Pelagic response of a humic lake to three years of phosphorus addition

Three years of whole-lake phosphorus (P) fertilization, conducted in a 12-ha boreal forest lake, revealed significant changes in epilimnion nutrients, biomasses, and primary production. As a time average for all three treatment years, primary production increased 257% relative to the reference basin value of 16.5 mg·m –3 ·day –1 , whereas the carbon masses of both nanoalgae (>2 µm) and zooplankton increased roughly 130% from their initial values of 15 mg·m –3 and 17 mg·m –3 , respectively. Calculated from a difference, the absolute increase in the sum of ciliates and picoplankton (heterotrophic bacteria and picoalgae) was more than six times as large as for algae larger than 2 µm, indicating that most added P ended in this compartment. Moreover, fertilization did not change the species inventory among nanoalgae and zooplankton, although the biomass composition changed somewhat. Only the former dominant species, the chrysophytes Dinobryon crenulatum, D. sociale v. americanum, Mallomonas allorgei, and Ochromonas sp. and the calanoid copepod Eudiaptomus gracilis increased substantially in biomass owing to the added P. Surprisingly, we observed a substantial delay in the food web response to fertilization, where most variables increased monotonously in size from year to year during the fertilization period. The underlying mechanisms for these delayed increases remains to be explained.