [DEB-1754389] ; Published version ; This manuscript is a product of the Dry Rivers Research Coordination Network, which was supported by funding from the US National Science Foundation (DEB-1754389). Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. This manuscript was improved by constructive feedback from Kristin Jaeger and three anonymous reviews.

Streams play a key role in the global carbon cycle. The balance between carbon intake through photosynthesis and carbon release via respiration influences carbon emissions from streams and depends on temperature. However, the lack of a comprehensive analysis of the temperature sensitivity of the metabolic balance in inland waters across latitudes and local climate conditions hinders an accurate projection of carbon emissions in a warmer future. Here, we use a model of diel dissolved oxygen dynamics, combined with high-frequency measurements of dissolved oxygen, light and temperature, to estimate the temperature sensitivities of gross primary production and ecosystem respiration in streams across six biomes, from the tropics to the arctic tundra. We find that the change in metabolic balance, that is, the ratio of gross primary production to ecosystem respiration, is a function of stream temperature and current metabolic balance. Applying this relationship to the global compilation of stream metabolism data, we find that a 1 degrees C increase in stream temperature leads to a convergence of metabolic balance and to a 23.6% overall decline in net ecosystem productivity across the streams studied. We suggest that if the relationship holds for similarly sized streams around the globe, the warming-induced shifts in metabolic balance will result in an increase of 0.0194 Pg carbon emitted from such streams every year. ; National Science Foundation (NSF)National Science Foundation (NSF) [EF-1258994]; NSFNational Science Foundation (NSF) [EF-1065255, EF-1065286, EF-1065055, EF-1065682, EF-1065267, EF-1064998, EF-1065377]; Northern Australian Environmental Resources Hub of the National Environmental Science Program; Scale, Consumers and Lotic Ecosystem Rates project ; The authors thank K. Gido for his contribution in obtaining funding and designing the field experiments. K. Gido, J. Drake, C. Osenberg and J. Minucci provided comments on earlier versions of this paper. Georgia Advanced Computing Resource Center provided the computing facility. This study was supported by the National Science Foundation (NSF, grant EF-1258994) and is part of the Scale, Consumers and Lotic Ecosystem Rates project supported by NSF grant EF-1065255. Data collection at each site was supported by NSF grants EF-1065286, EF-1065055, EF-1065682, EF-1065267, EF-1064998 and EF-1065377, and the Northern Australian Environmental Resources Hub of the National Environmental Science Program. ; Public domain authored by a U.S. government employee

Studies of trophic-level material and energy transfers are central to ecology. The use of isotopic tracers has now made it possible to measure trophic transfer efficiencies of important nutrients and to better understand how these materials move through food webs. We analyzed data from thirteen N-15-ammonium tracer addition experiments to quantify N transfer from basal resources to animals in headwater streams with varying physical, chemical, and biological features. N transfer efficiencies from primary uptake compartments (PUCs; heterotrophic microorganisms and primary producers) to primary consumers was lower (mean 11.5%, range 100%). Total N transferred (as a rate) was greater in streams with open compared to closed canopies and overall N transfer efficiency generally followed a similar pattern, although was not statistically significant. We used principal component analysis to condense a suite of site characteristics into two environmental components. Total N uptake rates among trophic levels were best predicted by the component that was correlated with latitude, DIN:SRP, GPP:ER, and percent canopy cover. N transfer efficiency did not respond consistently to environmental variables. Our results suggest that canopy cover influences N movement through stream food webs because light availability and primary production facilitate N transfer to higher trophic levels. ; National Science FoundationNational Science Foundation (NSF) [NSF-DEB 1052399, DBI-1401954]; Department of Energy's Office of Science, Biological and Environmental Research; U.S. DOEUnited States Department of Energy (DOE) [DE-AC05-00OR22725]; U.S. Department of EnergyUnited States Department of Energy (DOE) [DE-AC05-00OR22725] ; We thank everyone who participated in the individual tracer experiments used in this analysis. We are grateful for the leadership and friendship of the late Pat Mulholland, whose legacy continues to inspire. This manuscript is the product of a workshop funded by a National Science Foundation grant (NSF-DEB 1052399) to M. R. Whiles and W. K. Dodds. Partial support during manuscript preparation to N. A. Griffiths was from the Department of Energy's Office of Science, Biological and Environmental Research. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. S. M. Collins was supported by a National Science Foundation Postdoctoral Research Fellowship in Biology (DBI-1401954). ; Public domain authored by a U.S. government employee