Global Surface Water Nutrient Reduction Targets

Water-quality nutrient-reduction targets were estimated using the global river nutrient models developed by McDowell et al. (2025). These models were developed from approximately 1.2 million river-water-quality measurements collected between 2005 and 2013 and provide estimates of present-day and reference nutrient concentrations for 14,241 HydroBASINS Level 6 catchments worldwide. The analysis focused on total nitrogen (N) and total phosphorus (P), two nutrients strongly associated with eutrophication and undesirable algal growth in rivers.

The present-day concentration represents the estimated nutrient concentration under current conditions. It includes both the concentration expected under reference conditions and the additional contribution associated with human activities. Reference concentrations represent conditions with minimal anthropogenic influence while retaining the natural characteristics of each catchment. In the reference scenario, human-pressure predictors were modified by setting population density to approximately zero, removing cropland, pasture, and urban land, assigning the available land to forest or rangeland, and setting soil Olsen phosphorus to a low background value.

The human contribution to the present-day nutrient concentration was calculated as:

\[ C_{\mathrm{human}} = C_{\mathrm{current}} - C_{\mathrm{reference}}, \]

where \(C_{\mathrm{current}}\) is the modeled present-day nutrient concentration and \(C_{\mathrm{reference}}\) is the modeled concentration under reference conditions. Consequently, the reduction target applies only to the estimated human contribution, rather than to the total nutrient concentration.

Ecological thresholds associated with undesirable algal growth were established at 0.800 mg/L for total N and 0.046 mg/L for total P. When the present-day concentration exceeded the corresponding threshold, the percentage reduction in the human-derived nutrient contribution required to bring the total concentration to the threshold was calculated as:

\[ R = \frac{C_{\mathrm{current}} - C_{\mathrm{threshold}}} {C_{\mathrm{current}} - C_{\mathrm{reference}}} \times 100, \]

where \(R\) is the required reduction in the human contribution, expressed as a percentage, and \(C_{\mathrm{threshold}}\) is the ecological threshold for N or P. If the present-day concentration was at or below the applicable threshold, no reduction was required.

The modeled molar N:P ratio was used to identify the nutrient most likely to limit algal growth. Catchments with an N:P ratio below 7 were classified as nitrogen-limited, catchments with a ratio from 7 to less than 15 were classified as co-limited, and catchments with a ratio equal to or greater than 15 were classified as phosphorus-limited. The relevant nutrient-reduction target was interpreted in combination with this limitation class. Nitrogen reduction was prioritized in N-limited catchments, phosphorus reduction was prioritized in P-limited catchments, and both nutrient targets were considered in co-limited catchments.

Required reductions between 0% and 100% indicate that the ecological threshold could theoretically be reached by partially or completely eliminating the modeled human contribution. A value greater than 100% indicates that removing the full estimated human contribution would not be sufficient to reach the threshold. This generally occurs when the modeled reference concentration is already greater than the ecological threshold or when the estimated human contribution is very small relative to the amount by which the present-day concentration exceeds the threshold. These catchments should therefore be interpreted separately rather than as locations where reductions exceeding 100% could be implemented.

Results were screened for values greater than 100%, extremely large values such as those exceeding 1,000%, missing values, and infinite values. Extremely large or infinite results can occur when the denominator—the difference between the present-day and reference concentrations—is zero or very close to zero. Such results were flagged as undefined or unreliable rather than interpreted as actionable nutrient-reduction targets.

The resulting indicator represents the modeled percentage reduction in the human-derived nutrient contribution required to meet the applicable ecological concentration threshold. It should be interpreted as a global, catchment-scale screening indicator rather than as a precise local nutrient-load allocation. The results are conditional on the underlying nutrient models, reference-condition assumptions, concentration thresholds, N:P limitation rules, and HydroBASINS Level 6 spatial aggregation. Local monitoring data and watershed-specific analyses should therefore be used to refine management targets before implementation.

1 References

McDowell, R. W., Luo, D., Pletnyakov, P., Upsdell, M., & Dodds, W. K. (2025). Anthropogenic nutrient inputs cause excessive algal growth for nearly half the world’s population. Nature Communications, 16, 1830.