Application of stable isotopes in freshwater nutrient management: opportunities and challenges

Nutrient pollution of freshwaters remains a pervasive global challenge, with a substantial proportion of monitored water bodies continuing to fail quality standards despite sustained regulatory investment. Nutrient concentration monitoring data, whilst standardised and threshold-interpretable, offer limited insight into nutrient provenance, transformation, and legacy accumulation within complex catchment systems. Additional geochemical methods could address these gaps and enable more reliable, diagnostic assessments of water quality. This policy and practice review synthesises the published evidence base for stable isotope analysis of dissolved nitrogen and phosphorus compounds in English freshwaters, focusing on nitrate (δ 15 N-NO 3 and δ 18 O-NO 3 ), ammonium (δ 15 N-NH 4 ), and phosphate-oxygen isotopes (δ 18 O-PO 4 ). Here, nitrogen and phosphorus isotope systems are evaluated in combination, reflecting their co-driving role in eutrophication and the diagnostic advantages of multi-isotope approaches. Stable isotope approaches can provide empirical diagnostic information across four policy-relevant domains, including: supporting nutrient source apportionment for constraining catchment models, identifying internal loading and legacy nutrient sources, assessing nutrient attenuation zones, and evaluating the effectiveness of management actions. Multi-isotope frameworks can often reduce ambiguity in source identification where the selected tracers provide independent and complementary information on sources or processes. Stable isotope methods are currently absent from regulatory and management frameworks examined here; viable integration options are identified and assessed, from near-term deployment as supplementary investigative tools in persistently failing water bodies, through development of shared protocols and guidance, to longer-term contribution to management and investment frameworks. Current legislative reform provides a relevant, if uncertain, window for incorporating these methods more systematically within routine environmental governance.