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Nutrients in an oligotrophic boundary current: Evidence of a new role for the Leeuwin Current


Thompson, PA and Wild-Allen, K and Lourey, M and Rousseaux, C and Waite, AM and Feng, M and Beckley, LE, Nutrients in an oligotrophic boundary current: Evidence of a new role for the Leeuwin Current, Progress in Oceanography, 91, (4) pp. 345-359. ISSN 0079-6611 (2011) [Refereed Article]

Copyright Statement

Copyright 2011 Elsevier

DOI: doi:10.1016/j.pocean.2011.02.011


New observations along the continental shelf of Western Australia provide a novel explanation for the established ~60years relationship between Leeuwin Current (LC) strength and greater winter nitrate concentrations at 32S plus the inter-annual variation in the magnitude of the annual, shelf-scale, phytoplankton bloom. The potential source of dissolved nitrogen to support the annual shelf scale phytoplankton bloom was identified as thin layers of an unprecedented areal extent, nitrate concentration and shallow nature that were observed off the northwest of Australia. We propose that the dissolved inorganic nitrogen (DIN) in these layers enters the LC at depth and then enters the euphotic zone via by three mechanisms: instability that results in a warm core eddy, cooling that deepens the surface mixed layer and shallowing of the thin layer. During the onset of the annual phytoplankton bloom along the west coast of Australia from 22S to 34S the poleward flowing LC was clearly evident as a surface intensified ocean boundary current transporting warmer, lower-salinity, greater-silicate waters in a shallow mixed layer rapidly southward. Between 24 and 26S the core of the LC was present as a 50-100m deep layer over one or more thin layers, 15-50m thick, with high nitrate and low dissolved oxygen (DO). These layers were of lower salinity, cooler water with markedly reduced DO, high nitrate concentrations and distinct nitrate:silicate (NO3:Si(OH)4) nutrient ratios. As the LC flowed south it cooled and deepened thereby entraining the thin layers of high nitrate water into the euphotic zone. The LC also formed large (greater than 100km diameter) warm core eddies with a deep surface mixed layer that also entrained nitrate from these thin layers. In some locations as far south as 32S the LC was still present with the thin layer of high nitrate intact but now within the euphotic zone. Thus, the available evidence suggests the LC arises under conditions that favour rapid and shallow nitrification. This nitrification fuels a shelf-scale bloom on a downwelling favourable coast. Depending upon the rate of nitrification the source of the particular organic matter may be local or delivered from the tropics via horizontal advection in a subsurface layer of the LC.

Item Details

Item Type:Refereed Article
Keywords:Australia, boundary currents, continental shelf, deep layer
Research Division:Environmental Sciences
Research Group:Other environmental sciences
Research Field:Other environmental sciences not elsewhere classified
Objective Division:Environmental Management
Objective Group:Other environmental management
Objective Field:Other environmental management not elsewhere classified
UTAS Author:Thompson, PA (Dr Peter Thompson)
UTAS Author:Waite, AM (Dr Anya Waite)
UTAS Author:Feng, M (Dr Ming Feng)
ID Code:119686
Year Published:2011
Web of Science® Times Cited:39
Deposited By:Oceans and Cryosphere
Deposited On:2017-08-03
Last Modified:2017-09-27

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