Ponds 0.04 ha in bottom area and 1.2 m uniform depth with similar densities of the submerged aquatic plant Elodea canadensis Michx. but with different previous nutrient additions were treated with 1 mg/liter diquat (6,7‐dihydrodipyrido [1,2‐a:2′,1′‐c] pyrazinediium ion) on 17 Nov. 1972. This was done to simulate normal weedkill conditions and to measure subsequent carryover effects of past nutrient inputs. Before treatment and for 1 year thereafter, soluble P, particulate P, NO3−, NO2−, NH4+, K+, and chlorophyll a were measured in treated and similar control ponds. After herbicide treatment soluble P concentrations rose dramatically within 24 hours but were reduced to control levels of 10 to 20 µg/liter within 3 weeks. All P forms in the fertilized treated pond remained low until the following June, when particulate and organic P increased to 100 to 300 µg/liter as a result of mineralization processes. Chlorophyll a rose soon after to about 200 µg/liter and NH2+‐N to 100 µg/liter. There were no other differences between control or treated ponds in NH4+ or NO3− concentrations, but both increased several fold each fall in all ponds. Definite carryover effects of previous nutrient inputs were demonstrated in the herbicidetreated ponds, but not in untreated ponds. The compartment in the pond which stored these nutrients and then released them upon herbicide treatment was the rooted macrophytes.