Evaluating the eReefs Great Barrier Reef marine model against observed emergent properties

Abstract

The eReefs marine models simulate hydrodynamics, sediment dynamics, biogeochemistry and optical conditions in three dimensions on 1 km and 4 km grid scales across the whole Great Barrier Reef (GBR) Lagoon. The models are designed to support management of the Great Barrier Reef through operational provision of near real-time hydrodynamic, water quality and optical conditions over the reef, as well as supporting prognostic scenarios to facilitate catchment management policy. During development, the hydrodynamic and biogeochemical models were calibrated and validated against remotely sensed temperature, in situ measured physical properties, and in situ sampled dissolved and particulate nutrient species at sparsely distributed monitoring sites, most of which are located in near-shore waters along the length of the GBR Lagoon. A range of evaluation metrics provided a baseline degree of confidence in the model results. An additional level of confidence can be achieved by evaluating the model's performance in reproducing a range of system-scale emergent properties. These are large-scale patterns and relationships that are neither directly coded into the model nor used in model calibration -ideally, relationships that are not obvious from an inspection of the model algorithms, but arise from the complex system that these algorithms combine to create. This paper describes the evaluation of the 4 km grid-scale eReefs marine models against a range of emergent properties. Properties used in this evaluation included (a) the relationship between chlorophyll a concentrations and phytoplankton community size-structure; (b) the relationship between river discharge and annual mean photic depth on the mid-and outer-shelf; and (c) the relationship between flood-plume optical class and water quality. The results show good agreement across all three sets of emergent properties, enhancing confidence that the eReefs suite of process-based models is producing "the right results for the right reasons," correctly simulating a complex range of physical and biogeochemical processes across the Great Barrier Reef.