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Nutrient loading and nonstationarity: The importance of differentiating the independent effects of tributary flow and nutrient concentration

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Abstract The “phosphorus loading concept,” or more generally the “nutrient loading concept,” arose from Richard Vollenweider's work in the 1960–1970s that showed correlations between phosphorus loads and various eutrophication symptoms. The initial success of target loads developed for the Great Lakes solidified the concept that nutrient loading causes eutrophication, and load targets have become common tools to reduce eutrophication. Using concepts from the field of causality, we offer additional context to the nutrient loading concept to show that the correlation between nutrient load and eutrophication is spurious; load and eutrophication have common drivers, tributary flow and tributary nutrient concentration, but load itself is not causal. Consequently, in‐lake conditions are not invariant to the same load delivered at differing flow‐concentration combinations. We then use a simulation model to evaluate the consequences of delivering the same load at various flow‐concentration combinations from the Maumee River into Lake Erie. We show that load reductions under increased tributary flows may cause in‐lake phosphorus concentration increases, potentially offsetting the anticipated effect of the load reduction. Thus, particularly under a scenario where climate change may cause systematic flow changes, it will be important to expand the nutrient loading concept to consider the independent effects of tributary flow and nutrient concentrations, to assess the effectiveness of nutrient reduction strategies. This article is categorized under: Science of Water > Hydrological Processes Water and Life > Stresses and Pressures on Ecosystems Science of Water > Water Quality
Panel (a) Causal structure depicting nutrient load as the direct cause of lake nutrient concentration, with influent flow and influent nutrient concentration as the causes of nutrient load. The dashed arrow between influent flow and influent nutrient concentration indicates that concentration may be dependent on flow. For simplicity, eutrophication is represented as lake nutrient concentration; a more complex diagram could also show influent flow influencing factors including water residence time and water clarity, which, in turn, influence phytoplankton and macrophyte levels. Panel (b) Causal structure depicting influent flow and influent nutrient concentration as the direct causes of both lake nutrient concentration and influent nutrient load. The dashed arrow between influent flow and influent nutrient concentration indicates that concentration may be dependent on flow. For simplicity, eutrophication is represented as lake nutrient concentration; a more complex diagram could also show influent flow influencing factors including water residence time and water clarity, which, in turn, influence phytoplankton and macrophyte levels
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Panel (a) A hypothetical causal structure in which event A causes event B, which causes event C. The ellipses indicate events and the directed arrows indicate causal pathways. Panel (b) A hypothetical causal structure in which event A causes event B, which causes event C. In addition to the indirect effect of A on C via B, A also has a direct effect on C. The ellipses indicate events and the directed arrows indicate causal pathways
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Differences in phosphorus mass and concentration at the differing loads with differing concentration‐flow relationships. Both sets of four panels depict a bigger load at lower flow, higher concentration—a smaller load at higher flow, lower concentration. Panels (a–d) show experiment A—experiment F, panels (e–h) show experiment B–experiment G
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Differences in phosphorus mass and concentration at the same load with differing concentration‐flow relationships. Top four panels depict a comparison of experiments along the upper off‐diagonal of experiment matrix, bottom four panels depict a comparison of experiments along the lower off‐diagonal (Table ). Panels (a–d) show experiment E‐experiment B, panels (e–h) show experiment F–experiment C
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Differences in phosphorus mass and concentration at the same load with differing concentration‐flow relationships. Each set of four panels depicts a comparison of experiments along the main diagonal of experiment matrix (Table ). Panels (a–d) show experiment D‐experiment A, panels (e–h) show experiment G–experiment D, panels (i–l) show experiment G–experiment A
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Panel (a) Depth (meters) of Lake Erie's western basin showing the three transects along which model results are displayed. Panel (b) 2008 flow and total phosphorus load from the Maumee River
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