Global methane emissions from rivers and streams.

Methane (CH4) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH4 emissions from freshwater ecosystems1,2, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH4 emissions remain highly uncertain3,4. Here we report a spatially explicit global estimate of CH4 emissions from running waters, accounting for 27.9 (16.7-39.7) Tg CH4 per year and roughly equal in magnitude to those of other freshwater systems5,6. Riverine CH4 emissions are not strongly temperature dependent, with low average activation energy (EM = 0.14 eV) compared with that of lakes and wetlands (EM = 0.96 eV)1. By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land-water connections in regulating CH4 supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land.

Northern forest winters have lost cold, snowy conditions that are important for ecosystems and human communities.

Winter is an understudied but key period for the socioecological systems of northeastern North American forests. A growing awareness of the importance of the winter season to forest ecosystems and surrounding communities has inspired several decades of research, both across the northern forest and at other mid- and high-latitude ecosystems around the globe. Despite these efforts, we lack a synthetic understanding of how winter climate change may impact hydrological and biogeochemical processes and the social and economic activities they support. Here, we take advantage of 100 years of meteorological observations across the northern forest region of the northeastern United States and eastern Canada to develop a suite of indicators that enable a cross-cutting understanding of (1) how winter temperatures and snow cover have been changing and (2) how these shifts may impact both ecosystems and surrounding human communities. We show that cold and snow covered conditions have generally decreased over the past 100 years. These trends suggest positive outcomes for tree health as related to reduced fine root mortality and nutrient loss associated with winter frost but negative outcomes as related to the northward advancement and proliferation of forest insect pests. In addition to effects on vegetation, reductions in cold temperatures and snow cover are likely to have negative impacts on the ecology of the northern forest through impacts on water, soils, and wildlife. The overall loss of coldness and snow cover may also have negative consequences for logging and forest products, vector-borne diseases, and human health, recreation, and tourism, and cultural practices, which together represent important social and economic dimensions for the northern forest region. These findings advance our understanding of how our changing winters may transform the socioecological system of a region that has been defined by the contrasting rhythm of the seasons. Our research also identifies a trajectory of change that informs our expectations for the future as the climate continues to warm.

Nitrous Oxide and Dinitrogen: The Missing Flux in Nitrogen Budgets of Forested Catchments?

Most forest nitrogen budgets are imbalanced, with nitrogen inputs exceeding nitrogen outputs. The denitrification products nitrous oxide (N2O) and dinitrogen (N2) represent often-unmeasured fluxes that may close the gap between explained nitrogen inputs and outputs. Gaseous N2O and N2 effluxes, dissolved N2O flux, and traditionally measured dissolved nitrogen species (i.e., nitrate, ammonium, and dissolved organic nitrogen) were estimated to account for the annual nitrogen output along hillslope gradients from two catchments in a temperate forest. Adding N2O and N2 effluxes to catchment nitrogen output not only reduced the discrepancy between nitrogen inputs and outputs (9.9 kg ha-1 yr-1 and 6.5 or 6.3 kg ha-1 yr-1, respectively), but also between nitrogen outputs from two catchments with different topographies (6.5 kg ha-1 yr-1 for the catchment with a large wetland, 6.3 kg ha-1 yr-1 for the catchment with a very small wetland). Dissolved N2O comprised a very small portion of the annual nitrogen outputs. Nitrogen inputs exceeded nitrogen outputs throughout the year except during spring runoff, and also during autumn storms in the catchment with the large wetland. Failing to account for denitrification products, especially during summer rainfall events, may lead to underestimation of annual nitrogen losses.

High-speed limnology: using advanced sensors to investigate spatial variability in biogeochemistry and hydrology.

