MEPS 254:3-9 (2003)  -  doi:10.3354/meps254003

Nitrate supply and demand in the mixed layer of the ocean

Trevor Platt1,*, Shubha Sathyendranath1,2, Andrew M. Edwards1,2, David S. Broomhead3, Osvaldo Ulloa4

1Biological Oceanography Section, Bedford Institute of Oceanography, Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
2Department of Oceanography, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada
3Department of Mathematics, University of Manchester Institute of Science and Technology, PO Box 88, Manchester M60 1QD, United Kingdom
4Centro de Investigación Oceanográfica en el Pacífico sur-Oriental and Departamento de Oceanografía, Universidad de Concepción, Casilla 160-C, Concepción, Chile

ABSTRACT: We define a new dimensionless number, S, to be the ratio of nitrogen supply to nitrogen demand of new primary production in the pelagic ecosystem. When S > 1, we expect high-nutrient, low-chlorophyll (HNLC) conditions. Using the results of a new model of nitrogen input and consumption for the mixed layer of the ocean, we calculate S for selected oceanic regimes. Those generally accepted to be HNLC are characterised by S > 1. The bio-optical terms in this model (specific absorption of pigments, parameters of the light-saturation curve) are known to respond to addition of iron. Using these known responses, we recalculated the expected value of S under hypothetical enrichments of the selected regimes with iron. In each case, the magnitude of S was reduced, but not always below unity. The maximum value of chlorophyll biomass that can be sustained in a given mixed layer may be calculated from consideration of either the bio-optics or the nitrogen supply. The maximum realised biomass will be the smaller of these 2 estimates.

KEY WORDS: High-nutrient low-chlorophyll · HNLC · Iron fertilisation · Plankton models · IronEx

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