The impact of diquat on macrophytes and water quality in Battle Ground Lake, Washington.

A nearly monotypic population of egeria ( Egeria densa Planch.) was interfering with recreation in a popular lake in southwest Washington. In June 2003 the littoral zone was treated with the contact herbicide diquat (6,7-dihydrodipyrido [1,2-a2’,1’-c] pyrazinediium dibromide). Aquatic plant frequency and biomass data were collected on all submersed species before treatment, and eight weeks, one year and two years after treatment. Water quality and herbicide dissipation data were also collected before and for one season after the treatment. Results from the aquatic plant data showed a significant reduction in egeria frequency and biomass after the herbicide treatment, although the species did not disappear entirely. Two native submersed species, water moss ( Fontinalis antipyretica Hedw.) and stonewort ( Nitella sp.), increased after treatment. However their increase was not enough to offset the egeria population reduction, as total plant abundance was significantly reduced after treatment. The herbicide dissipation data illustrated the dispersal of diquat throughout the lake and persistence at low concentrations (up to 10.4 ppb) in the water column for at least two weeks after treatment. Water quality data demonstrated a slight decrease in dissolved oxygen and water transparency following the herbicide treatment, potentially due to plant die-off and subsequent plant decomposition.