The Ross Sea region is one of the most productive areas of the Southern Ocean. Low microzooplankton grazing is considered a key feature underpinning the massive phytoplankton blooms observed during spring and summer.  Growth and grazing measurements are, however, scarce and constrained to the surface mixed-layer. Here we present depth-resolved rates of phytoplankton growth and microzooplankton grazing in the euphotic zone (56 profiles) obtained from 2-point dilution experiments (n=336) carried out in the Ross Sea continental shelf, shelf-slope and off-shelf regions during 3 Antarctic summer voyages conducted as part of the New Zealand Ross Sea Research And Monitoring Programme (Ross-RAMP). Phytoplankton growth was higher than grazing in the upper euphotic zone, and decreased with depth, while grazing remained relatively constant vertically and led to net consumption in the lower euphotic zone. Depth-averaged growth and net growth rates (i.e., growth - grazing) tended to be positive and substantially higher in the shelf compared to off-shelf waters, where phytoplankton growth was lower and kept in check by microzooplankton grazing. Pigment-specific analysis of dilution experiments showed that diatoms growth and net growth rates were consistently higher than that of prymnesiophytes across all regions and depths. Understanding the factors that control phytoplankton growth, microzooplankton grazing variability is crucial to predict the effects that projected environmental changes will have on the trophic and biogeochemical functioning of the Ross Sea region pelagic ecosystem.

Primary Presenter: Andrés Gutiérrez Rodríguez, IEO - CSIC (andres.gutierrez@ieo.csic.es)

Authors:

Karl Safi, National Institute of Water and Atmospheric research  (Karl.Safi@niwa.co.nz)

Mark Gall, National Institute of Water and Atmospheric research  (Mark.Gall@niwa.co.nz)

Antonia Cristi, National Institute of Water and Atmospheric research  (Antonia.Cristi@niwa.co.nz)

Stacy Deppeler, National Institute of Water and Atmospheric research  (Stacy.Deppeler@niwa.co.nz)

Alexander Hayward, National Institute of Water and Atmospheric research  (Alex.Hayward@niwa.co.nz)

Sarah Searson, National Institute of Water and Atmospheric research  (Sarah.Searson@niwa.co.nz)

Matt Walkington, National Institute of Water and Atmospheric research  (Matt.Walkington@niwa.co.nz)

Youngju Lee, Korea Polar Research Institute  (yjlee@kopri.re.kr)

Mikel Latasa, IEO - CSIC  (Mikel.Latasa@ieo.csic.es)

Matt Pinkerton, National Institute of Water and Atmospheric research  (Matt.Pinkerton@niwa.co.nz)