Relationships between nutrient stocks and inventories and phytoplankton physiological status along an oligotrophic meridional transect in the Tasman Sea

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 (∼33°S; 165–175°E). 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 0.70nmolL−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.