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Widespread deoxygenation in warming rivers

Abstract

Deoxygenation is commonly observed in oceans and lakes but less expected in shallower, flowing rivers. Here we reconstructed daily water temperature and dissolved oxygen in 580 rivers across the United States and 216 rivers in Central Europe by training a deep learning model using temporal weather and water quality data and static watershed attributes (for example, hydro-climate, topography, land use, soil). Results revealed persistent warming in 87% and deoxygenation in 70% of the rivers. Urban rivers demonstrated the most rapid warming, whereas agricultural rivers experienced the slowest warming but fastest deoxygenation. Mean deoxygenation rates (−0.038 ± 0.026 mg l−1 decade−1) were higher than those in oceans but lower than those in temperate lakes. These rates, however, may be underestimated, as training data are from grab samples collected during the day when photosynthesis peaks. Projected future rates are between 1.6 and 2.5 times higher than historical rates, indicating significant ramifications for water quality and aquatic ecosystems.

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Fig. 1: DO patterns and dynamics.
Fig. 2: Historical trends of WT, DOsat (mg l−1), DO (mg l−1) and DO%sat.
Fig. 3: Deoxygenation rates versus warming rates in US and CE rivers.
Fig. 4: Projected trends and spatial patterns of warming and deoxygenation under SSP2–4.5 and SSP5–8.5 scenarios.
Fig. 5: Projected trends and decadal increases in annual and seasonal days of stress and hypoxia conditions under SSP2–4.5 and SSP5–8.5 scenarios.
Fig. 6: Rates of warming and deoxygenation in different waters.

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Data availability

Discharge and water quality data in the United States were downloaded from the USGS National Water Information System (NWIS) at https://waterdata.usgs.gov/nwis. The historical meteorological datasets in the United States are available from the NLDAS-2 (https://ldas.gsfc.nasa.gov/nldas/v2/forcing) and DAYMET (https://daymet.ornl.gov). Basin characteristics in the United States are from GAGES-II archived at https://water.usgs.gov/GIS/metadata/usgswrd/XML/gagesII_Sept2011.xml. The LamaH-CE paper and dataset including meteorological forcing, discharge and basin attributes is available at https://doi.org/10.5194/essd-13-4529-2021. Due to limits in sharing raw water quality data from providers in the CE region, we recommend accessing data directly from their websites: Water quality data for Austria were obtained from the Federal Ministry of Agriculture, Regions and Tourism at https://wasser.umweltbundesamt.at/h2odb/fivestep/abfrageQdPublic.xhtml. Water quality data in Switzerland were obtained from the Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Federal Office for the Environment (FOEN) at https://doi.org/10.25678/0004AV. Water quality data for Germany were obtained from the State Agency for the Environment Baden-Württemberg at https://udo.lubw.baden-wuerttemberg.de/public/index.xhtml, and the Bavarian State Office for the Environment at https://www.gkd.bayern.de/en/rivers/chemistry. Supporting data are deposited at https://github.com/LiReactiveWater/WT-DO-US-CE-dataset. The projected downscaled forcing data from the NEX-GDDP-CMIP6 database19 can be found at https://doi.org/10.7917/OFSG3345.

Code availability

The deep learning code and instruction are available on GitHub at https://github.com/LiReactiveWater/WT-DO-US-CE-LSTM. The ‘streamMetabolizer’ R package for calculating DO saturation concentration is available on GitHub at https://github.com/USGS-R/streamMetabolizer. The ‘dataRetrieval’ R package for downloading discharge and water quality data for the United States is available on GitHub https://github.com/USGS-R/dataRetrieval. The ‘bestNormalize’ R package for transforming model inputs is available on GitHub at https://github.com/cran/bestNormalize.

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Acknowledgements

This study was supported by the Barry and Shirley Isett professorship endowment to L.L. and a seed grant from the Institute of Computation and Data Science at Penn State University to L.L.

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Authors and Affiliations

Authors

Contributions

W.Z. conceived the idea and compiled data for 580 US rivers. C.K. acquired and prepared data from Central Europe. J.L. downloaded and processed the daily downscaled forcing data from the NEX-GDDP-CMIP6. W.Z. trained the deep learning model. W.Z. wrote the first draft of the paper together with L.L. W.Z. and L.L. iterated and edited multiple versions to shape the ideas, figures and main message of the paper. C.K. edited the paper. L.L. finalized the paper.

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Correspondence to Li Li.

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Nature Climate Change thanks Emily Bernhardt, Guillaume Durand, Danlu Guo, Michael Hutchins and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Monthly dynamics and variations of WT and DO in US and CE rivers.

Lines and dots are the model and measured data of WT (a) and DO (b), respectively, for the means of the temporally (that is, 1981–2019) and spatially averaged monthly values from all US and CE rivers. Shade areas and error bars are mean ± standard deviation to indicate monthly variability across US (n = 580) and CE (n = 216) rivers.

Extended Data Fig. 2 Model performance of DO and WT in the testing period.

Lower magnitude values closer to 0 in the percent bias (Pbias, a) and root mean square error (RMSE, b) indicate more accurate model results. Values closer to 1 in the Pearson’s correlation coefficient (Pcorr, c) indicates positive correlations and better capture of seasonality. Boxes show the median values (middle line) and the interquartile range (IQR), which is the range between the first quartile (Q1) and third quartile (Q3). The lower and upper whiskers extend to Q1 − 1.5 × IQR and Q3 + 1.5 × IQR, respectively.

