AME 71:239-256 (2014)  -  DOI: https://doi.org/10.3354/ame01679

ABSTRACT: Experimental determinations of phytoplankton growth and mortality were carried out in a freshwater pond in Mississauga, Canada, during May, July, and October 2011. Modified Landry-Hassett-type dilution assays were performed to assess grazing and virus-induced mortality for 4 distinct phytoplankton populations via taxon-specific quantitative polymerase chain reaction (qPCR) assays, and for the entire algal community via fluorometric determination of chlorophyll a (chl a) concentrations. The sources and amounts of mortality observed for the phytoplankton populations varied throughout the year, and often differed from those of the whole community and each other. In several cases, the effects of viruses in the system appeared to counteract the effects of grazing mortality on phytoplankton. In these instances, mortality was greater in the dilution series generated with filtrates that excluded grazers relative to the dilution series created with filtrates that excluded both viruses and grazers. Relative to the grazer-free dilution series, mortality in the virus- and grazer-free dilution series was reduced for the whole community in May by 0.29 d^-1, and in July for the Prymnesiales, Chlamydomonadales I, and Chlorellales populations by 0.93, 1.31, and 1.87 d^-1, respectively. Although grazing mortality was not observed for the Chlamydomonadales II population during the May experiment, the population’s apparent growth rate increased in proportion with the presumed abundance of viruses in the incubation bottles. Our results reinforce the notion that individual phytoplankton taxa can have dynamic roles in aquatic food webs and suggest that viruses can potentially stimulate primary production, counteracting grazing mortality to the point of tipping a phytoplankton population’s status from decline to growth.

KEY WORDS: Algae · Viruses · Grazers · qPCR · Modified dilution assay · Seasonal · Growth enhancement