Dissolved Inorganic (Carbon, Nitrogen & Phosphorus) (GBR4 BGC baseline)

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    This set of visualisations represents estimates of Dissolved Inorganic Nitrogen (DIN), Dissolved Inorganic Carbon (DIC) and Dissolved Inorganic Phosphorus (DIP) from the 4 km resolution eReefs BioGeoChemical (BGC) model along the Queensland coast covering the Great Barrier Reef out into the Coral Sea.

    The primary production (plankton growth) in Australian coastal environments is generally limited by dissolved nitrogen in marine environments. Photosynthetic growth is determined by concentrations of dissolved nutrients (nitrogen and phosphate) and available light.

    The BGC model simulates the cycling of carbon, nitrogen, phosphorus, and oxygen across multiple compartments—such as phytoplankton, zooplankton, detritus, dissolved organic and inorganic substances—distributed over various water column and sediment layers. The model conservatively tracks these nutrient flows based on mechanistic equations.

    The ratio of carbon, nitrogen, and phosphorus in the organisms in the models is modelled as structural material with a Redfield ratio (O:C:N:P of 110:106:16:1 for plankton and animals; 554:550:30:1 for benthic plants) and a reserve within each organism. Growth is restricted by the limiting nutrient and light.

    More details about how DIN, DIC, and DIP are calculated in the model can be found in Baird et al. (2020)

    Dissolved Inorganic Nitrogen (DIN)

    Nitrogen is critical for the plant and animal growth. It is needed for protein synthesis, DNA and RNA, and a major component in the development of chlorophyll needed for photosynthesis. Nitrogen is cycled through organisms but is eventually converted back into nitrogen gas. In the sediment nitrogen in the porewaters is denitrified and lost as nitrogen gas.

    Within the GBR lagoon the main source of DIN is from land run off during flooding events. These flood plumes are generally stay close to the coast, pushed northwards by the wind. The nitrogen in the plumes leads to rapid growth of phytoplankton. This algal rich water absorbs light leading to less light reaching the seagrass meadows and inshore coral reefs (See Secchi depth, vertical attenuation at 490nm & light intensity above seagrass product) lowering their growth.

    In the Coral Sea the nitrogen levels in the surface waters of the ocean are generally very low as it is quickly taken up by the growth of phytoplankton. As these plankton die their detritus slowly sink and the nitrogen is released back into deeper waters. Westward ocean currents coming from the Coral Sea divide north of Cairns, leading to northern (Gulf of Papua Current) and southern currents (East Australian Current). These flow along the outer edge of the Great Barrier Reef and the continental shelf. On the outer edge of the ribbon reefs on the eastern side of Torres Strait and on the outer edge of the southern GBR these currents result in significant upwelling, drawing up nitrogen rich deeper water closer to the surface. This results in increased concentrations of DIN in these regions.

    Model specifics

    This visualisation shows the modelled Concentration of dissolved inorganic nitrogen (DIN) in mg N m-3. DIN is the sum of nitrate and ammonium concentrations, [NO3]+[NH4].

    The model contains two forms of dissolved inorganic nitrogen (DIN) used by photosynthetic organisms, dissolved ammonium (NH4) and dissolved nitrate (NO3). In the model, the ammonium component of the DIN pool is taken up first, followed by the nitrate, with the caveat that the uptake of ammonium is limited by diffusion.

    For nitrogen the main sources are river inputs, the atmosphere by nitrogen fixing Trichodesmium cyanobacteria and from upwelling of deeper nutrient rich waters. Nitrogen fixation (conversion of nitrogen gas to ammonium) occurs by trichodesmium algae when DIN is low (4 to 20 mg N m−3; Robson et al., 2013) and carbon and phosphorus are available to support nitrogen uptake.

    An assessment of the BGC model shows it has a skill of (bias ± Root Mean Square Error) of nitrate of −0.70±3.86 mg N m−3 and ammonium of −0.77±1.63 mg N m−3 (Baird e. al., 2020). This represents the difference between the model values and 36 long-term water quality monitoring sites along the Queensland coast.

