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1. Long‐Term Changes in Concentration and Yield of Riverine Dissolved Silicon From the Poles to the Tropics

   https://doi.org/10.1029/2022GB007678  

   Jankowski, Kathi Jo; Johnson, Keira; Sethna, Lienne; Julian, Paul; Wymore, Adam S.; Shogren, Arial J.; Thomas, Patrick K.; Sullivan, Pamela L.; McKnight, Diane M.; McDowell, William H.; et al (September 2023, Global Biogeochemical Cycles)

   Abstract Riverine exports of silicon (Si) influence global carbon cycling through the growth of marine diatoms, which account for ∼25% of global primary production. Climate change will likely alter river Si exports in biome‐specific ways due to interacting shifts in chemical weathering rates, hydrologic connectivity, and metabolic processes in aquatic and terrestrial systems. Nonetheless, factors driving long‐term changes in Si exports remain unexplored at local, regional, and global scales. We evaluated how concentrations and yields of dissolved Si (DSi) changed over the last several decades of rapid climate warming using long‐term data sets from 60 rivers and streams spanning the globe (e.g., Antarctic, tropical, temperate, boreal, alpine, Arctic systems). We show that widespread changes in river DSi concentration and yield have occurred, with the most substantial shifts occurring in alpine and polar regions. The magnitude and direction of trends varied within and among biomes, were most strongly associated with differences in land cover, and were often independent of changes in river discharge. These findings indicate that there are likely diverse mechanisms driving change in river Si biogeochemistry that span the land‐water interface, which may include glacial melt, changes in terrestrial vegetation, and river productivity. Finally, trends were often stronger in months outside of the growing season, particularly in temperate and boreal systems, demonstrating a potentially important role of shifting seasonality for the flux of Si from rivers. Our results have implications for the timing and magnitude of silica processing in rivers and its delivery to global oceans. 

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2. Aquatic biomass is a major source to particulate organic matter export in large Arctic rivers

   https://doi.org/10.1073/pnas.2209883120  

   Behnke, Megan I.; Tank, Suzanne E.; McClelland, James W.; Holmes, Robert M.; Haghipour, Negar; Eglinton, Timothy I.; Raymond, Peter A.; Suslova, Anya; Zhulidov, Alexander V.; Gurtovaya, Tatiana; et al (March 2023, Proceedings of the National Academy of Sciences)

   Arctic rivers provide an integrated signature of the changing landscape and transmit signals of change to the ocean. Here, we use a decade of particulate organic matter (POM) compositional data to deconvolute multiple allochthonous and autochthonous pan-Arctic and watershed-specific sources. Constraints from carbon-to-nitrogen ratios (C:N), δ 13 C, and Δ 14 C signatures reveal a large, hitherto overlooked contribution from aquatic biomass. Separation in Δ 14 C age is enhanced by splitting soil sources into shallow and deep pools (mean ± SD: −228 ± 211 vs. −492 ± 173‰) rather than traditional active layer and permafrost pools (−300 ± 236 vs. −441 ± 215‰) that do not represent permafrost-free Arctic regions. We estimate that 39 to 60% (5 to 95% credible interval) of the annual pan-Arctic POM flux (averaging 4,391 Gg/y particulate organic carbon from 2012 to 2019) comes from aquatic biomass. The remainder is sourced from yedoma, deep soils, shallow soils, petrogenic inputs, and fresh terrestrial production. Climate change-induced warming and increasing CO 2 concentrations may enhance both soil destabilization and Arctic river aquatic biomass production, increasing fluxes of POM to the ocean. Younger, autochthonous, and older soil-derived POM likely have different destinies (preferential microbial uptake and processing vs. significant sediment burial, respectively). A small (~7%) increase in aquatic biomass POM flux with warming would be equivalent to a ~30% increase in deep soil POM flux. There is a clear need to better quantify how the balance of endmember fluxes may shift with different ramifications for different endmembers and how this will impact the Arctic system. 

