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.

Evaluation of wetland ecological condition requires quantitative biological indices for measuring anthropogenic impairment. We implemented a modified floristic quality assessment index (FQAI) protocol for 75 isolated, depressional herbaceous wetland systems, exploring refinements of FQAI standard methods. Species encountered during sampling (n = 397) were assigned coefficients of conservatism (CC) by ten expert botanists working independently. A quantitative summary metric of adjacent site buffer (up to 100 m) land use intensity, called the landscape development intensity (LDI) index, was calculated for each wetland system to quantify expected anthropogenic impairment. The association between LDI and wetland community mean CC scores was strong and conditionally independent of ecoregion. Weaker associations with LDI were observed for other community summary metrics, including richness‐weighted FQAI. We inverted LDI to compute an intensity coefficient (IC), which quantifies observed buffer development intensity tolerated by each species. IC scores were significantly associated with CC scores on a species basis and strongly associated on a site mean basis. Growing interest in floristic quality assessment for regulatory purposes provides opportunities for formally linking expert opinion and ground observations of species‐specific disturbance tolerance.

Hypoxia in coastal waters and lakes is widely recognized as a detrimental environmental issue, yet we lack a comparable understanding of hypoxia in rivers. We investigated controls on hypoxia using 118 million paired observations of dissolved oxygen (DO) concentration and water temperature in over 125,000 locations in rivers from 93 countries. We found hypoxia (DO < 2 mg L⁻¹) in 12.6% of all river sites across 53 countries, but no consistent trend in prevalence since 1950. High‐frequency data reveal a 3‐h median duration of hypoxic events which are most likely to initiate at night. River attributes were better predictors of riverine hypoxia occurrence than watershed land cover, topography, and climate characteristics. Hypoxia was more likely to occur in warmer, smaller, and lower‐gradient rivers, particularly those draining urban or wetland land cover. Our findings suggest that riverine hypoxia and the resulting impacts on ecosystems may be more pervasive than previously assumed.

Elemental cycles are coupled directly and indirectly to ecosystem metabolism at multiple time scales. Understanding coupling in lotic ecosystems has recently advanced through simultaneous high‐frequency measurements of multiple solutes. Using hourly in situ measurements of soluble reactive phosphorus (SRP), specific conductance (SpC), and dissolved oxygen (DO), we estimated phosphorus (P) retention pathways and dynamics in a large (discharge, Q ≈ 7.5 m ³ /s) spring‐fed river (Ichetucknee River, Florida, USA). Across eight multi‐day deployments, highly regular diel SRP variation of 3–9 μg P/L (mean ∼50 μg P/L) was strongly correlated with DO variation, suggesting photosynthetic control directly via assimilation, and/or indirectly via geochemical reactions. Consistent afternoon SRP maxima and midnight minima suggest peak removal lags gross primary production (GPP) by ∼8 hours. Two overlapping processes were evident, one dominant with maximum removal near midnight, the other smaller with maximum removal near midday. Hourly [Ca] measurements during three 24‐hour deployments showed consistent afternoon minima, suggesting that calcite precipitates as GPP increases pH and mineral saturation state. Resulting P co‐precipitation was modeled using SpC as a [Ca] proxy, yielding a diel P signal adjusted for the primary geochemical retention pathway. P assimilation, the dominant diel signal, was computed by interpolating between daily concentration maxima, both with and without geochemical adjustment, and converting concentration deficits to fluxes using discharge and benthic area. Adjusting for calcite co‐precipitation yielded assimilation rates (13.7 ± 5.8 mg P·m ⁻² ·d ⁻¹ ) strongly correlated with GPP, accounting for 72% ± 9% of gross removal, and ecosystem C:P stoichiometry (466 [± 12]:1) consistent with dominant vascular autotrophs (478 [± 24]:1). Without adjusting for co‐precipitation, covariance with GPP was weaker, and C:P implausibly high. Consistent downstream P accumulation, likely from P‐rich porewater seepage, varied across deployments (3–22% of total flux) and co‐varied with discharge and respiration. Asynchronous N and P assimilation may arise from differential timing of protein and ribosome production, with the latter requiring maximal carbohydrate stores and minimal photolytic interference. This study demonstrates direct and indirect coupling of biological, hydrological, and geochemical processes and the utility of high‐resolution time series of multiple solutes for understanding these linkages.

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.