    Dissolved Inorganic Phosphorus (DIP)

    Dissolved Inorganic Phosphorus is used for various biological functions including ATP synthesis and DNA. In the BGC model DIP is needed for growth of marine phytoplankton, benthic microalgae, trichodesmium, seagrass, coral and macroalgae. Zooplankton receive their phosphorus from consuming phytoplankton. As organisms die, they become detritus and the phosphorus is liberated as DIP. DIP converts to and from Particulate Inorganic Phosphorus (PIP) at a rate dependant on the level of oxygen. A proportion of the phosphorus is adsorbed into the sediment and is permanently immobilised.

    The phosphorus cycle is different to the nitrogen cycle as there is no exchange with the atmosphere. The final sink for phosphorus is the ocean sediment. The deeper ocean floor sediment is rich in phosphorus. Upwelling from the Coral Sea brings up nutrient-rich waters along the edge of the continental shelf.

    Model specifics

    This visualisation shows the modelled concentration of dissolved inorganic phosphorus (DIP), also referred to as orthophosphate or soluble reactive phosphorus, SRP, composed chiefly of HPO42- ions, with a small percentage present as PO43-.

    Assessment of the predicted DIP indicates that the model has a skill of (bias ± RMSE) of -0.88 ± 2.17 mg P m−3 (Baird e. al., 2020). This is an assessment of the model against the AIMS long term water quality monitoring sites.

    Dissolved Inorganic Carbon (DIC)

    The major pools of dissolved inorganic carbon (DIC) species in the ocean are HCO3-, CO3-, and dissolved CO2, which influence the speciation of H+ and OH− ions, and therefore pH.

    The change in the surface DIC concentration is dependent on the partial pressure of carbon dioxide in the atmosphere and the dissolved carbon dioxide concentration, which is in turn determined from the DIC and the total alkalinity. At pH values around 8, dissolved CO2 makes up only approximately 1∕200 of DIC in seawater.

    In the water column DIC is up taken by phytoplankton as part of photosynthesis and released as part of respiration.

    Model specifics

    The model contains two state variables to represent the state of carbon chemistry, dissolved inorganic carbon and alkalinity, which when combined with temperature and salinity allow the pH and aragonite saturation state to be calculated.

    Assessment of carbon chemistry properties along the entire length of the GBR shows a bias ± RMSE of DIC of -7.7 ± 34.2 mmol m−3 (Baird e. al., 2020).

    Reference

    Baird, M. E., Wild-Allen, K. A., Parslow, J., Mongin, M., Robson, B., Skerratt, J., Rizwi, F., Soja-Woźniak, M., Jones, E., Herzfeld, M., Margvelashvili, N., Andrewartha, J., Langlais, C., Adams, M. P., Cherukuru, N., Gustafsson, M., Hadley, S., Ralph, P. J., Rosebrock, U., … Steven, A. D. L. (2020). CSIRO Environmental Modelling Suite (EMS): scientific description of the optical and biogeochemical models (vB3p0). Geoscientific Model Development, 13(9), 4503–4553. https://doi.org/10.5194/gmd-13-4503-2020

    Robson, B. J., Baird, M. E., and Wild-Allen, K. A.: A physiological model for the marine cyanobacteria, Trichodesmium, in: MODSIM2013, 20th International Congress on Modelling and Simulation, edited by: Piantadosi, J. R. S. A. and Boland, J., Modelling and Simulation Society of Australia and New Zealand, 1652–1658, available at: https://www.mssanz.org.au/modsim2013/H3/robson.pdf (last access: 25 October 2023), 2013. 

    Source data

    The videos/images on this page are based on the 4km eReefs BioGeoChemical model (v3.1) (GBR4_H2p0_B3p1_Cq3b_Dhnd) run with SOURCE Catchments using Baseline catchment conditions. Detailed information about the model can be found in the paper: CSIRO Environmental Modelling Suite (EMS): Scientific description of the optical and biogeochemical models (vB3p0). The raw model data is available from the NCI THREDDS server (daily, in curvilinear NetCDF format) and the aggregate data from the AIMS eReefs THREDDS server (daily, monthly, yearly, in in regular rectangular grid NetCDF format).

    Data span

    These results are based on a fixed time period (Dec 2010 - Apr 2019) hind-cast analysis developed for comparing changes in land practices. The river run off used to drive the BGC model were provided by the SOURCE Catchments modelling.