Extended Data Fig. 3 Model NSE performance comparison against Gaussian Process Regression (GPR) model.

Three GPR model scenarios (that is, GPR-r200, GPR-r100, GPR-ind) were compared to the long-short term memory (LSTM) for DO (a, c) and WT (b, d) performance. In the top panel of Empirical Cumulative Distribution Function (ECDF), the lower LSTM curve tending toward the right side (NSE = 1.0) indicates better model performance for both DO and WT. The bottom panel similarly shows higher LSTM performances. The boxes show the median values (middle line) and the interquartile range (IQR), which is the range between the first quartile (Q1) and third quartile (Q3). The lower and upper whiskers extend to Q1 − 1.5 × IQR and Q3 + 1.5 × IQR, respectively. The three GRP scenarios refer to two regional models trained with 200 and 100 basins (that is, GPR-r200 and GPR-r100) and one individual model (that is, GPR-ind) trained for each individual basin (Methods).

Extended Data Fig. 4 Historical warming trends in air temperature over 1981 to 2019.

The top, middle, and bottom panels are daily average temperature (Tavg), daily maximum temperature (Tmax), and daily minimum temperature (Tmin), respectively. The spatial patterns of WT warming rates (Fig. 2a) are similar to Tmin warming rates. The side boxes show the median values (middle line) and the interquartile range (IQR), which is the range between the first quartile (Q1) and third quartile (Q3). The lower and upper whiskers extend to Q1 − 1.5 × IQR and Q3 + 1.5 × IQR, respectively. A previous study showed that water temperature no longer increases linearly with the increase in air temperature when it rises above 25 °C as heat is increasingly lost as evaporative cooling increase71. It is therefore possible that water temperature has a closer relationship with Tmin as it tends not to increase above 25 °C as Tmax and Tavg.

Extended Data Fig. 5 Basin attributes of US and CE rivers.

The attributes include mean elevation (top row), relative humidity (2nd row), stream order (3rd row), drainage area (4th row), and dominant land use (last row). Stream order is the modified Strahler order that accounts for flow splits55. The side boxes show the median values (middle line) and the interquartile range (IQR), which is the range between the first quartile (Q1) and third quartile (Q3). The lower and upper whiskers extend to Q1 − 1.5 × IQR and Q3 + 1.5 × IQR, respectively.

Extended Data Fig. 6 Correlations between changing rates and basin attributes.

The correlations include warming (a, b) and deoxygenation (c, d) rates with basin area (left column) and basin slope (right column).

Extended Data Fig. 7 Projected temperature and precipitation variables under SSP2-4.5 and SSP5-8.5 scenarios.

Variables include daily average (a), maximum (b), and minimum (c) air temperature and precipitation (d). The historical period covers 1981 to 2019. The solid line as the average of all 796 basins. The projection period spans from 2020 to 2100 with color lines from 10 CMIP6 models. The solid line and black shading represent the mean and two standard deviations of the 10 CMIP6 models, respectively. Temperature (a-c) shows consistent warming trends while the projected precipitation (d) generally exhibits a stable but decreased trends compared to historical period.

Extended Data Fig. 8 Model inputs for historical prediction and future projection.

Two types of inputs are required to run the model for DO and WT, that is, time-series of daily hydro-meteorological forcing and constant basin attributes. In the historical prediction, meteorological forcing data are from the NLDAS-2, DAYMET, and LamaH-CE (see Methods). Basin attributes are from the GAGES-II and LamaH-CE. In the future projection, historical variables of temperature (Tavg, Tmax, Tmin) and precipitation were replaced (red box) with these projected variables from the NEX-GDDP-CMIP6 dataset while others that are not available remained the same as in the historical periods (blue box). With these new projected temperature and precipitation variables, their long-term means were updated as new basin attributes (red box) in the future scenarios.

Extended Data Fig. 9 Modeled seasonal trends of warming and deoxygenation under SSP2-4.5 and SSP5-8.5 scenarios.

The top and bottom panels show warming (a, b) and deoxygenation (c, d), respectively. The historical period covers 1981 to 2019 with the solid line as the average of all 796 basins. The projection period spans from 2020 to 2100 with thin color lines from the 10 CMIP6 models and bold black lines as the model mean ± 2 std. In each panel, the long-dash, dotted, dashed, and dot-dash lines represent the seasons of summer, autumn, spring, and winter, respectively.

Extended Data Fig. 10 Projected changing rates of stress and hypoxia days under SSP2-4.5 and SSP5-8.5 scenarios.

Stress (red) and hypoxia (blue) days were counted with daily DO concentration less than 5.0 and 3.0 mg/L, respectively. In the top panel (a), the long-dash, dotted, dashed, and dot-dash lines represent the seasons of summer, autumn, spring, and winter, respectively. In the maps, annual day rate (b) and seasonal day rates (cf) are the average of 10 CMIP6 models. The side boxes show the median values (middle line) and the interquartile range (IQR), which is the range between the first quartile (Q1) and third quartile (Q3). The lower and upper whiskers extend to Q1 − 1.5 × IQR and Q3 + 1.5 × IQR, respectively. Note a river can exhibit both stress and hypoxia conditions. CE is not included because all rivers have zero stress and hypoxia days in both scenarios.

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Zhi, W., Klingler, C., Liu, J. et al. Widespread deoxygenation in warming rivers. Nat. Clim. Chang. 13, 1105–1113 (2023). https://doi.org/10.1038/s41558-023-01793-3

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