Collaborative Research: Biogeochemical Modeling of Carbon Partitioning in the Pacific: the Role of Si and Fe in Regulating Production by Siliceous and Calcifying Phytoplankton

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Physical and biological interactions play a complex role in the partitioning of carbon between the atmosphere, upper ocean, deep ocean and sediments. At present, interdisciplinary models offer the best means to test hypotheses about how carbon partitioning is regulated in various oceanic regions on time scales of years to centuries. A new interdisciplinary model will integrate advances in several areas to test two related hypotheses: 1) that switches in community and productivity dominance between diatoms and other phytoplankton groups (e.g. non?siliceous picoplankton and calcifying phytoplankton) significantly affect carbon partitioning, vary spatially and temporally and are regulated by a combination of Si and Fe in combination; 2) that changes in Si trapping in the Southern Ocean affect Si(OH)4 concentrations in the "mode" waters that feed equatorial upwelling, and that Si and C uptake by equatorial phytoplankton alters air?sea exchange of carbon dioxide at the equator.An interdisciplinary science team of biological and chemical oceanographers and modelers will address these issues via four approaches. First, a calcifying phytoplankton component will be added to an existing model and used to incorporate water column production and dissolution of CaC03. Second, model experiments will be designed and executed to explore the regulation of switching between siliceous (diatoms) and non?siliceous (pico and calcifying) plankton. Third, an improved blogeochemical model developed for the equatorial Pacific will be spatially expanded to include the Southern Ocean, and used to conduct a series of simulation experiments on the processes that link high latitude "mode" water regions with the source waters for equatorial upwelling. Finally, iron?sensitive growth parameters of the phytoplankton components of the model will be manipulated to test the ecosystem response to iron enrichment in both the equatorial Pacific Ocean and the Southern Ocean. These objectives represent a significant step in the development of coupled physical biogeochemical models for exploring the response of marine biogeochemical processes to climate change.