Seamless water quality modelling under extreme weather conditions: Linking land nitrogen outputs to coastal ecosystem health

Details

Supervisors

Hanert, Emmanuel ; De Le Court, Miguel

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en sciences et technologies de l'environnement, à finalité spécialisée

Abstract

enTropical cyclones can generate major hydrological and biogeochemical disturbances in coastal ecosystems through extreme rainfall, river discharge, and associated nutrient inputs. These processes are particularly relevant for the Great Barrier Reef (GBR), where coral reefs and seagrass meadows are exposed to variations in coastal water quality. This study investigates how TC-associated nitrogen inputs and hydrodynamic processes shaped phytoplankton dynamics during and following TC Jasper and TC Kirrily in the 2023–24 wet season, and the resulting potential ecosystem exposure. A three-dimensional hydrodynamic model of the GBR region was developed using SLIM3D and coupled to a simplified NPZD biogeochemical model incorporating dissolved inorganic nitrogen inputs from four rivers affected by the two cyclones. Model performance was evaluated against hydrodynamic observations, satellite-derived chlorophyll-a, and in situ DIN measurements. Simulated phytoplankton concentrations were then used to quantify cumulative exposure of coral reefs and seagrass meadows, complemented by a River–No-river sensitivity experiment. The hydrodynamic model reproduced the main regional circulation and hydrographic structures, whereas the biogeochemical model underestimated the observed post-Jasper coastal DIN enrichment and phytoplankton response. Nevertheless, contrasting exposure patterns emerged between the two cyclone periods. Exposure increased strongly in the northern GBR following Kirrily, with mean values in Cape York South reaching 123.3 mmol N m^-2 d for coral reefs and 103.7 mmol N m^-2 d for seagrass meadows, compared with 47.7 and 21.4 mmol N m^-2 d, respectively, following Jasper. This intensification coincided with an episodic intrusion of DIN-rich subsurface waters and hydrodynamic conditions potentially favouring local retention. Overall, cyclone-driven phytoplankton exposure in the GBR appears to result from the interaction of riverine nitrogen inputs, the pre-existing biogeochemical state, and hydrodynamic processes controlling nutrient supply, transport, and retention.