Limited progress in nutrient pollution in the U.S. caused by spatially persistent nutrient sources

Abstract

Human agriculture, wastewater, and use of fossil fuels have saturated ecosystems with nitrogen and phosphorus, threatening biodiversity and human water security at a global scale. Despite efforts to reduce nutrient pollution, carbon and nutrient concentrations have increased or remained high in many regions. Here, we applied a new ecohydrological framework to ~12,000 water samples collected by the U.S. Environmental Protection Agency from streams and lakes across the contiguous U.S. to identify spatial and temporal patterns in nutrient concentrations and leverage (an indicator of flux). For the contiguous U.S. and within ecoregions, we quantified trends for sites sampled repeatedly from 2000 to 2019, the persistence of spatial patterns over that period, and the patch size of nutrient sources and sinks. While we observed various temporal trends across ecoregions, the spatial patterns of nutrient and carbon concentrations in streams were persistent across and within ecoregions, potentially because of historical nutrient legacies, consistent nutrient sources, and inherent differences in nutrient removal capacity for various ecosystems. Watersheds showed strong critical source area dynamics in that 2–8% of the land area accounted for 75% of the estimated flux. Variability in nutrient contribution was greatest in catchments smaller than 250 km2 for most parameters. An ensemble of four machine learning models confirmed previously observed relationships between nutrient concentrations and a combination of land use and land cover, demonstrating how human activity and inherent nutrient removal capacity interactively determine nutrient balance. These findings suggest that targeted nutrient interventions in a small portion of the landscape could substantially improve water quality at continental scales. We recommend a dual approach of first prioritizing the reduction of nutrient inputs in catchments that exert disproportionate influence on downstream water chemistry, and second, enhancing nutrient removal capacity by restoring hydrological connectivity both laterally and vertically in stream networks.

Spatial Heterogeneity and Temporal Stability of Baseflow Stream Chemistry in an Urban Watershed

“…Long‐term residence times of subsurface solute masses drive the temporal persistence observed in baseflow chemistry patterns, for example, chloride in the urban critical zone (Mazumder et al., 2021) or nitrogen from agricultural land use (Van Meter et al., 2017). The persistence of stream chemistry patterns has been recognized in recent literature (Frei et al., 2021; Gu et al., 2021) but that persistence has not been directly tied to an array of specific solute sources. Of particular note in the urban critical zone is that the spatial distribution of sources differs among solutes.…”

“…Synoptic stream sampling can be used to characterize spatial solute patterns along a stream length at a snapshot in time, which can complement temporal stream water chemistry measured at fixed locations. Recent work has highlighted that data from synoptic (longitudinal) stream sampling can yield insights into watershed processes and solute sources (Abbott et al, 2018;Dupas et al, 2019;Floriancic et al, 2019;Frei et al, 2021;Gu et al, 2021;Lee-Cullin et al, 2018;McGuire et al, 2014;Pardo et al, 2022;Rhea et al, 2022;Sonne et al, 2017). For example, Pardo et al (2022) used synoptic sampling to demonstrate the connection of nitrification hot spots in upland soils to the stream strongly influenced stormwater nitrate dynamics.…”

Resistance, recovery, and resilience: rethinking the three Rs of survival in the Anthropocene

“…Collectively, complex-systems tools comprise a variety of approaches including machine-learning algorithms, nonparametric statistics, network analysis, Bayesian inference, stochastic models, and evolutionary computation Frei et al 2021). They can be used for classification, regression, and prediction tasks in the analysis of ecological dynamics across scales.…”

Human induced fish declines in North America, how do agricultural pesticides compare to other drivers?

“…A number of states have seen a decline in fertilizer consumption (e.g., Rhode Island, Massachusetts), while others have seen a considerable increase (e.g., South Dakota, Indiana, Kansas) (USDA 2022 ). Not all of the responsibility should not be laid at the feet of agriculture, urban land use can also be a significant contributor to nutrient pollution in freshwater ecosystems (Frei et al 2021 ). As mentioned earlier, agricultural land use in the USA has been steady decreasing since the 1980s and mainly being replaced by urban and residential land use (Freegood et al 2020 ).…”