Fisiologia

Vol. 254: 3–9, 2003

MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser

Published June 4

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
1

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

ABSTRACT: We define a newdimensionless 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 characterisedby 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 ofchlorophyll 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
Resale or republication not permitted without written consent of the publisherINTRODUCTION Among the issues that have captured the interest of ocean biogeochemists during the past decade, the significance of areas with high residual nutrients (in particular nitrate) and low chlorophyll concentration (the so-called HNLC regime) is prominent (Chisholm & Morel 1991). It has been inferred that such systems are, in some way, biologically limited (Martin et al. 1990b): otherwise, thenutrients would be consumed, with a concomitant increase in phytoplankton biomass (as indexed by chlorophyll concentration). Iron, an essential element for autotrophic physiology, has been identified as a potentially limiting resource (Martin & Fitzwater 1988), with the corollary that
*Email: tplatt@dal.ca

addition of iron to the ocean would enhance photosynthesis by phytoplankton (Martin etal. 1990a, Coale et al. 1996), leading to complete utilisation of the ambient nitrate. Here, we show that HNLC regimes need not be considered as anomalous marine ecosystems; rather, they should be seen as elements of a continuum in which the biological dynamics are controlled by the ratio of nutrient supply to the nutrient demand of the local primary production, which is determined by thebio-optical characteristics of the pelagic ecosystem. We introduce a new dimensionless number, S, that captures the dynamics and diagnoses whether a given region will be HNLC, either in its pristine condition or after treatment with iron.
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Mar Ecol Prog Ser 254: 3–9, 2003

That the nitrate is not exhausted (by definition) in HNLC regions is an indicationthat it is resupplied, if not continuously then at least fast enough (on the average) to exceed the demands of new production (Dugdale 1967, Platt et al. 1989). An important mechanism for nutrient resupply to the upper mixed layer is entrainment through deepening of the layer (Gardner et al. 1993), for example following passage of a storm system (or a mesoscale eddy). Against this background, we...