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Journal of Plankton Research plankt.oxfordjournals.org J. Plankton Res. (2015) 37(2): 1 – 17. doi:10.1093/plankt/fbv011 Grazer and viral impacts on microbial growth and mortality in the southern California Current Ecosystem ALEXIS L. PASULKA 1 *, TY J. SAMO 2 AND MICHAEL R. LANDRY 1 9500 GILMAN DRIVE , LA JOLLA , CA 92037, USA AND 2 DEPARTMENT OF OCEANOGRAPHY , UNIVERSITY OF 1000 POPE RD , HONOLULU , HI 96822, USA SCRIPPS INSTITUTION OF OCEANOGRAPHY , UCSD , HAWAI ’ I AT MANOA , * CORRESPONDING AUTHOR : apasulka@caltech.edu Received November 25, 2014; accepted February 11, 2015 Corresponding editor: John Dolan Protistan grazers and viruses are major agents of mortality in marine microbial communities with substantially differ- ent implications for food-web dynamics, carbon cycling and diversity maintenance. While grazers and viruses are typ- ically studied independently, their impacts on microbial communities may be complicated by direct and indirect interactions of their mortality effects. Using a modification of the seawater dilution approach, we quantified growth and mortality rates for total phytoplankton and picophytoplankton populations (Prochlorococcus, Synechococcus, picoeu- karyotes) at four contrasting sites in the California Current Ecosystem. Grazing mortality was significant in 10 of 15 cases, while viral effects were significant for 2 cases. Nonetheless, mortality estimates for the entire phytoplankton community based on chlorophyll a were 38 + 13% higher when viral effects were included, relative to grazing alone. Mortality estimates for picophytoplankton varied in space and among groups. We also explored a potential methodo- logical constraint of this method and hypothesize that heterotrophic bacteria may be affected by the dilution of their growth-sustaining substrates. For all picophytoplankton, estimates of grazing and viral mortality were inversely related within and across experiments. Indirect interactions among grazers and viruses may be important to consider if there are tradeoffs in the grazing and virus resistance strategies of prey/host cells. KEYWORDS: microzooplankton grazing; viral lysis; picophytoplankton; mortality I N T RO D U C T I O N Mortality processes within microbial communities play key roles in regulating biomass, species composition and elemental cycling (Pomeroy et al., 2007; Suttle, 2007). Protistan grazers, the primary consumers of phytoplank- ton and bacteria (Sherr and Sherr, 1994, 2000), account, available online at www.plankt.oxfordjournals.org # Th A h 2015 P bli h d b O f d U i i P All i h d F i i l il j l i i