Phytoplankton Community Composition and Taxon-Specific Growth and Microzooplankton Grazing Rates in Subantarctic HNLC Waters Under Contrasting Mixing and Iron Limiting Conditions

Andres Gutierrez-Rodriguez1, Mikel Latasa2, Antonia Cristi-Martínez3, Priscillia Gourvil4, Karl Safi5, Dominique Marie4, Adriana Lopes dos Santos6, Daniel Vaulot4 and Scott D Nodder7, (1)National Institute of Water and Atmospheric Research, New Zealand, Marine Biogeochemistry, Wellington, New Zealand, (2)Instituto Español de Oceanografía, Gijon, Spain, (3)Universidad Catolica de la Santisima Concepcion, Concepci�n, Chile, (4)Station Biologique de Roscoff, Roscoff, France, (5)National Institute of Water and Atmospheric Research, Hamilton, New Zealand, (6)Nanyang Technological University, Singapore, Singapore, (7)National Institute of Water and Atmospheric Research (NIWA), Marine Biogeochemistry, Wellington, New Zealand
Abstract:
The Subantarctic (SA) zone is one of the largest HNLC zones of the Southern Ocean, yet, phytoplankton proliferations often occurs near bathymetric features such as on the Campbell Plateau, southeast of New Zealand. In this study we investigated environmental and biological controls on phytoplankton community structure and production rates in SA HNLC waters of this region. We compared phytoplankton community composition, taxon-specific growth and microzooplankton grazing rates from dilution experiments conducted on Campbell Plateau and deeper waters south of it.

Chemtax analysis showed the dominance of Prymnesiophyceae over diatoms and Prasinophytes, although the contribution of the latter increased on C. Plateau. 18S rRNA analysis of phytoplankton community and flow-cytometrically sorted populations confirmed the prevalence of these groups, although it rendered Prasinophytes, and Chloropicophyceae in particular, more abundant than Prymnesiophyceae on C. Plateau. Here, phytoplankton photochemical efficiency and growth rate were enhanced (Fv/Fm=0.45±0.05, m=0.50±0.10 day-1) relative to surrounding waters (Fv/Fm=0.37±0.04, 0.29±0.07 day-1) suggesting an alleviation of iron limitation. All taxa shared the same growth pattern, although diatoms (0.75 day-1) and Prasinophytes (0.5 day-1) showed the strongest boost. Grazing rate remained more stable across regions and taxa (~0.3 day-1) leading to positive (³ 0.2 day-1) and negligible net growth rates on C. Plateau and south of it, respectively. Prymnesiophyceae and Pelagophyceae were the exception having relatively constant and low intrinsic (0.25 day-1) and negligible net growth rates across all region. Our preliminary results suggest that low growth rates characteristic of HNLC SA waters are similar among groups, some (e.g. Diatoms and Prasinophytes) are prone to respond to favorable conditions, while others (e.g. Prymnesiophyceae) constitute a less reactive, and dominant background of the community.

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