Prediction of the spreading of an unprecedented harmful algal bloom in South Australian gulfs

Using a coupled physical-biological model, this work investigates the spreading of an unprecedented harmful algal bloom that since March 2025 has been causing massive kills of marine animals in Gulf St. Vincent, which alongside Spencer Gulf is one of the two South Australian gulfs. The gulfs are large inverse estuaries with limited physical connectivity to the ambient coastal ocean. The biological model accounts for algal growth, mortality due to shear flow, a sink term representing other effects like zooplankton grazing, and diel vertical migration. The growth rate is based on the known physiology of Karenia mikimotoi , initially thought to dominate the algal bloom. For simplicity, limitations of algal growth due to lack of nutrients or oxygen are ignored. The model hindcast reproduces the timeline of the initial bloom spreading, which started in mid-March 2025 just outside Gulf St. Vincent, from where the bloom spread across this entire gulf within 2-3 months, not affecting Spencer Gulf. Seasonal cooling of gulf waters during winter reduces the growth rate and hence the cell densities of K. mikimotoi . Nonetheless, the worst- case model scenario for the next summer predicts spreading of the algal bloom to Spencer Gulf to become a persistent feature of both gulfs with high cell densities. Surprisingly and in contrast to the model prediction, actual cell counts of Karenia species in Gulf St. Vincent revealed the existence high concentrations >1000 cells/mL throughout winter months. Moreover. a recent laboratory analysis of water samples revealed that, while K. mikimotoi had the expected low cell densities, another Karenia species, known as K. cristata , dominated the algal bloom mix during winter. Additional model simulations address this missing Karenia species under the assumption that it grows well in colder gulf water during winter. • An unprecedented harmful algal bloom has evolved in South Australian gulfs since mid-March 2025 • A coupled physical-biological model is used to predict the spreading of this bloom. • The model hindcast largely reproduces the reported initial spreading of the bloom. • Model forecasts predict that the harmful algal bloom will become a persistent feature of both gulfs.