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3. Shifted sediment-transport regimes by climate change and amplified hydrological variability in cryosphere-fed rivers

   https://doi.org/10.1126/sciadv.adi5019  

   Zhang, Ting; Li, Dongfeng; East, Amy E; Kettner, Albert J; Best, Jim; Ni, Jinren; Lu, Xixi (November 2023, Science Advances)

   Climate change affects cryosphere-fed rivers and alters seasonal sediment dynamics, affecting cyclical fluvial material supply and year-round water-food-energy provisions to downstream communities. Here, we demonstrate seasonal sediment-transport regime shifts from the 1960s to 2000s in four cryosphere-fed rivers characterized by glacial, nival, pluvial, and mixed regimes, respectively. Spring sees a shift toward pluvial-dominated sediment transport due to less snowmelt and more erosive rainfall. Summer is characterized by intensified glacier meltwater pulses and pluvial events that exceptionally increase sediment fluxes. Our study highlights that the increases in hydroclimatic extremes and cryosphere degradation lead to amplified variability in fluvial fluxes and higher summer sediment peaks, which can threaten downstream river infrastructure safety and ecosystems and worsen glacial/pluvial floods. We further offer a monthly-scale sediment-availability-transport model that can reproduce such regime shifts and thus help facilitate sustainable reservoir operation and river management in wider cryospheric regions under future climate and hydrological change. 

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4. Improving Predictions of Stream CO ₂ Concentrations and Fluxes Using a Stream Network Model: A Case Study in the East River Watershed, CO, USA

   https://doi.org/10.1029/2021GB006972  

   Saccardi, Brian; Winnick, Matthew (December 2021, Global Biogeochemical Cycles)

   Abstract Inland waters are an important component of the global carbon budget. However, our ability to predict carbon fluxes from stream systems remains uncertain, aspCO₂varies within streams at scales of 1–100 m. This makes direct monitoring of large‐scale CO₂fluxes impractical. We incorporate CO₂input and output fluxes into a stream network advection‐reaction model, representing the first process‐based representation of stream CO₂dynamics at watershed scales. This model includes groundwater (GW) CO₂inputs, water column (WC), benthic hyporheic zone (BHZ) respiration, downstream advection, and atmospheric exchange. We evaluate this model against existing statistical methods including upscaling and multiple linear regressions through comparisons to high‐resolution streampCO₂data collected across the East River Watershed in the Colorado Rocky Mountains (USA). The stream network model accurately captures GW, evasion, and respiration‐drivenpCO₂variability and significantly outperforms multiple linear regressions for predictingpCO₂. Further, the model provides estimates of CO₂contributions from internal versus external sources suggesting that streams transition from GW‐ to BHZ‐dominated sources between 3rd and 4th Strahler orders, with GW, BHZ, and WC accounting for 49.3%, 50.6%, and 0.1% of CO₂fluxes from the watershed, respectively. Lastly, stream network model atmospheric CO₂fluxes are 4‐12x times smaller than upscaling technique predictions, largely due to relationships between streampCO₂and gas exchange velocities. Taken together, this stream network model improves our ability to predict stream CO₂dynamics and efflux. Furthermore, future applications to regional and global scales may result in a significant downward revision of global flux estimates. 

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5. Light and flow regimes regulate the metabolism of rivers

   https://doi.org/10.1073/pnas.2121976119  

   Bernhardt, Emily S.; Savoy, Phil; Vlah, Michael J.; Appling, Alison P.; Koenig, Lauren E.; Hall, Robert O.; Arroita, Maite; Blaszczak, Joanna R.; Carter, Alice M.; Cohen, Matt; et al (February 2022, Proceedings of the National Academy of Sciences)

   Mean annual temperature and mean annual precipitation drive much of the variation in productivity across Earth's terrestrial ecosystems but do not explain variation in gross primary productivity (GPP) or ecosystem respiration (ER) in flowing waters. We document substantial variation in the magnitude and seasonality of GPP and ER across 222 US rivers. In contrast to their terrestrial counterparts, most river ecosystems respire far more carbon than they fix and have less pronounced and consistent seasonality in their metabolic rates. We find that variation in annual solar energy inputs and stability of flows are the primary drivers of GPP and ER across rivers. A classification schema based on these drivers advances river science and informs management. 

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