Effectiveness of diluting Moses Lake, Washington, with low phosphorus Columbia River water

Abstract

Welch, EB and Brattebo SK. 2024. Effectiveness of diluting Moses Lake, Washington, with low phosphorus Columbia River water. Lake Reserv. Manage. 40:xxxx-xxxx. Moses Lake has been rehabilitated by dilution with low total phosphorus (TP) inflow to improve lake quality in 46 of the past 47 years. Inflows of low TP (7 µg/L) Columbia River water (CRW), which enters during April–September, have ranged from around 75 to over 400 × 106 m3 (mcm), representing 0.5–2.5 total lake volumes annually. The CRW inflows substantially lowered the May–September average lower lake TP from 152 µg/L during 1969–1970 before dilution to 25 µg/L during the past 24 years when CRW input averaged 250 mcm. Total P decreased to an average of 65 µg/L during 11 of the first 12 years following the start of large CRW inputs with the Clean Lakes Project in 1977. Increased CRW inflow since 2000 reduced TP to less than 30 µg/L and has greatly improved lake quality and lowered the cyanobacteria fraction of phytoplankton biomass. Cyanobacteria were minimal at 5–22% of phytoplankton biomass in 2 years (2001, 2021) with 284 mcm of CRW inflow and average TPs of 19 and 20 µg/L. Average TP was 20 and 25 µg/L and cyanobacteria were 12 and 22% in 2022 and 2023 with less CRW (235 and 197 mcm). Even in the more enriched upper lake, TP in 2021–2023 averaged about half that of the previous 4 years (39 vs. 73 µg/L) and cyanobacteria averaged 33% of phytoplankton biomass. Unless the internal TP load is reduced substantially, CRW input should be at least 250 mcm to dilute external and internal loading and to hold TP below 30 µg/L and minimize cyanobacteria, as occurred in 2021–2023.