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Dissolved organic matter (DOM) concentrations and composition within wet deposition are rarely monitored despite contributing a large input of bioavailable dissolved organic carbon (DOC) and nitrogen (DON) to the Earth's surface. Lacking from the literature are spatially comprehensive assessments of simultaneous measurements of wet deposition DOC and DON chemistry and their dependencies on metrics of climate and environmental factors. Here, we use archived precipitation samples from the US National Atmospheric Deposition Program collected in 2017 to 2018 from 17 sites across six ecoregions to investigate variability in the concentration and composition of depositional DOM. We hypothesize metrics of DOM chemistry vary with ecoregion, season, large‐scale climate drivers, and precipitation geographic source. Findings indicate differences in DOC and DON concentrations and loads among ecoregions. The highest wet deposition concentrations are from sites in the Northern Forests and lowest concentrations from sites in Marine West Coast Forests. Summer and autumn samples contained the highest DOC concentrations and DON concentrations that were consistently above detection limit, corresponding with seasonality of peak air temperatures and the phenology of the growing season in the northern hemisphere. Compositional trends suggest lighter DOM molecules in autumn and winter and heavier molecules in spring and summer. Climate drivers explain 51% of variation in DOM chemistry, revealing differing drivers on the concentrations and loads of DOC versus DON in wet deposition. This study highlights the necessity of incorporating DOC and DON measurements into national deposition monitoring networks to understand spatial and temporal feedbacks between climate change, atmospheric chemistry and landscape biogeochemistry. Plain Language Summary: Organic matter concentrations in wet deposition–or rain–are rarely reported, even though they contribute significantly to the dissolved organic matter (DOM) reaching the Earth's surface. We particularly lack insight into trends in the dissolved organic nitrogen (DON) portion of DOM. In this study, we examine how wet deposition DOM concentration, composition, and source change over time and across different regions. We hypothesize that the concentrations and composition of dissolved organic carbon (DOC) and DON in wet deposition will vary by region, show seasonal patterns, and be influenced by large‐scale climate factors. To test these hypotheses, we use archived samples from the US National Atmospheric Deposition Program to analyze wet deposition DOM from samples collected in 2017 to 2018 across the U.S. Our results show that the concentrations of wet deposition DOM varied among regions, with most sites showing the highest values during the summer and fall seasons. Climate factors explained much of the variation in DOM wet deposition, but the carbon and nitrogen fraction of organic matter were related to different climate factors. Monitoring the exchange of organic matter between the atmosphere and the land surface provides valuable insights into the role of deposition in carbon and nitrogen cycles. Key Points: Archived wet deposition samples from the US National Atmospheric Deposition Program are viable for hindcasting dissolved organic carbon (DOC) and dissolved organic nitrogen (DON)Highest concentrations of DOC and DON wet deposition correspond with the growing season in the northern hemisphereClimatic variables such as airmass range, air temperature, and precipitation seasonality explain variability in the spatial heterogeneity of Dissolved organic matter wet deposition [ABSTRACT FROM AUTHOR]

The seasonal behavior of fluvial dissolved silica (DSi) concentrations, termed DSi 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. Plain Language Summary: The amount of dissolved silicon (DSi) in rivers is an important control on numerous ecological and biogeochemical processes, such as types of algae that bloom and rates of carbon sequestration. Compared to our knowledge of other nutrients, such as nitrogen and phosphorus, we have limited understanding of what controls the timing and concentration of DSi in rivers. Previous work identified five distinct seasonal patterns of DSi concentrations in rivers across the Northern Hemisphere; here we look at the environmental variables that control these seasonal patterns. We found that rivers often have one to five seasonal patterns over time due to interannual shifts in temperature, evapotranspiration, and streamflow. In addition, we found that the average shape of the seasonal pattern for a given river, specifically minimum and maximum DSi concentrations, was related to nitrogen (N) and phosphorus (P) concentrations, highlighting linkages between N, P, and DSi cycling in rivers. This work identifies why river DSi concentrations exhibit both within and between year variability, highlighting that temperature, streamflow, and nutrient availability control the timing of river DSi availability for biological uptake. Key Points: Seasonal variations in annual riverine dissolved silica concentrations (DSi regime) were correctly classified 80% of the timeClimate and primary productivity emerge as the most important drivers in differentiating among average DSi regimesMedian nitrogen and phosphorus concentrations strongly predicted minimum and maximum DSi concentration, regardless of regime type [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nitrogen (N) wet deposition chemistry impacts watershed biogeochemical cycling. The timescale and magnitude of (a)synchrony between wet deposition N inputs and watershed N outputs remains unresolved. We quantify deposition‐river N (a)synchrony with transfer entropy (TE), an information theory metric enabling quantification of lag‐dependent feedbacks in a hydrologic system by calculating directional information flow between variables. Synchrony is defined as a significant amount of TE‐calculated reduction in uncertainty of river N from wet deposition N after conditioning for antecedent river N conditions. Using long‐term timeseries of wet deposition and river DON, NO3−, and NH4+ concentrations from the Lamprey River watershed, New Hampshire (USA), we constrain the role of wet deposition N to watershed biogeochemistry. Wet deposition N contributed information to river N at timescales greater than quick‐flow runoff generation, indicating that river N losses are a lagged non‐linear function of hydro‐biogeochemical forcings. River DON received the most information from all three wet deposition N solutes while wet deposition DON and NH4+ contributed the most information to all three river N solutes. Information theoretic algorithms facilitated data‐driven inferences on the hydro‐biogeochemical processes influencing the fate of N wet deposition. For example, signals of mineralization and assimilation at a timescale of 12 to 21‐weeks lag display greater synchrony than nitrification, and we find that N assimilation is a positive lagged function of increasing N wet deposition. Although wet deposition N is not the main driver of river N, it contributes a significant amount of information resolvable at time scales of transport and transformations. Plain Language Summary: Nitrogen (N) dissolved within precipitation (e.g., wet deposition) is an important input of N from the atmosphere to biosphere. Whether wet deposition N is synchronized with watershed biogeochemical processes, including N losses to rivers, remains unresolved at fine temporal scales. Synchrony is a phenomenon observed between variables displaying complex interactions with potentially lagged temporal relationships. Information theory algorithms, like transfer entropy (TE), quantify directional information flow between variables, enabling the quantification of lag‐dependencies and feedback cycles in a hydrologic system, informative for creating conceptual models of watershed‐scale biogeochemical processes. Here, wet deposition‐river N synchrony between pair‐wise combinations of dissolved organic N, NO3−, and NH4+ is expressed as the timing and magnitude of TE at the same time step or pre‐determined lag. Synchrony was calculated using 17‐years of weekly paired wet deposition‐river N observations from the Lamprey River watershed. Precipitation and streamflow were most synchronized at no lag, while biogeochemical analyses were synchronized at lags between 12 and 21‐weeks, indicating N losses are a function of hydrologic and biogeochemical forcings. We quantified strength and timing of synchrony for each pair of N solutes. Signals of inorganic and organic N incorporation into biomass and conversion of organic N to inorganic N displayed the greatest synchrony, suggesting that N wet deposition inputs may contribute to uptake and further cycling of N within terrestrial and aquatic biomass. Key Points: Wet deposition‐river N synchrony was resolved at timescales greater than hydrologic processes such as surface flow runoffThe timing of peak synchrony indicates that N deposition may magnify processes like assimilation and mineralization in the watershedWhile deposition N contributed information to river N, the magnitude suggests sources besides wet deposition are driving river N [ABSTRACT FROM AUTHOR]