Hydrodynamic modelling and algal bloom tracking in Hong Kong's coastal waters in March-April 1998

Hong Kong lies on the coastal shelf of Southern China. Despite the frequent occurrence of red tides and associated fishkills over the past two decades, the relation between the water movement and algal blooms/red tides has not been studied. In this study, a calibrated three-dimensional hydrodynamic model, Delft3D, is used to study the tidal circulation in Hong Kong's coastal waters. Particular emphasis is placed on understanding the possible cause of the April 1998 massive red tide, and to develop a predictive harmful algal bloom (HAB) tracking model. Based on the tidal boundary conditions and the measured wind velocity during the period March - April 1998, the 3D flow field and surface drogue tracking for a release in different parts of the northeast coastal waters are computed. The results show that a bloom initiated in Mirs Bay would likely move along northeast to southwest direction and possibly be transported to the southeast coastal waters under the combined action of tidal current and wind. Computed drogue tracking patterns are generally consistent with the reported sequence of red tide events in Spring 1998. Extensive simulations show that the major cause of the bloom being transported into the East Lamina Channel (and causing severe fishkills) is the unusually strong wind in March 1998 coupled with the change of wind direction during almost diurnal tidal conditions at the beginning of April.