Abstract
A global three‐dimensional marine ecosystem model with several key phytoplankton functional groups, multiple limiting nutrients, explicit iron cycling, and a mineral ballast/organic matter parameterization is run within a global ocean circulation model. The coupled biogeochemistry/ecosystem/circulation (BEC) model reproduces known basin‐scale patterns of primary and export production, biogenic silica production, calcification, chlorophyll, macronutrient and dissolved iron concentrations. The model captures observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic and equatorial Pacific. Spatial distributions of nitrogen fixation are in general agreement with field data, with total N‐fixation of 55 Tg N. Diazotrophs directly account for a small fraction of primary production (0.5%) but indirectly support 10% of primary production and 8% of sinking particulate organic carbon (POC) export. Diatoms disproportionately contribute to export of POC out of surface waters, but CaCO3 from the coccolithophores is the key driver of POC flux to the deep ocean in the model. An iron source from shallow ocean sediments is found critical in preventing iron limitation in shelf regions, most notably in the Arctic Ocean, but has a relatively localized impact. In contrast, global‐scale primary production, export production, and nitrogen fixation are all sensitive to variations in atmospheric mineral dust inputs. The residence time for dissolved iron in the upper ocean is estimated to be a few years to a decade. Most of the iron utilized by phytoplankton is from subsurface sources supplied by mixing, entrainment, and ocean circulation. However, owing to the short residence time of iron in the upper ocean, this subsurface iron pool is critically dependent on continual replenishment from atmospheric dust deposition and, to a lesser extent, lateral transport from shelf regions.
Highlights
Atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1610 Global Change: Atmosphere (0315, 0325); 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: ecosystem model, nutrient limitation, iron cycle, phytoplankton community
Under strongly nutrient limiting conditions, ecosystems are often characterized by small pico- and nano-sized phytoplankton, strong grazing pressure from microzooplankton, high particulate organic carbon (POC) recycling, and low vertical carbon export
The mineral ballast produced by diatoms and coccolithophores may enhance carbon export to the deep ocean [Armstrong et al, 2002; Klaas and Archer, 2002]
Summary
[2] Growing appreciation for the key roles played in ocean biogeochemistry by phytoplankton functional groups and trace metal limitation is leading to a new generation of more sophisticated, coupled ocean biological, chemical, and physical models [e.g., Doney, 1999; Doney et al, 2001, 2003]. [3] Shifts in the size-structure and composition of phytoplankton communities can strongly influence carbon cycling in surface waters and export to the deep ocean. Coccolithophores are another common component of blooms, in temperate waters These phytoplankton form external platelets of calcium carbonate (CaCO3), a key process affecting carbonate cycling in the oceans that can influence surface water pCO2 concentrations and air-sea flux [Holligan et al, 1993; Robertson et al, 1994]. We assume that the dissolved iron pool is completely bioavailable to all phytoplankton without regard to speciation or ligand dynamics These assumptions are necessary at present to begin to incorporate iron dynamics into models of ocean biogeochemistry.
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