Catchment controls on the use of dissolved organic nutrients by river phytoplankton in the United Kingdom

•1. Dissolved organic matter (DOM) affects the structure and function of aquatic ecosystems and can supply nitrogen and phosphorus to meet the stoichiometric needs of biota. Anthropogenic alteration of the nature and rates of DOM inputs to freshwaters may be increasing the risk of eutrophication, but most of our understanding of DOM use in phytoplankton is based on site specific assessments; little is known about landscape‐scale controls on DOM bioavailability to river phytoplankton communities, across varying landscape character. It is important to test whether our understanding of DOM use at individual sites is generalisable at larger scales and if simple measures of catchment characteristics can be used as an effective proxy to predict growth responses of riverine phytoplankton to DOM. •2. We used bioassays to investigate the ability of phytoplankton to use dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) compounds to support growth in rivers in the United Kingdom (UK). We then assessed the extent to which statistical models built upon these data can be used to provide generalisable insights into phytoplankton growth responses at new sites based on riverine nutrient concentrations and catchment land cover. •3. We found the greatest phytoplankton growth responses to DOM compounds occurred at lower concentrations of inorganic nutrients, although organic nutrients continued to stimulate growth even at higher inorganic nutrient concentrations. Water quality models based on bioassay responses developed in two catchments of contrasting character could adequately predict growth responses across the UK ( r 2 = 0.56 for DON; r 2 = 0.33–0.51 for DOP). However, better model fit and performance was generated by models developed at the national scale using national monitoring and bioassay datasets (overall r 2 = 0.90, fixed effects r 2 = 0.82 for DON and overall r 2 = 0.82–0.92, fixed effects r 2 = 0.39–0.51 for DOP). Models using only a lowland to upland land cover gradient could adequately predict DON use by river phytoplankton (overall r 2 = 0.90, fixed effects r 2 = 0.56) but by contrast, their DOP use was influenced more strongly by site‐ and DOM compound‐specific factors (overall r 2 = 0.83–0.92, fixed effects r 2 = 0.17–0.25). •4. Growth stimulation of river phytoplankton by organic nutrient enrichment was widespread across the UK, even at nutrient concentrations considered non‐limiting to growth. The extent to which catchment land cover could predict DON versus DOP growth responses differed, but nutrient concentrations remained a key driver in predicting growth stimulation in UK rivers. •5. Human‐induced perturbations to organic matter inputs to freshwaters may therefore be altering these ecosystems and changing the composition of the communities that they support. The models generated here are useful tools to identify which rivers may be more at risk from the eutrophication potential of dissolved organic nutrient inputs, such as those resulting from predicted future changes in nutrient management and land cover.