Water quality and phytoplankton dynamics in Moreton Bay, south-eastern Queensland. II. Mathematical modelling.

Abstract

A coupled hydrodynamic water-quality model of Moreton Bay was developed to enable better management of nutrient loads and to predict eutrophication-related problems. The hydrodynamic submodel was calibrated to available tidal data, and the transport submodel was calibrated to a salinity dataset. A 15-month time-series of field data was used together with historical data to calibrate and validate the water-quality submodel. Model simulations suggest that denitrification removes over half the external N inputs with about one-third exported to the open ocean. Approximately 8% of N and 20% of the P loading accumulates in the sediments. Short-term variations due to hydrodynamic effects tend to mask the seasonal cycle in phytoplankton biomass except at inshore localities. The model predictions confirm the field data that indicate that water quality in the western bay is severely affected while the better flushed eastern region remains relatively unaffected. Future increases in nutrient loading due to population pressure are predicted to extend the severely affected region eastwards. Over most of the bay, algal productivity is N-limited except for the western margins where nutrients are saturating and light availability regulates growth. Model hindcasts suggest that system-wide mean algal production has increased by a factor of ten since European settlement.