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Abstract Small estuaries link uplands and nearshore marine environments but are understudied in many regions. We quantified spatial and temporal variability in CH₄, CO₂, nutrients, and organic matter in a small, deep, bar‐built tropical estuary in Fajardo, Puerto Rico. The estuary was typically impounded and strongly stratified at all three sampling locations. Across 5 sampling dates, the pycnocline was sharp, with salinity of 0 in the upper meter and up to 17 ppt at 3–5 m. Methane concentrations at the surface were extraordinarily high for a mangrove‐fringed tropical estuary, averaging from 26 to 59 μM at three stations. Surface methane flux was 17.1 mM/m²/day. Methane concentrations were high throughout the water column, declined temporarily following estuarine flushing that increased salinity, and were correlated with CO₂(r² = 0.48,p < 0.01). Small bar‐built estuaries are common in Puerto Rico and other high‐wave energy environments and should be included in assessments of greenhouse gas emissions. 

ABSTRACT Methane (CH4) accounts for about one-third of the current anthropogenic greenhouse gas-driven warming. Rivers, particularly those draining human-impacted landscapes, are important sources of CH4 to the atmosphere. Yet, effective mitigation of this important flux remains elusive due to the lack of accurate quantification of human-induced emissions and a poor understanding of its key aquatic drivers. Here, we demonstrate that CH4 emission rates from global nutrient-enriched rivers in human-impacted regions more than double (2.5-fold) those from pristine rivers. Importantly, a strong association is identified between CH4 flux and concentrations of total phosphorus and ammonium nitrogen in these systems. This relationship is mediated by increased autochthonous and exogenous supplies of labile organic substrates, enhanced methanogen abundance and diversity, and extended anoxia in nutrient-enriched rivers. Quantitative modeling incorporating nutrient effects estimates global CH4 emissions from nutrient-enriched rivers at 9.7 ± 1.1 Tg CH4 yr−1, of which 22%–49% could be mitigated by reducing current nutrient concentrations to levels constrained by sustainable development goals, half of their current levels, or pristine conditions. These findings highlight the substantial potential for effective mitigation of riverine CH4 emissions via coordinated riverine nutrient management at the global scale. 

Abstract The production, transport, loss, and export of dissolved organic carbon (DOC) are closely connected with hydrometeorological drivers. While terrestrial production depends on biogeochemical variables like soil moisture, air temperature, and the contact area between water and organic matter sources, export is determined by water availability and flow paths within soil storage. This study aims to quantify how hydrometeorological drivers jointly control DOC production and export across spatial scales in a temperate snow‐dominated forested catchment. Using a parsimonious model to simultaneously optimize discharge and DOC concentrations, we leveraged over 14 years of high‐resolution (15‐min) hydrological data, along with weekly to monthly measurements of DOC concentrations, across nested catchments of varying sizes from small (7 km²), medium (170 km²), to large (480 km²). The simulations replicated daily hydrological (KGE ∼ 0.60) and DOC (KGE ∼ 0.60) concentration dynamics at all sites. Our findings indicate that shallow soil storage and antecedent precipitation (>45 days) significantly influence DOC dynamics across all three spatial scales, emphasizing the importance of connectivity in catchments' responses to hydrometeorological conditions. The influence of antecedent precipitation is of greater magnitude at larger scales, while pulses of DOC are more common at smaller scales due to rapid source activation. Our results highlight that model performance and the ability to realistically represent catchment processes are enhanced when hydrological and biogeochemical processes are considered simultaneously. Parsimonious models are crucial to evaluate how hydro‐biogeochemical processes vary across spatial scales and ultimately respond to human perturbations, such as climate change and extreme events. 

Abstract Riverine silicon (Si) plays a vital role in governing primary production, water quality, and carbon cycling. Climate and land cover change have altered how dissolved Si (DSi) is processed on land, transported to rivers, and cycled through aquatic ecosystems. The Global Aggregation of Stream Silica (GlASS) database was constructed to assess changes in river Si concentrations and fluxes, their relationship to other nutrients (nitrogen (N) and phosphorus (P)), and to evaluate mechanisms driving the availability of Si. GlASS includes concentrations of DSi, dissolved inorganic N (NO₃, NO_x, and NH₄), and dissolved inorganic P (as soluble reactive P or PO₄-P) at daily to quarterly time steps from 1963 to 2024; daily discharge; and watershed characteristics for 421 rivers spanning eight climate zones. Original data sources are cited, data quality assurance workflows are public, and input files to a common load model are provided. GlASS offers critical data to address questions about patterns, controls, and trajectories of global river Si biogeochemistry and stoichiometry. 

