P. Parekh

Decoupling of iron and phosphate in the global ocean

[1] We formulate a mechanistic model of the coupled oceanic iron and phosphorus cycles. The iron parameterization includes scavenging onto sinking particles, complexation with an organic ligand, and a prescribed aeolian source. Export production is limited by the availability of light, phosphate, and iron. We implement this biogeochemical scheme in a coarse resolution ocean general circulation model using scavenging rates and conditional stability constants guided by laboratory studies and a suite of box model sensitivity studies. The model is able to reproduce the broad regional patterns of iron and phosphorus. In particular, the high macronutrient concentrations of the Southern Ocean, tropical Pacific, and subarctic Pacific emerge from the explicit iron limitation of the model. In addition, the model also qualitatively reproduces the observed interbasin gradients of deep, dissolved iron with the lowest values in the Southern Ocean. The ubiquitous presence of significant amounts of free ligand is also explicitly captured. We define a tracer, Fe* which quantifies the degree to which a water mass is iron limited, relative to phosphorus. Surface waters in high-nutrient, lowchlorophyll regions have negative Fe* values, indicating Fe limitation. The extent of the decoupling of iron and phosphorus is determined by the availability and binding strength of the ligand relative to the scavenging by particulate. Global iron concentrations are sensitive to changes in scavenging rate and physical forcing. Decreasing the scavenging rate 40% results in � 0.1 nM increase in dissolved iron in deep waters. Forcing the model with weaker wind stresses leads to a decrease in surface [PO4] and [Fe ]i n the Southern Ocean due to a reduction in the upwelling strength.

Interactions of the iron and phosphorus cycles: A three-dimensional model study: INTERACTIONS OF THE IRON AND PHOSPHORUS CYCLES

[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.