Four decades of diluting phosphorus to maintain lake quality

Dilution with low-nutrient Columbia River Water (CRW) has markedly improved Moses Lake quality for 42 years. There were two phases of CRW volume input, which proportionately lowered total phosphorus (TP). Initially, spring-summer inputs averaged 130 × 106 m³ during 1977-1988 reducing average TP from 152 to 65 µg/L in half the lake proximal to the inputs. That input represented 1.5 volumes of that half-lake volume. Inputs doubled through the mid-1990s, and nearly 2.5 times since 2000, decreasing TP to a 18-year average of 24 µg/L. Chlorophyll a (chl) decreased further from 18 µg/L during the early dilution years to about 6 µg/L as TP declined. Cyanobacteria biovolume declined to 57% of total biovolume during 1977-1988 from 98% before dilution. Less (65%) CRW water since 2016 led to higher TP (41 µg/L) and chl (18 µg/L) in 2018, while cyanobacteria averaged 87% of total biovolume. More TP and cyanobacteria in 2018 are attributed to more internal TP loading. Increased N:P ratios have possibly given more advantage to the non-N-fixing cyanobacteria Microcystis, which comprised 82% and 74% of maximum cyanobacteria biovolume in 2017 and 2018. PRACTITIONER POINTS: Lake total phosphorus (TP) was reduced 57% in the 1970s-1980s by adding large volumes of low-nutrient Columbia River water (CRW). Total P was further reduced by 65% since 2000 by more than doubling the earlier CRW input to an average spring-summer concentration of 24 μg/L. Less (65%) CRW during 2017-2018 led to higher lake TP (41 μg/L) and a worse cyanobacteria bloom in 2018. Microcystis, an non-nitrogen fixer, was the dominant cyanobacteria in 2017-2018 likely related to higher N:P ratios.