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Nutrient Chemistry in Herbicide-treated Ponds of Differing Fertility¹

ABSTRACT

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.

Key Words: algal bloom • aquatic plants • diquat • Elodea • mineralization

¹ Agronomy Paper no. 1260. Contribution from the Dep. of Agronomy, New York State College of Agriculture and Life Sciences, Cornell Univ., Ithaca, NY 14853. Supported in part by a grant from Chevron Chemical Co., Ortho Division, Richmond, CA 93804.

² Assistant Professor of Soil Science and Research Support Specialist, Cornell Univ., respectively.

Received for publication July 31, 1978.