Evaluating stream nutrient trends: A new statistical approach to compensate for changing streamflow

Session: Great Lakes Tributaries: Connecting Land and Lakes (1)

Robert Hirsch, U.S. Geological Survey, [email protected]
Anne Choquette, US Geological Survey, [email protected]
Jennifer Murphy, U.S. Geological Survey, [email protected]
Laura Johnson, Heidelberg University, National Ctr for Water Quality Res., [email protected]
Remegio Confesor, Heidelberg College, NCWQR, [email protected]

Abstract

Tracking changes in stream nutrient inputs to Lake Erie over multidecadal time scales depends on the use of statistical methods that can remove the influence of year-to-year variability of streamflow, but also consider the influence of long-term trends in streamflow.  Earlier in 2019, a paper (doi:10.1016/j.jglr.2018.11.012) and associated software release (the EGRET 3.0 ‘R’ package) defined and implemented an extended version of Weighted Regressions on Time, Discharge, and Season (WRTDS) modeling that explicitly considers nonstationary streamflow.  Using this method, we evaluated soluble reactive phosphorus (SRP; dissolved orthophosphate) and streamflow records of approximately 40 years duration at six sites in the Lake Erie basin.  These results extend those previously published for 1995 – 2015 and provide a more current estimate of the SRP changes taking place in these watersheds.  Reductions in tributary SRP-loading are considered critical to reducing cyanobacteria HABs in western Lake Erie.  These results allow for the partitioning of the observed SRP trends into two components: the portion associated with the changing relationship between concentration and discharge, and the portion due to the changing probability distribution of streamflow.  The methods described here could provide ongoing measures of progress, or lack of progress, towards nutrient-reduction goals for Great Lakes watersheds.