Introduction

A simple vertical stability theory has been devised to determine the conditions necessary for red tide formation based on considerations on algal growth, loss and depth regulation in a water column. It is shown that red tides occur only under certain stable water column conditions and with sufficient nutrient and light supply. Furthermore, in very tranquil waters, motile species are more competitive in utilizing bottom nutrient. A criterion for predicting whether a red tide would be caused by motile or non-motile species is established.

Theory

1. Non-motile species

	For non-motile species, blooms are caused by rapid growth of the species. An environment favouring the growth of non-motile species is therefore considered. 　 Considering the effect of turbulent diffusion, sinking and growth/mortality on the algal concentration 　 Criterion for red tides caused by non-motile species is given by

2. Motile species
	For motile species, blooms are mainly caused by aggregation instead of growth. An environment favouring the depth regulation of motile species is therefore considered. Examinating the concentration profile, the condition for bloom formation (by aggregation) is

3. Competition threshold
	When the water body becomes very calm, the motile species become more competitive in utilising the bottom nutrient through downward migration

4. Triggering Nutrient Level
By considering the efficiency of the algae in utilising the sediment nutrient source, the red tide triggering nutrient level is shown to be around 100-200μg/L for nitrogen

Flow diagram of the red tide prediction model

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Verification

The model has been validated against 13 observed red tide occurrences at the two field monitoring stations in Kat O and Lamma Island (2000-2003). The model has also been verified using independent data from Yung Shue Au, Tolo Harbour. The theory predicts the triggering factors for red tides in Hong Kong, and helps explain the occurrence of spring dinoflagellate blooms in the northeastern waters and summer diatom blooms in the southern waters.

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