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

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.