Global trends in river eutrophication and oligotrophication: A systematic review

Trophic status serves as a fundamental water quality indicator, directly affecting biodiversity and ecosystem functioning in aquatic environments. While riverine nutrient pollution has remained a critical global challenge, recent evidence suggests a widespread transition from eutrophic towards oligotrophic states. However, the recovery is spatially heterogenous driven by contrasting anthropogenic pressures and management efficacies. Consequently, the true spatial extent and ecological implications of oligotrophication remained insufficiently explored, a deficit further compounded by inconsistent definitions and indicator parameters.

To address these gaps, we systematically reviewed 1,034 scientific publications (Scopus: 955; Web of Science: 526) and conducted a full-text analysis of 102 studies to address the following questions: (1) How are eutrophication and oligotrophication defined, and what are the most commonly associated variables? (2) What is the global distribution of oligotrophication and eutrophication in river systems? We identified key indicator variables used to analyse long-term trends in trophic status (≥ 10 years), and reporting either eutrophication, no change, or oligotrophication.

The global distribution of long-term nutrient trends exposes a stark regional divergence between mature and emerging economies. Rivers of European countries predominantly demonstrate a declining trajectory (oligotrophication), attributed to successful legislative intervention. France and Germany exhibit significant nitrate reductions in 74% and 91% of long-term observations, respectively, alongside a "resounding success" in phosphorus decrease driven by tertiary wastewater treatment and detergent bans. Conversely, Asian basins display a pronounced upward trajectory (eutrophication); in Japan, 55% of rivers show increasing nitrate levels, meanwhile the Yangtze River (China) exhibits continuous increases in nitrate flux, driven by intensive agriculture and forest loss. Degradation extends to the African continent, where nearly 60% of South African catchments exhibit significant phosphate increases linked predominantly to mining and dysfunctional sewage infrastructure. North America presents a complex, transitional profile characterised by "stalled recovery" in urbanised systems (e.g. the Charles River) and hydrological decoupling of concentration and load (e.g. the Maumee River). Superimposed on anthropogenic impact, climate factors are increasingly modulating long-term trends, with intensified drought driving "chemodynamic" behaviour (e.g. the Spree River, Germany) and warming temperatures amplifying natural denitrification (e.g. the Po River, Italy).

Riverine ecosystems divide along with two distinct trajectories globally: widespread oligotrophication in developed regions (e.g. in Europe), contrasted against intensifying eutrophication in developing regions (e.g. in Asia and Africa). While climate change increasingly alters long-term nutrient baselines, a critical research asymmetry persists. Although oligotrophication trends become more prevalent in riverine datasets, the scientific literature remains heavily skewed towards eutrophication assessments. Future research need to urgently redress the imbalance and investigate the ecological implications of trophic status recovery. As an example, greater attention is required to understand how river communities respond and may restructure both in terms of composition and functionally.