Relationships between nutrient stocks and inventories and phytoplankton physiological status along an oligotrophic meridional transect in the Tasman Sea Michael J. Ellwood a,n , Cliff S. Law b , Julie Hall b , E. Malcolm S. Woodward c , Robert Strzepek d , Joma Kuparinen e , Karen Thompson f , Stuart Pickmere f , Philip Sutton b , Philip W. Boyd g a Research School of Earth Sciences, The Australian National University, Building 61, Canberra, ACT 0200, Australia b National Institute of Water and Atmospheric Research (NIWA), Greta Point, Wellington, New Zealand c Plymouth Marine Laboratory, Plymouth, Devon, United Kingdom d Department of Chemistry, University of Otago, Dunedin, New Zealand e Department of Environmental Sciences, University of Helsinki, FI-00014, Finland f National Institute of Water and Atmospheric Research, Hamilton, New Zealand g Centre for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin, New Zealand article info Article history: Received 27 June 2012 Received in revised form 25 October 2012 Accepted 3 November 2012 Available online 27 November 2012 Keywords: Nitrogen limitation Oligotrophic Tasman Sea Synechococcus Prochlorococcus Iron Dust Nano-nutrients Phytoplankton abstract The waters of the north Tasman Sea are adjacent to the arid Australian sub-continent and are north of the circumpolar Subtropical convergence. Nutrient and phytoplankton stocks in this region were investigated on two voyages during February 2005 and March 2006 to establish the spatial extent and magnitude of oligotrophy in the region. Surface nitrate, phosphate, ammonium and nitrite were all in the nanomolar concentration range north and south of the Tasman Front (  331S; 165–1751E). The location of the nitracline was found to be at or above the 1% light level. The distributions of pico-eukaryotic cells, Synechococcus and Prochlorococcus showed strong latitudinal and vertical gradients, with higher abundances south of the Tasman Front that decreased northward, but consistent with changes in nutrient concentration. A nitrite maximum was observed at and below the 1% light level and results from incomplete assimilatory nitrate reduction by phytoplankton. Mixed layer dissolved iron concentrations varied between 0.05 and 0.70 nmol L 1 , and were dependent on the vertical supply rate of iron from below and on sporadic atmospheric dust deposition. Based on the rate of iron supply, phytoplankton located south of the Tasman Front were unlikely to be iron limited whereas phytoplankton located north of the Tasman Front were near the threshold for iron limitation. Deck-board incubation experiments involving the addition of macro- (ammonium, nitrate and phosphate) and micro- (iron, dust and zinc) nutrients confirm nitrogen availability to be the primary control on biological production, with the potential for secondary phosphate, silicate and dissolved organic carbon limitation, when nitrogen limitation was alleviated. The form of nitrogen required to stimulate the phytoplankton community also varied; ammonium stimulated Prochlorococcus growth whereas nitrate stimulated Synechococcus growth. Predator-free incubation experiments indicate that grazing was an important constraint on phytoplankton production. Water column observations and incubation results confirm that the supply of dissolved inorganic nitrogen into the euphotic zone was the primary factor controlling new primary production in the northern Tasman Sea region. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction The subtropical ocean and its transitional waters make up  60% of total ocean area thus making it the largest cohesive community of plants and animals on our planet (Emerson et al., 1997; Eppley and Peterson, 1979). Estimates of vertical organic-carbon export to the ocean interior from these waters accounts for about half of the global carbon export flux, making subtropical waters a significant contributor to the ocean’s biological pump (Emerson et al., 1997). However, much of the subtropical ocean is either seasonally or permanently oligotrophic (Jenkins and Doney, 2003), with the supply of nitrogen into the euphotic zone determining the magni- tude of new primary production. In oligotrophic regions, the supply of nitrogen has typically been defined by the upward vertical flux of nitrate across the permanent thermocline (Dugdale and Goering, 1967; Eppley and Peterson, 1979; Lewis et al., 1986). One external Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/dsri Deep-Sea Research I 0967-0637/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dsr.2012.11.001 n Corresponding author. E-mail address: michael.ellwood@anu.edu.au (M.J. Ellwood). Deep-Sea Research I 72 (2013) 102–120