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Widespread increases in soluble phosphorus concentrations in streams across the transboundary Great Lakes Basin

Nature Geoscience volume 16, pages 893–900 (2023)Cite this article

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Abstract

Excess phosphorus from agricultural intensification has contributed to the eutrophication of rivers and lakes worldwide, including the transboundary Laurentian Great Lakes Basin. Algal blooms have surged in the past decade, threatening ecosystems, drinking water supplies and lake-dependent tourism economies in both large lakes (for example, Lake Erie) and smaller water bodies. Whereas previous research has focused mainly on phosphorus loads to Lake Erie, a comprehensive analysis of phosphorus species across the basin is lacking. Here we analyse changes in soluble reactive phosphorus and total phosphorus concentrations in over 370 watersheds across the Great Lakes Basin from 2003 to 2019. We find widespread increases in soluble phosphorus concentrations (83% of watersheds, with 46% showing significant increase), while total phosphorus concentrations are decreasing or non-significant. Utilizing random forest models, we identify small, forested watersheds at higher latitudes as the areas experiencing the largest relative increases in soluble phosphorus concentrations. Furthermore, we find winter temperatures to be a key driver of winter concentration trends. We propose that the increasing soluble phosphorus concentrations across the basin, along with warming temperatures, might be contributing to the increasing frequency and intensity of algal blooms, emphasizing the need for management strategies to prevent further water-quality degradation.

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Fig. 1: Phosphorus concentrations in streams across the Great Lakes Basin.
Fig. 2: Temporal trends in soluble reactive phosphorus and total phosphorus concentrations across the Great Lakes Basin.
Fig. 3: Trajectories of soluble reactive phosphorus and total phosphorus concentrations for selected watersheds.
Fig. 4: SRP:TP ratios across the Great Lakes Basin.
Fig. 5: Random forest models describing trends in TP and SRP concentrations as a function of watershed attributes.

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Data availability

Phosphorus (total and orthophosphate-P) concentrations can be obtained from https://data.ontario.ca/dataset/provincial-stream-water-quality-monitoring-network (Canada) and https://waterdata.usgs.gov/nwis (United States). Flow datasets are available from https://wateroffice.ec.gc.ca/mainmenu/historical_data_index_e.html (Canada) and https://waterdata.usgs.gov/nwis/dv/?referred_module=sw (United States). Monthly climate (precipitation totals, minimum and maximum air temperatures) datasets for Canada and the United States are available through https://doi.org/10.3334/ORNLDAAC/2131. Land-use land-cover maps are available from https://open.canada.ca/data/en/dataset/3688e7d9-7520-42bd-a3eb-8854b685fef3 (Canada) and https://www.sciencebase.gov/catalog/item/5d4c6a1de4b01d82ce8dfd2f (United States). Tillage datasets are available from https://www150.statcan.gc.ca/n1/en/type/data?MM=1 (Canada) and https://www.ctic.org/CRM (United States). Tile-drainage maps are available from https://geohub.lio.gov.on.ca/datasets/lio::tile-drainage-area/about (Canada) and https://figshare.com/articles/dataset/AgTile-US/11825742 (United States). Soils texture datasets are available from https://www.fao.org/soils-portal/data-hub/soil-maps-and-databases/harmonized-world-soil-database-v12/en/ (Canada) and https://water.usgs.gov/GIS/metadata/usgswrd/XML/ds866_ssurgo_variables.xml#stdorder (United States). Population data can be downloaded from https://www12.statcan.gc.ca/census-recensement/2011/dp-pd/index-eng.cfm (Canada) and https://www.census.gov/programs-surveys/decennial-census/decade.2010.html (United States). Great Lakes Basin map can be obtained from https://www.glc.org/greatlakesgis.

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Acknowledgements

The research published in this paper was supported by the ‘Lake Futures’ project under the Global Water Futures programme, funded by the Canada First Research Excellence Fund. We thank B. Tolson and M. Han at University of Waterloo for access to and advice regarding data published in https://lake-river-routing-products-uwaterloo.hub.arcgis.com/.

Author information

Authors and Affiliations

  1. Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario, Canada

    Nitin K. Singh & Nandita B. Basu

  2. Department of Geography, Pennsylvania State University, University Park, PA, USA

    Kimberly J. Van Meter

  3. Civil and Environmental Engineering, University of Waterloo, Waterloo, Ontario, Canada

    Nandita B. Basu

  4. Water Institute, University of Waterloo, Waterloo, Ontario, Canada

    Nandita B. Basu

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  1. Nitin K. Singh
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Contributions

N.K.S., N.B.B. and K.J.V.M. conceptualized the project. N.K.S. conducted the analyses. N.K.S. and N.B.B. analysed the figures and results. N.K.S., N.B.B. and K.J.V.M. wrote the manuscript. N.B.B. supervised and acquired funding.

