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Dissolved inorganic nitrogen, nitrate & ammonia (GBR4 BGC v3.1 baseline)

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    This visualisation shows modelled nitrogen levels from the CSIRO eReefs Biogeochemical model. Nitrogen is a key limiting nutrient in the growth of plants and algae. Coral reefs are generally nutrient poor environments with much of the nutrients being locally recycled. Increased nutrient levels promote the growth of phytoplankton, turf algae and macroalgae, inhibiting the rate of recovery of coral reefs.

    The BGC model tracks the transformation of nitrogen through the nitrogen cycle, from fixing of nitrogen gas into ammonia by tricodesmium, to its uptake by plants, propagation through the food chain to zooplantkon, to release due to mortality and finally denitrification back into nitrogen gas.

    The BGC model contains two forms of dissolved inorganic nitrogen (DIN), dissolved ammonia (NH4) and dissolved nitrate (NO3): DIN = [NH4] + [NO3]. In the model, the ammonia component of the DIN pool is assumed to be taken up first by all primary producers (phytoplankton, tricodesmium, macroalgae and seagrass), followed by the nitrate, with the caveat that the uptake of ammonia cannot exceed the diffusion limit for ammonia. The underlying principle of this assumption is that photosynthetic organisms can entirely preference ammonia, but that the uptake of ammonia is still limited by diffusion to the organism’s surface.

    Nitrification is the oxidation of ammonia (NH4) with oxygen, to form nitrite followed by the rapid oxidation of these nitrites into nitrates (NO3). In the BGC model nitrification occurs in the water column and sediment depending on oxygen levels.

    Photosynthetic growth is determined by concentrations of dissolved nutrients (nitrogen and phosphate) and photosynthetically active radiation. Autotrophs (seagrass, phytoplankton, macroalgae and tricodesmium) take up dissolved ammonium, nitrate, phosphate and inorganic carbon. Ammonia is preferentially taken up as a source of nitrogen over nitrate as it can be directly used by plants. Nitrogen is transported through the phytoplankton to zooplankton food chain. Nitrogen is released as ammonia when an organism decays. Tricodesmium fixes nitrogen, converting nitrogen gas N2 into ammonia NH3 when the DIN concentration falls below a critical concentration.

    Dissolved Inorganic Nitrogen

    Concentration of dissolved inorganic nitrogen (DIN). DIN = [NH4] + [NO3].


    Concentration of nitrate. In the absence of nitrite [NO2-] in the model, nitrate represents [NO3-] + [NO2-].


    Concentration of ammonia ions [NH4+].