Seasonal and spatial dynamics of Noctiluca scintillans in a semi-enclosed bay in the northeastern part of Hong Kong

Seasonal and spatial patterns in the occurrence of Noctiluca scintillans , a dinoflagellate responsible for most of the red tides in the coastal waters of Hong Kong, were studied in Tolo Harbour and Mirs Bay from July 2003 to June 2005. N. scintillans density and chlorophyll a concentration were highest in the inner part of the semi-enclosed Tolo Harbour and decreased towards Mirs Bay, which is exposed to currents from the ocean. N. scintillans became abundant during winter and early spring, but densities recorded between December 2004 and April 2005 were much higher than those recorded between December 2003 and March 2004. The density of N. scintillans did not correlate with chlorophyll a concentration, but cell size tended to decrease as chlorophyll a concentration increased. Cells collected from Tolo Harbour were more likely to contain food than those collected from Mirs Bay. Dense populations of N. scintillans usually appeared first in Mirs Bay, but peak densities were always much higher in Tolo Harbour than in Mirs Bay. These observations suggest that N. scintillans was prevented from reaching high abundance in Mirs Bay by insufficient food and strong water currents. Conversely, nutritional and hydrographical conditions inside Tolo Harbour allow N. scintillans populations to reach high densities.