Advanced sensor technology is widely used in aquatic monitoring and research. Most applications focus on temporal variability, whereas spatial variability has been challenging to document. We assess the capability of water chemistry sensors embedded in a high-speed water intake system to document spatial variability. This new sensor platform continuously samples surface water at a range of speeds (0 to >45 km h(-1)) resulting in high-density, mesoscale spatial data. These novel observations reveal previously unknown variability in physical, chemical, and biological factors in streams, rivers, and lakes. By combining multiple sensors into one platform, we were able to detect terrestrial-aquatic hydrologic connections in a small dystrophic lake, to infer the role of main-channel vs backwater nutrient processing in a large river and to detect sharp chemical changes across aquatic ecosystem boundaries in a stream/lake complex. Spatial sensor data were verified in our examples by comparing with standard lab-based measurements of selected variables. Spatial fDOM data showed strong correlation with wet chemistry measurements of DOC, and optical NO3 concentrations were highly correlated with lab-based measurements. High-frequency spatial data similar to our examples could be used to further understand aquatic biogeochemical fluxes, ecological patterns, and ecosystem processes, and will both inform and benefit from fixed-site data.

The fate of 17ß-estradiol in snowmelt from a field with a history of manure application: A laboratory simulation and field study.

17ß-estradiol is a naturally occurring estrogen, and livestock manure applied to agricultural fields is a major source to the environment. Liquid swine manure is widely applied to agricultural fields in the Canadian Prairies, a region where the majority of the annual runoff occurs during a brief snowmelt period over frozen soil. Transport of estrogens from manure amendments to soil during this important hydrological period is not well understood but is critical to mitigating the snowmelt-driven offsite transport of estrogens. This study quantified the concentration and load of 17ß-estradiol in snowmelt from an agricultural field with a history of manure application under manure application methods: no manure applied, manure applied on the sub-surface, and on the surface, using a laboratory simulation study with flooded intact soil cores and a field study during snowmelt. A higher concentration of 17ß-estradiol was in the laboratory simulation than in the field (mean laboratory pore water = 1.65 ± 1.2 µg/L; mean laboratory flood water = 0.488 ± 0.58 µg/L; and mean field snowmelt = 0.0619 ± 0.048 µg/L). There were no significant differences among manure application methods for 17ß-estradiol concentration. Laboratory pore water concentrations significantly increased over time, corresponding with changes in pH. In contrast, there was no significant change in the field snowmelt concentrations of 17ß-estradiol over time. However, for both laboratory simulation experiments and field-based snowmelt experiments, mean concentrations of 17ß-estradiol were higher with subsurface than surface-applied manure, and the cumulative load of 17ß-estradiol was significantly higher in the sub-surface than in surface applied. The mean cumulative load from the field study across all treatments (6.91 ± 3.7 ng/m2) approximates the magnitude of 17ß-estradiol that could be mobilized from manured fields. The sub-surface application of manure seems to increase the persistence of 17ß-estradiol in soil, thus enhancing the potential loss to snowmelt runoff.

Release of phosphorus and metal(loid)s from manured soils to floodwater during a laboratory simulation of snowmelt flooding.

Phosphorus (P) and metal accumulation in manured agricultural soils and subsequent losses to waterways have been extensively studied; however, the magnitudes and the factors governing their losses during spring snowmelt flooding are less known. We examined the P and metal release from long-term manured soil to floodwater under simulated snowmelt flooding with recent manure additions. Intact soil columns collected from field plots located in Randolph, Southern Manitoba, 2 weeks after liquid swine manure treatments (surface-applied, injected, or control with no recent manure addition) were flooded and incubated for 8 weeks at 4 ± 1°C to simulate snowmelt conditions. Floodwater (syringe filtered through 0.45 µm) and soil porewater (extracted using Rhizon-Mom samplers) samples were periodically extracted and analyzed for dissolved reactive phosphorus (DRP), pH, zinc (Zn), manganese (Mn), iron (Fe), magnesium (Mg), calcium (Ca), and arsenic (As). Mean floodwater DRP concentrations (mg L-1) for manure injected (2.0 ± 0.26), surface-applied (2.6 ± 0.26), and control (2.2 ± 0.26) treatments did not differ significantly. Despite manure application, DRP loss to floodwater did not significantly increase compared to the control, possibly due to the elevated residual soil P at this site from the long-term manure use. At the end of simulated flooding, the DRP concentrations increased by 1.5-fold and 5-fold in porewater and floodwater, respectively. Metal(loid) concentrations were not affected by manure treatments in general, except for Zn and Mg on certain days. Unlike DRP, where porewater and floodwater concentrations increased with time, metalloid concentration in porewater and floodwater did not show consistent trends with flooding time.