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Correspondence to Nandita B. Basu.

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Nature Geoscience thanks Robert Mooney and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Xujia Jiang, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Seasonal total phosphorus (a-d) and soluble reactive phosphorus (e-h) concentrations over the last five years across the Great Lakes Basin.

Winter (a, e), Spring (b, f), Summer (c, g), Fall (d, h). Violin plots show the distributions of seasonal concentrations for (a-d) TP and (e-h) SRP across the six major subbasins (see Supplementary Fig. 1 for basin locations). Number of streams in each subbasin is denoted by n. Within the violins, white-filled circles show the median value, thick lines the interquartile range, and whiskers extend to a maximum of 1.5 times the interquartile range. The dashed lines indicate the eutrophic threshold of 76 μg l−1 for TP (a-d) and 50 μg l−1 for SRP (e-h).

Extended Data Fig. 2 The distribution of land use across the Great Lakes Basin.

We categorized the land use and land cover maps (Canada and the United States; Supplementary Table 3) into four classes21: Urban [Urban >12.5% & Crop <25% & (Crop + Pasture) < 30% & Undeveloped <70%], Forested/Undeveloped [(Forest+BarrenLand+Shrubland+Wetland+OpenWater+Grassland)>70%], Agricultural (Crop>12.5%) & (Crop +Pasture>40%) & (Undeveloped <70%), and Mixed [watersheds that do not fall into first three classes].Mixed [watersheds that do not fall into first three classes].

Extended Data Fig. 3 The distribution of watershed typologies across the Great Lakes Basin.

Quadrant 1 indicates watersheds where both SRP and TP are increasing, Quadrant 2 indicates watersheds where SRP is increasing and TP is decreasing, Quadrant 3 indicates watersheds where both SRP and TP are decreasing, and Quadrant 4 indicates watersheds where SRP is decreasing and TP is increasing. Typologies are for all watersheds with significant (p < 0.05) or non-significant (p > 0.05) trends in concentrations.

Extended Data Fig. 4 The distribution of (a) population density and (b) forest coverage across the three typologies of trends.

Note that we focused only on watersheds with significant trends, and thus Quadrant 4 is not presented. Violin plots show the distributions of observations with significant trends (p < 0.05), white-filled circles show the medians, thick lines the interquartile range, and whiskers extend to a maximum of 1.5 times the interquartile range. Sample sizes for (a) and (b): Quadrant 1 (n = 14), Quadrant 2 (n = 23) and Quadrant 3 (n = 17).

Extended Data Fig. 5 Distribution of watersheds with forest area and mean topographic slope along the latitudinal gradient.

Larger size of circles indicates greater forest cover. Overall, forest cover is greater at higher latitudes in our study watersheds.

Extended Data Fig. 6 Variable importance plots for seasonal concentrations of total phosphorus and soluble reactive phosphorus.

The ranked importance of key predictors for (a–d) total phosphorus and (e–h) soluble reactive phosphorus seasonally across 100 (n) model runs is represented by box and whisker plots, which show the median values (horizontal line in each box), the interquartile range (width of the box), and the whiskers that extend to a maximum of 1.5 times the interquartile range. Adjacent boxes of the same color indicate that there is no significant difference in importance between those predictors (p < 0.05) (Supplementary Data).

Extended Data Table 1 The proportion of watersheds (%) showing increasing and decreasing trends in annual and seasonal concentrations
Full size table
Extended Data Table 2 Median changes (% per decade) of annual and seasonal concentrations
Full size table
Extended Data Table 3 Correlations (spearman coefficients and p values*) between changes in seasonal and annual TP concentrations and watershed attributes
Full size table
Extended Data Table 4 Correlations (spearman coefficients and p values*) between seasonal and annual changes in SRP concentrations and watershed attributes
Full size table

Supplementary information

Supplementary Information

Supplementary Tables 1–3, Figs. 1–3 and References for Supplementary Tables.

Supplementary Data

Anova p values for RF models of SRP and TP concentrations.

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Singh, N.K., Van Meter, K.J. & Basu, N.B. Widespread increases in soluble phosphorus concentrations in streams across the transboundary Great Lakes Basin. Nat. Geosci. 16, 893–900 (2023). https://doi.org/10.1038/s41561-023-01257-5

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