Factors driving changes in phosphorus flux since 2011 in 24 U.S. Great Lakes tributaries

Extensive efforts have been made to reduce phosphorus inputs to the Great Lakes, yet tracking progress remains a challenge. Water quality trend analysis with Weighted Regressions on Time, Discharge, and Season (WRTDS) can remove the influence of discharge variability on concentration and flux estimates through flow-normalization. The results provide a view of underlying trends that can be used to infer watershed management effectiveness. WRTDS was used to evaluate the cause of changes in flow-normalized phosphorus fluxes in 24 U.S. Great Lakes tributaries from 2011-2020. In particular, we evaluated phosphorus concentration-discharge relationships, seasonal patterns and changes in total and dissolved phosphorus flux, and changes in point-source phosphorus inputs. Flow-normalized phosphorus flux decreased in all watersheds with low phosphorus yield (< 50 kg km^-2 yr^-1) and increasing mean annual discharge over the study period, suggesting the development of supply limitation. Seasonal fluxes of both total and dissolved phosphorus were generally highest in spring, but this pattern was more pronounced for total phosphorus. In all but two tributaries, percent changes in total phosphorus were more negative than percent changes in dissolved phosphorus, indicating potential increases in phosphorus bioavailability. These results may help guide ongoing phosphorus reduction efforts in Great Lakes tributaries.

Primary Presenter: Dustin Kincaid, U.S. Geological Survey (dustinkincaid@usgs.gov)

Authors:

Dustin Kincaid, U.S. Geological Survey  (dwkincaid@usgs.gov)

Matthew Diebel, U.S. Geological Survey  (mdiebel@usgs.gov)

Luke Loken, U.S. Geological Survey  (lloken@usgs.gov)