Interactions of the iron and phosphorus cycles: A three‐dimensional model study

Abstract

[1] We use an ocean circulation, biogeochemistry, and ecosystem model to explore the interactions between ocean circulation, macro-and micro-nutrient supply to the euphotic layer, and biological productivity. The model suggests a tight coupling between the degree of iron limitation in the upwelling subpolar and tropical oceans and the productivity of the adjacent subtropical gyres. This coupling is facilitated by lateral Ekman transfer of macro-nutrients in the surface ocean. We describe a coarse resolution configuration of the MIT ocean circulation and biogeochemistry model in which there are fully prognostic representations of the oceanic cycles of phosphorus, iron, and silicon. The pelagic ecosystem is represented using two functional groups of phytoplankton and a single grazer. Using present-day forcing, the model qualitatively captures the observed basin and gyre scale patterns of nutrient distributions and productivity. In a suite of sensitivity studies we find significant regional variations in response to changes in the aeolian iron supply. In a dustier (model) world, the Southern Ocean and Indo-Pacific upwelling regions are more productive, but there is a decrease in productivity in the subtropical gyres and throughout the Atlantic basin. These results can be described most easily by a simple conceptual classification of the Southern and Indo-Pacific oceans into two regimes: (1) upwelling, iron limited regions and (2) macro-nutrient limited, oligotrophic subtropical gyres. Enhancing the aeolian iron supply to the upwelling regions relieves iron limitation and increases local primary and export production, but reduces the Ekman transfer of phosphate to the neighboring subtropical gyres. Consequently, over time, the gyres become further depleted in macro-nutrients and productivity decreases in response to global scale iron fertilization. In a large-scale analogy, the macro-nutrient budget of the Atlantic is maintained by lateral transfer of nutrients in the upper ocean. Enhanced aeolian supply of iron leads to increased productivity in the Southern Ocean and Indo-Pacific upwelling regions, reducing the lateral transfer of macro-nutrients to the Atlantic basin, which becomes increasingly macro-nutrient limited throughout.Citation: Dutkiewicz, S., M. J. Follows, and P. Parekh (2005), Interactions of the iron and phosphorus cycles: A three-dimensional model study, Global Biogeochem. Cycles, 19, GB1021,

Modeling the coupling of ocean ecology and biogeochemistry

“…We direct the reader to the online supplemental material of that paper for additional discussion. The most significant change is that the grazing term is now includes variable palatability of phytoplankton and sloppy feeding as treated by Dutkiewicz et al [2005]. Additionally the nitrogen limitation term (equation (A10)) has been slightly modified.…”

Winners and losers: Ecological and biogeochemical changes in a warming ocean

“…Atmospheric delivery of dust is a major source of iron to the ocean, and in this model the dust supply does not change. Though iron is also supplied from depth, in the subtropical regions lateral supplies are more important [Dutkiewicz et al, 2005b]. An increase in biomass occurs in regions where lateral iron supply from upstream becomes higher.…”

Ocean fertilization: a potential means of geoengineering?

“…Peng & Broecker 1985) indicated a possible reduction in atmospheric CO 2 of 50-100 ppm; however, recent studies with higher resolution three-dimensional models coupled to ecosystem dynamics including iron have suggested that addition of iron is much less efficient (order of 10 ppm) because the other limiting factors of light and grazing become dominant (e.g. Dutkiewicz et al 2005;. The link between nutrient supply and ecosystem dynamics is complex, especially for micronutrients.…”

“…In the open oceans, observations have been limited to a purposeful release of phosphate and iron (Rees et al 2006) and to various modelling studies that highlight the intimate and intricate relationships between the various essential elements and the forms in which they are present in the ocean (Dutkiewicz et al 2005;Parekh et al 2005).…”