Changing competitive interactions between salps and protistan grazers during salp bloom evolution

As large mesozooplankton that often co-vary in abundance with euphausiids in the Southern Ocean, salps are often considered competitors of krill. However, with fine mesh feeding filters, these organisms may primarily compete with protistan grazers (ciliates, dinoflagellates, and nanoflagellates) for pico- and nanophytoplankton prey. We quantified size-selective grazing rates by salps and the bulk protistan community during Lagrangian experiments near the Chatham Rise. Grazing was quantified using the microzooplankton grazing dilution approach (for protists) and incubations conducted in plankton kreisels (for salps) with samples analyzed using flow cytometry, epifluorescence microscopy, and flowcam. Preliminary results show that protists were consistent grazers on Synechococcus, picoeukaryotes, and nanoflagellates. Salp prey was determined by organism stage and size. Although salps were never significant grazers on Synechococcus, small (~10-mm) Salpa thompsoni blastozooids were substantial grazers of picoeukaryotes. All stages and sizes of Salpa thompsoni fed on nanoflagellates, although Thetys vagina was not found to consume any phytoplankton groups. Our results show that the competitive interactions between dense salp blooms and protistan grazers can change during the evolution of a salp bloom.