Abstract Estuaries are threatened by eutrophication due to anthropogenic nutrient loading. The Great Bay Estuary of New Hampshire and Maine has been designated as nitrogen impaired primarily due to a 65% loss in seagrass coverage between 1996 and 2023. This region has also experienced multiple consecutive years of low annual precipitation and more recent record precipitation years. The loss of seagrass, increased precipitation variability, and continued anthropogenic influence have biogeochemical consequences for the estuarine filter. Water quality budgets for a subregion of Great Bay Estuary were developed at annual timescales for nitrogen, orthophosphate (PO₄³⁻), dissolved organic carbon (DOC), and total suspended solids (TSS). Results show annual total nitrogen (TN) input loads, including tidal flux into Great Bay, (3,900 kg ha⁻¹ year⁻¹) are less than output loads whereas dissolved inorganic nitrogen (DIN) inputs (1,410 kg ha⁻¹ year⁻¹) exceed outputs, indicating net TN export and net DIN retention. PO₄³⁻loads were nearly balanced, with a mean input and output of 257 kg ha⁻¹ year⁻¹and 238 kg ha⁻¹ year⁻¹, respectively. Seagrass acreage negatively correlated with DOC inputs (33,300 kg ha⁻¹ year⁻¹). High TSS output loads (227,000 kg ha⁻¹ year⁻¹) resulted in net TSS export. Despite critical seagrass loss and recent point source nitrogen load reductions, the system continues to retain bioavailable forms of nutrients, suggesting the importance of other primary producers to estuarine biogeochemical cycles. Water quality budgets for Great Bay provide useful insight into the biogeochemical capacity of an estuary during a time of habitat degradation and subsequent coastal management efforts. 

Abstract Root distributions are typically based on root mass per soil volume. This plant‐focused approach masks the biogeochemical influence of fine roots, which weigh little. We assert that centimeter‐scale root presence‐absence data from soil profiles provide a more soil‐focused approach for probing depth distributions of root‐regolith interfaces, where microsite‐scale processes drive whole‐ecosystem functioning. In 75 soil pits across the continental USA, Puerto Rico, and the Alps, we quantified fine and coarse root presence as deep as 2 m. In 70 of these pits we estimated root mass and created standardized metrics of both data sets to compare their depth distributions. We addressed whether: (a) depth distributions of root presence‐absence data differ from root mass data, thus implying different degrees of root‐regolith interactions with depth; and (b) if root presence or any depth‐dependent differences between these data sets vary predictably with environmental conditions. Presence of fine roots exhibited diverse depth‐dependent patterns; root mass generally declined with depth. In B and C horizons, standardized root presence was greater than standardized root mass; random forest analyses suggest these discrepancies are greater in B horizons with increasing mean annual precipitation and in C horizons with increasing mean annual temperature. Our work suggests that deep in the subsurface, biogeochemical and reactive transport processes result from more numerous root‐regolith interfaces than mass data suggest. We present a new paradigm for discerning patterns in depth distributions of root‐regolith interfaces across multiple biomes and land uses that promotes understanding of the roles of those interfaces in driving key critical zone processes. 

