Salp Grazing Effects on the Lower Food Web and Carbon Export, in Subtropical and Subantartic Waters East of New Zealand

Moira Décima, National Institute of Water and Atmospheric Research, Marine Biogeochemistry, Wellington, New Zealand, Michael R Stukel, Florida State University, Tallahassee, FL, United States, Karen E Selph, University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, Andres Gutierrez-Rodriguez, National Institute of Water and Atmospheric Research, New Zealand, Marine Biogeochemistry, Wellington, New Zealand, Karl Safi, National Institute of Water and Atmospheric Research, Hamilton, New Zealand, Scott D Nodder, National Institute of Water and Atmospheric Research (NIWA), Marine Biogeochemistry, Wellington, New Zealand, Florian Lüskow, University of British Columbia, Department of Earth, Ocean and Atmospheric Sciences, Vancouver, BC, Canada, Christian Fender, Florida State University, Earth, Ocean, and Atmospheric Science, United States and Morgan Meyers, San Francisco State University - Romberg Tiburon Center, Biology, Tiburon, CA, United States
Salps are known to rapidly proliferate under certain ocean conditions, which can dramatically alter phytoplankton growth-grazing dynamics, and associated export fluxes. We investigated how salps and non-salp zooplankton affect the phytoplankton community, in both subtropical and subantarctic waters, and compare their impact on microplankton standing stocks and carbon flux. During five lagrangian experiments (three in areas with high abundances of salps and two in areas of low salp abundance), we quantified the major growth and grazing fluxes through micro- and mesozooplankton, and used drifting sediment traps and 238U-234Th deficiency to quantify vertical carbon flux.

The dominant salp species encountered was Salpa thompsoni, which we sampled in three different stages of bloom development. Preliminary results indicate high grazing pressure on phytoplankton by salps, removing substantial portions of the standing stocks, especially when large oozooids were present. Highest grazing by salps occurred when the community was dominated by large diatoms, but also corresponded to the onset of the salp bloom. Despite different phytoplankton communities in other experimental cycles, salps also grazed heavily but lower abundances resulted in lower grazing impacts. Finally, removal of phytoplankton by zooplankton could be substantial in areas without salps, but tended to be very low if salps were present. Vertical carbon export was enhanced by a factor of ~2 to 4-fold beneath the salp bloom relative to adjacent waters, as a result of the flux of rapidly-sinking salp fecal pellets. Overall, we show how salps significantly alter the transfer of material from phytoplankton to zooplankton, and explore the implications for both export production and transfer up the marine food-web.