The limited effect of salps on the coupling between microzooplankton and phytoplankton in subtropical and subantarctic oceanic waters east of New Zealand

Siobhan O'Connor, Victoria University of Wellington, School of Biological Sciences, Wellington, New Zealand, Andres Gutierrez-Rodriguez, National Institute of Water and Atmospheric Research, New Zealand, Marine Biogeochemistry, Wellington, New Zealand, Natalie Yingling, Florida State University, Tallahassee, United States, Thomas Bryce Kelly, Florida State University, Earth Ocean and Atmospheric Science Department, Tallahassee, United States, Michael R Stukel, Florida State University, Tallahassee, FL, United States, Karen E Selph, University of Hawaii at Manoa, Oceanography, Honolulu, HI, United States, Karl Safi, National Institute of Water and Atmospheric Research, Hamilton, New Zealand, Adriana Lopes dos Santos, Nanyang Technological University, Singapore, Singapore, Alexia Saint-Macary, National Institute of Water and Atmospheric Research, Wellington, New Zealand, Maxim Y Gorbunov, Rutgers University, Department of Marine and Coastal Sciences, New Brunswick, NJ, United States, Scott D Nodder, National Institute of Water and Atmospheric Research (NIWA), Marine Biogeochemistry, Wellington, New Zealand and Moira Decima, NIWA National Institute of Water and Atmospheric Research, Wellington, New Zealand
Rapid proliferations of salps can transiently alter the microbial foodweb structure and associated trophic and biogeochemical flows. The aim of this study was to investigate the effect of salps on the community structure and production of phytoplankton and microzooplankton consumption under different oceanographic regimes. To do so, we tracked five selected water parcels for several days (i.e. Cycles, C1-C5), each of which harbored different densities and life-stages of salps in subtropical (ST) and subantarctic (SA) waters east of New Zealand. During each cycle, depth-resolved14C-incorporation and dilution grazing experiments were conducted in situ using a drifting experimental array and initial phytoplankton stocks and photophysiology were quantified using bottle and underway sampling.

Phytoplankton biomass and production were lower and dominated by picophytoplankton in High Nutrient-Low Chlorophyll SA waters of C2 and C5 compared to more coastal C1, where the alleviation of iron stress enhanced the contribution of larger cells. Coinciding with the highest salp abundances during C1, phytoplankton growth exceeded microzooplankton grazing, supporting alternative pathways for a substantial fraction of daily primary production. However, despite differences in salp densities between C2 (moderate) and C5 (very low), phytoplankton growth rates were balanced by microzooplankton grazing, which consumed most primary production in both cycles. Phytoplankton biomass and production were higher in nutrient-replete ST waters during C3, where despite the dominance of small phytoplankton, community growth was not matched by microzooplankton grazing and densities of salps remained low.

Our preliminary results show no clear relation between salp abundance and the (im-) balance between phytoplankton growth and grazing, and thus do not support a direct effect of salps on the close coupling between both processes, which is characteristic of nutrient limited oceanic regions.