Experimental Manipulation of Algal Bloom Composition by Nitrogen Addition

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Abstract
Nitrogen (as ammonium, nitrate, or both) was added over an 8- to 12-wk period to enclosures in a small prairie lake and to a whole pond. In previous years, both had experienced seasonal anoxia as a result of collapses of Aphanizomenon flos-aquae blooms followed by a massive fish kill. Additions altered the bloom magnitude and the species composition if N was added prior to the appearance of A. flos-aquae. Low additions (~ 0.75–1.5 g m−8∙wk−1) initiated after the appearance of the cyanophyte bloom resulted in a significant reduction of the A. flos-aquae biomass, while the same addition at the time of a chlorophyte bloom (Scenedesmus and Oocystis sp.) caused a substantial shift to Microcystis aeruginosa. High additions (~ 7–14 g m−3∙wk−1) resulted in a total absence of cyanophytes and were followed by a complete shift to small chlorophytes and cryptomonads, with a total absence of A. flos-aquae and the fish kill. Low additions of N did not show any accumulation over 3–5 mo after discontinuation of the fertilization; high additions showed a significant accumulation of nitrate in the water. Both lake and pond returned to heavy A. flos-aquae blooms and fish kills the following summer when not fertilized.Key words: nutrient enrichment, nitrogen, algal blooms, fish kills, oxygen depletion, prairie lakes, biomanipulation, eutrophication

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