| Summary: | Primary productivity in aquatic systems relies on carbon (C), nitrogen (N), and phosphorus (P) availability, with a reference stoichiometric ratio of 106 C/16 N/1 P, known as the Redfield ratio. This paper presents a methodology to visualize river water C/N/P stoichiometry and examine phytoplankton response. Redfield total dissolved C/N/P concentration ratios (TDC/TDN/TDP) from five River Thames tributaries were plotted in a ternary diagram, allowing relationships between nutrient stoichiometry, total P concentrations, and chlorophyll a, as a surrogate for phytoplankton biomass, to be explored. Chlorophyll a concentrations above 100 μg L were not observed below 14% TDP, and concentrations above 30 μg L were not observed below 13% TDP. This indicates a potentially lower TDP limit for highly eutrophic waters. These rivers are C and N rich, and this methodology should be applied to a wider range of rivers to explore C, N and P thresholds across different river typologies.
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