Annual nitrate load-flow slopes showed no clear change after 2000 in two Gulf rivers through 2025

We tested whether annual nitrate plus nitrite load became more sensitive to flow after 2000 in the Mississippi and Atchafalaya rivers. We used USGS version 3.0 load estimates and complete annual flow records for 1980-2025. We fitted changes in log combined load against changes in log combined flow, with separate slopes for transitions ending in 1981-2000 and 2001-2025. Slopes were 1.021 and 1.037. The late minus early slope was 0.016, with conditional 95% three-transition block limits of -0.291 to +0.502. All nine planned checks also had limits that included zero. The data give no clear increase, but do not establish equal slopes. Load estimates use discharge as an input, so the relation is partly built into the source method. Annual averaging, two connected branches, short periods, and absent joint source-error draws limit physical inference. Code, small aggregate inputs, all fitted checks, bootstrap draws, a figure, source hashes, retrieval steps and an independent arithmetic audit are included.

nitrate · Mississippi · Atchafalaya · annual flow · slope change · null result · WRTDS