ABSTRACT Nitrous oxide (N₂O) reductase, the sole natural microbial sink for N₂O, exists in two microbial clades:nosZI andnosZII. Although previous studies have explored inter‐clade ecological differentiation, the intra‐clade variations and their implications for N₂O dynamics remain understudied. This study investigated both inter‐ and intra‐clade ecological differentiation among N₂O reducers, the drivers influencing these patterns, and their effects on N₂O emissions across continental‐scale river systems. The results showed that bothnosZI andnosZII community turnovers were associated with similar key environmental factors, particularly total phosphorus (TP), but these variables explained a larger proportion of variation in thenosZI community. The influence of mean annual temperature (MAT) on community composition increased for more widespread N₂O‐reducing taxa. We identified distinct ecological clusters within each clade of N₂O reducers and observed identical ecological clustering patterns across both clades. These clusters were primarily characterized by distinct MAT regimes, coarse sediment texture as well as low TP levels, and high abundance of N₂O producers, with MAT‐related clusters constituting predominant proportions. Intra‐clade ecological differentiation was a crucial predictor of N₂O flux and reduction efficiency. Although different ecological clusters showed varying or even contrasting associations with N₂O dynamics, the shared ecological clusters across clades exhibited similar trends. Low‐MAT clusters in both thenosZI andnosZII communities were negatively correlated with denitrification‐normalized N₂O flux and the N₂O:(N₂O + N₂) ratio, whereas high‐MAT clusters showed positive correlations. This contrasting pattern likely stems from low‐MAT clusters being better adapted to eutrophic conditions and their more frequent co‐occurrence with N₂O‐producing genes. These findings advance our understanding of the distribution and ecological functions of N₂O reducers in natural ecosystems, suggesting that warming rivers may have decreased N₂O reduction efficiency and thereby amplify temperature‐driven emissions. 

Abstract The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termedDSi regime, mediates the timing of DSi delivery to downstream waters and thus governs river biogeochemical function and aquatic community condition. Previous work identified five distinct DSi regimes across rivers spanning the Northern Hemisphere, with many rivers exhibiting multiple DSi regimes over time. Several potential drivers of DSi regime behavior have been identified at small scales, including climate, land cover, and lithology, and yet the large‐scale spatiotemporal controls on DSi regimes have not been identified. We evaluate the role of environmental variables on the behavior of DSi regimes in nearly 200 rivers across the Northern Hemisphere using random forest models. Our models aim to elucidate the controls that give rise to (a) average DSi regime behavior, (b) interannual variability in DSi regime behavior (i.e., Annual DSi regime), and (c) controls on DSi regime shape (i.e., minimum and maximum DSi concentrations). Average DSi regime behavior across the period of record was classified accurately 59% of the time, whereas Annual DSi regime behavior was classified accurately 80% of the time. Climate and primary productivity variables were important in predicting Average DSi regime behavior, whereas climate and hydrologic variables were important in predicting Annual DSi regime behavior. Median nitrogen and phosphorus concentrations were important drivers of minimum and maximum DSi concentrations, indicating that these macronutrients may be important for seasonal DSi drawdown and rebound. Our findings demonstrate that fluctuations in climate, hydrology, and nutrient availability of rivers shape the temporal availability of fluvial DSi. 

Abstract Fluvial silicon (Si) plays a critical role in controlling primary production, water quality, and carbon sequestration through supporting freshwater and marine diatom communities. Geological, biogeochemical, and hydrological processes, as well as climate and land use, dictate the amount of Si exported by streams. Understanding Si regimes—the seasonal patterns of Si concentrations—can help identify processes driving Si export. We analyzed Si concentrations from over 200 stream sites across the Northern Hemisphere to establish distinct Si regimes and evaluated how often sites moved among regimes over their period of record. We observed five distinct regimes across diverse stream sites, with nearly 60% of sites exhibiting multiple regime types over time. Our results indicate greater spatial and interannual variability in Si seasonality than previously recognized and highlight the need to characterize the watershed and climate variables that affect Si cycling across diverse ecosystems. 

Abstract Processes that drive variability in catchment solute sourcing, transformation, and transport can be investigated using concentration–discharge (C–Q) relationships. These relationships reflect catchment and in‐stream processes operating across nested temporal scales, incorporating both short and long‐term patterns. Scientists can therefore leverage catchment‐scale C–Q datasets to identify and distinguish among the underlying meteorological, biological, and geological processes that drive solute export patterns from catchments and influence the shape of their respective C–Q relationships. We have synthesized current knowledge regarding the influence of biological, geological, and meteorological processes on C–Q patterns for various solute types across diel to decadal time scales. We identify cross‐scale linkages and tools researchers can use to explore these interactions across time scales. Finally, we identify knowledge gaps in our understanding of C–Q temporal dynamics as reflections of catchment and in‐stream processes. We also lay the foundation for developing an integrated approach to investigate cross‐scale linkages in the temporal dynamics of C–Q relationships, reflecting catchment biogeochemical processes and the effects of environmental change on water quality. This article is categorized under:Science of Water > Hydrological ProcessesScience of Water > Water QualityScience of Water > Water and Environmental Change