Experimental evaluation of microzooplankton grazing methods in the lab and in the field

Microzooplankton grazing on picoplankton is a fundamental process within aquatic food webs, particularly in low-nutrient oligotrophic regions that are typically dominated by small picophytoplankton. Classical methods employed in the field have been used for decades without rigorous testing in a controlled laboratory setting. Two commonly employed methods for measuring microzooplankton grazing rates in nature were assessed in a controlled laboratory experiment designed to compare the estimated phytoplankton mortality rates by the experimental techniques to the picophytoplankton mortality observed directly in the culture based on changes in cell abundance. The Dilution and fluorescently labeled bacteria disappearance techniques were applied to a culture of the cyanobacterium subjected to a nanozooplanktonic grazer in the presence and absence of a cyanophage. Results indicated that, under laboratory conditions, these methods underestimated picoplankton mortality rates by 50-80%. The techniques were then applied in the field to compare results outside of the laboratory. There was an order of magnitude difference between mortality rates estimated using the techniques, which raises concerns of the validity of the assumptions underlying the techniques. Overall, both techniques provided reasonable, albeit somewhat underestimated microzooplankton-mediated mortality rates under controlled laboratory conditions, but caution should be exercised in interpreting these rates in field studies where the assumptions of the methods may be difficult to meet.

Presentation Preference: Oral

Primary Presenter: Jennifer Beatty, University of Southern California (jlbeatty@usc.edu)

Authors:

Brittany Stewart, University of Southern California  (bs70959@usc.edu)

Kendra Turk-Kubo, University of California, Santa Cruz  (kturk@ucsc.edu)

David Caron, University of Southern California  (dcaron@usc.edu)