Using a weight-structured oyster population dynamic model to explore top-down control of coastal water quality in a subtropical embayment

The natural filtering capacity of oysters and other suspension filter-feeders has seen them put forward as a potential water quality management option. However, the specifics of how many oysters would be required to clean a system are not necessarily straightforward to evaluate because of the size-dependence of oyster physiological rates along with the dynamic coupling that exists between the oysters and the environment. We use a weight-structured shellfish population model and a nutrient-phytoplankton-detritus model to answer the question of how many oysters it would take to clean a large estuary located in Queensland, Australia. Modelling results indicate that improvements in the water quality are not seen until the stocking density of oysters exceeds 0.09 ind.m(-3) and that local water quality guidelines are not fully met until the density exceeds 1.80 ind.m(-3). At these densities, the corresponding times to filter the entire volume of the study area are 52-38 d (0.09 ind.m(-3)) and 1.80-1.35 d (1.80 ind.m(-3)), respectively. This research highlights that regulatory feedback pathways exist between a shellfish population and the water quality constituents that they control through filtration. While the use of oysters and other filter-feeders may be an appealing approach to nutrient management and top-down control of phytoplankton, the practicalities of deploying oysters at a system-scale may be the greatest barrier to this option.