Modelling buoyancy-driven vertical movement of Trichodesmium application in the Great Barrier Reef

Trichodesmium cells aggregate and form single trichomes or larger colonies and possess strong intracellular gas vesicles that generate strong positive buoyancy and facilitate the vertical migration of colonies. Trichodesmium is proposed to be an important source of nitrogen in the Great Barrier Reef (GBR) with implications for nutrient cycling and eutrophication. To understand the dynamics of Trichodesmium in the GBR ecosystem, reliable model predictions of Trichodesmium growth, nitrogen fixation and distribution are required. The sinking rates of Trichodesmium colonies have been reported to be dependent on the shape and size of colonies, and the orientation of colonies in seawater. Therefore, to better simulate the vertical movement of Trichodesmium in the GBR, and subsequent biogeochemical dynamics, the Trichodesmium processes in the eReefs biogeochemical model was modified by applying the form resistance factor to the sinking velocities of tuft-shaped Trichodesmium colonies. Our model results compare well with observations from the Australian Institute of Marine Science Marine Monitoring Program sensor network sites and capture the emergent patterns of phytoplankton size spectrum observed in nature. The modified model formulations improve the physiological realism of the Trichodesmium growth submodel of the eReefs marine biogeochemical models, and can help to improve the understanding of Trichodesmium dynamics for effective GBR water quality management.