Nitrogen dynamics and nitrogen-to-phosphorus stoichiometry in cold region agricultural streams.

Cold agricultural regions are getting warmer and experiencing shifts in precipitation patterns, which affect hydrological transport of nutrients through reduced snowpack and higher annual proportions of summer rainfall. Previous work has demonstrated that the timing of phosphorus (P) concentrations is regionally coherent in streams of the northern Great Plains, suggesting a common climatic driver. There has been less investigation into patterns of stream nitrogen (N), despite its importance for water quality. Using high-frequency water quality data collected over 6 yr from three southern Manitoba agricultural streams, the goal of this research was to investigate seasonal patterns in N and P concentrations and the resultant impacts of these patterns on N/P stoichiometry. In the spring, high concentrations of inorganic N were associated with snowmelt runoff, while summer N was dominated by organic forms; inorganic N concentrations remained consistently low in the summer, suggesting increased biological N transformation and N removal. Relationships between N concentration and discharge showed generally weak model fits (r2 values for significant relationships ranging from .33 to .48), and the strength and direction of model fits differed among streams, seasons, and forms of N. Dissolved organic N concentrations were strongly associated with dissolved organic carbon. Nitrogen-to-phosphorus ratios varied among streams but were significantly lower during summer storm events (p < .0001). These results suggest that climate-driven shifts in temperature and precipitation may negatively affect downstream water quality in this region.

Hydrological and Seasonal Controls of Phosphorus in Northern Great Plains Agricultural Streams.

Controls on nutrient transport in cold, low-relief agricultural regions vary dramatically among seasons. The spring snowmelt is often the dominant runoff and nutrient loading event of the year. However, climate change may increase the proportion of runoff occurring with rainfall, and there is an urgent need to understand seasonal controls on nutrient transport to understand how patterns may change in the future. In this study, we assess patterns and drivers of total P (TP) dynamics in eight streams draining agriculturally dominated watersheds, located in southern Manitoba, Canada. Data from three years of monitoring revealed highly coherent patterns of TP concentrations in streams, with pronounced peaks in the spring and midsummer across the region. This coherent pattern was in spite of considerable interannual variability in the magnitude and timing of discharge; in particular, a major storm event occurred in summer 2014, which resulted in more discharge than the preceding spring melt. Concentration-discharge model fits were generally poor or not significant, suggesting that runoff generation is not the primary driver of TP dynamics in the majority of streams. Seasonal patterns of conductivity and stream temperature suggest that mechanisms controlling TP vary by season; a spring TP concentration maximum may be related to surface runoff over frozen soils, whereas the summer TP maximum may be related to temperature-driven biogeochemical processes, which are not well represented in current conceptual or predictive models. These findings suggest that controls on stream TP concentrations are dynamic through the year, and responses to increases in dormant and nondormant season temperatures may depend on seasonally variable processes.

Landscape Controls on Nutrient Export during Snowmelt and an Extreme Rainfall Runoff Event in Northern Agricultural Watersheds.

In the northern Great Plains, most runoff transport of N, and P to surface waters has historically occurred with snowmelt. In recent years, significant rainfall runoff events have become more frequent and intense in the region. Here, we examine the influence of landscape characteristics on hydrology and nutrient export in nine tributary watersheds of the Assiniboine River in Manitoba, Canada, during snowmelt runoff and with an early summer extreme rainfall runoff event (ERRE). All watersheds included in the study have land use that is primarily agricultural, but with differing proportions of land remaining as wetlands, grassland, and that has been artificially drained. Those watersheds with greater capacity for storage of water in surface depressions (noneffective contributing areas) exhibited lower rates of runoff and nutrient export with snowmelt. During the ERRE, higher export of total P (TP), but not total N, was observed from those watersheds with larger amounts of contributing area that had been added through artificial surface drainage, and this was associated primarily with higher TP concentrations. Increasing or restoring the storage of water on the landscape is likely to reduce nutrient export; however, the importance of antecedent conditions was evident during the ERRE, when small surface depressions were at or near capacity from snowmelt. Total P concentrations observed during the summer ERRE were as high as those observed with snowmelt, and N/P ratios were significantly lower. If the frequency of summer ERREs increases with climate change, this is likely to result in negative water quality outcomes.