The Spurious Correlation Between Phosphorus Load and Eutrophication

Session: 41b. - Great Lakes Harmful Algal Blooms Research from Watershed Influence to Ecosystem Effects

Craig Stow, NOAA Great Lakes Environmental Research Laboratory, [email protected]
Qianqian Liu, Grand Valley State Univ., [email protected]
Eric Anderson, NOAA, Great Lakes Environmental Research Laboratory (GLERL), [email protected]

Abstract

The “phosphorus loading concept”, or more generally the “nutrient loading concept”, arose from Richard Vollenweider’s work in the 1960-70s 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 mitigate eutrophication severity. Using concepts from the field of causality we demonstrate that the correlation between 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. Using a simulation model we evaluate potential consequences for Lake Erie of delivering loads at various flow-concentration combinations from the Maumee River, which has experienced recent flow increases. We show that load reductions under increased tributary flows may cause local in-lake phosphorus concentration increases, potentially offsetting the effect of the load reduction.  Thus, it will be important to track the independent effects of changing tributary flows and phosphorus concentrations, using a well-thought-out combination of models and monitoring, as measures to achieve the updated Lake Erie phosphorus targets are implemented.

1. Keyword
pollution load

2. Keyword
phosphorus

3. Keyword
eutrophication

4. Additional Keyword
load target

5. Additional Keyword
causality