In the 12 years since Dudgeon et al. (2006) reviewed major pressures on freshwater ecosystems, the biodiversity crisis in the world's lakes, reservoirs, rivers, streams and wetlands has deepened. While lakes, reservoirs and rivers cover only 2.3% of the Earth's surface, these ecosystems host at least 9.5% of the Earth's described animal species. Furthermore, using the World Wide Fund for Nature's Living Planet Index, freshwater population declines (83% between 1970 and 2014) continue to outpace contemporaneous declines in marine or terrestrial systems. The Anthropocene has brought multiple new and varied threats that disproportionately impact freshwater systems. We document 12 emerging threats to freshwater biodiversity that are either entirely new since 2006 or have since intensified: (i) changing climates; (ii) e-commerce and invasions; (iii) infectious diseases; (iv) harmful algal blooms; (v) expanding hydropower; (vi) emerging contaminants; (vii) engineered nanomaterials; (viii) microplastic pollution; (ix) light and noise; (x) freshwater salinisation; (xi) declining calcium; and (xii) cumulative stressors. Effects are evidenced for amphibians, fishes, invertebrates, microbes, plants, turtles and waterbirds, with potential for ecosystem-level changes through bottom-up and top-down processes. In our highly uncertain future, the net effects of these threats raise serious concerns for freshwater ecosystems. However, we also highlight opportunities for conservation gains as a result of novel management tools (e.g. environmental flows, environmental DNA) and specific conservation-oriented actions (e.g. dam removal, habitat protection policies, managed relocation of species) that have been met with varying levels of success. Moving forward, we advocate hybrid approaches that manage fresh waters as crucial ecosystems for human life support as well as essential hotspots of biodiversity and ecological function. Efforts to reverse global trends in freshwater degradation now depend on bridging an immense gap between the aspirations of conservation biologists and the accelerating rate of species endangerment.

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Geographically isolated wetlands (GIWs), those surrounded by uplands, exchange materials, energy, and organisms with other elements in hydrological and habitat networks, contributing to landscape functions, such as flow generation, nutrient and sediment retention, and biodiversity support. GIWs constitute most of the wetlands in many North American landscapes, provide a disproportionately large fraction of wetland edges where many functions are enhanced, and form complexes with other water bodies to create spatial and temporal heterogeneity in the timing, flow paths, and magnitude of network connectivity. These attributes signal a critical role for GIWs in sustaining a portfolio of landscape functions, but legal protections remain weak despite preferential loss from many landscapes. GIWs lack persistent surface water connections, but this condition does not imply the absence of hydrological, biogeochemical, and biological exchanges with nearby and downstream waters. Although hydrological and biogeochemical connectivity is often episodic or slow (e.g., via groundwater), hydrologic continuity and limited evaporative solute enrichment suggest both flow generation and solute and sediment retention. Similarly, whereas biological connectivity usually requires overland dispersal, numerous organisms, including many rare or threatened species, use both GIWs and downstream waters at different times or life stages, suggesting that GIWs are critical elements of landscape habitat mosaics. Indeed, weaker hydrologic connectivity with downstream waters and constrained biological connectivity with other landscape elements are precisely what enhances some GIW functions and enables others. Based on analysis of wetland geography and synthesis of wetland functions, we argue that sustaining landscape functions requires conserving the entire continuum of wetland connectivity, including GIWs.connectivity | navigable waters | significant nexus Understanding connectivity-patterns of matter, energy, and organism exchanges among landscape elements and across scales-is a challenge that unites the fields of ecology and hydrology (1). Connectivity enables dispersal of organisms and flows of water between landscape elements at multiple spatial and temporal scales

IntroductionClearly defined hydrologic response units (HRUs) that incorporate unifying concepts in hydrology-the complete hydrologic cycle and conservation of mass (Dooge, 1986)-are required to direct and integrate local, regional and continental scales of hydrologic research and management. The topographically defined watershed or catchment has been championed as the basic HRU (Dooge, 1968). However, catchment studies reveal large complexity and heterogeneity of runoff behaviour, resulting in a multitude of conceptual and numerical model structures. Recent reviews argue that a broad-scale classification of catchments is required to generalize dominant hydrologic processes, direct field methodologies, and apply hydrologic model structure (Sivapalan, 2003; McDonnell and Woods, 2004). However, protocols on defining such areas are presently lacking.Traditionally, researchers have disregarded large portions of the landscape in favour of areas amenable to 'hydrologic study', by relying on catchments where hydrologic boundaries can be easily defined. These catchments are often small and homogeneous, to 'control' for climatic and geologic features, which may have misled non-catchment-hydrologists (or up-and-coming hydrologists and managers) to believe that the first variable to consider in predicting hydrologic response is topography. This approach may provide a false sense of security about the effectiveness of topographically defined catchments as an approach to conduct research, assess regional hydrology, and generalize results to broad landscape scales. Recent reviews clearly illustrate the need for a thorough integration of surface water and groundwater processes (Winter, with respect to dominant hydrologic cycling and mass balance. We believe that asserting the topographically defined catchment as a standard hydrologic unit, or by assuming that the water table conforms to topography, is a methodological approach that has been overstated in importance for regional to national scales of water management.2001a Effective Delineation of a Catchment Using Dominant HRUs: a Boreal Plain ExampleThe impetus for this commentary comes from an interest in understanding hydrology on the subhumid glaciated plains of the western Boreal Forest, and our realization that traditional approaches for hydrologic research may actually serve to limit insights into hydrologic function in this region. Ongoing research at our Utikuma Research Study Area (URSA), Alberta, Canada, reveals that glaciated regions, such as the Boreal Plain, with deep glaciated substrates arguably result in some of the most complex surface and groundwater interactions (e.g. Winter, 1999Winter, , 2001a The difference in a hydrologist's perception of the effective catchment area determined by first considering topography, rather than climate and geology, is illustrated in the example in Figure 1 (Mink Lake, Alberta). From the data provided and the scale of the example, similar runoff contribution per unit area would often be assumed, and the hydrologic response time...

Abstract:A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration-discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM.