The Currents in Mirs Bay and the Drogue Tracking of 1998 Red Tide in Hong Kong Coastal Waters

In sub-tropical coastal waters around Hong Kong, year 1998 spring massive harmful algal blooms (HABs) invaded the northeaster and southern coastal waters of Hong Kong. Apart from multiple environmental reasons, diurnal tidal and strong wind caused surface currents with persistent NE to SW direction are the crucial causes of the HABs. The detailed clockwise gyre in northeast part of Mirs Bay is found and it lasted for one week in early April 1998. The consistent clockwise gyre created a ‘grey strip’ right from north part of Mirs Bay to Nan Au area then lead towards east of Lamma Channel. In the region, the surface current is much stronger than usual. A comparison has been done between 1998 and dry season in general by looking at the group drogue tracking and spreading of a dye tracer. These give more replicable conclusion of algal bloom movement in spring 1998 comparing to that of the typical dry season. A bloom originated in the northeaster waters of Hong Kong in the wet season would unlikely have a significant impact on the fish farms to the south of Hong Kong waters.