Effects of diffusion and upwelling on the formation of red tides

In this paper, records on the timing and location of specific red tides monitored once or twice a week in Mikawa Bay, Japan, are related to horizontal and vertical mixing rates determined from a numerical model. Horizontal (Kh) and vertical (Kz) diffusion coefficients, and upwelling veloc- ities, were estimated using a box model analysis. In the wind-mixed period and in the upper layer during the stratified period, Kh was estimated to be of the order of 102 m2 s-1. During the stratified period, Kz was estimated to be of the order of 10-5 m2 s-1. The upwelling velocity was calculated to be in the range 0.35-5.1 m day-1 with an average of 1.5 m day-1. Comparison between the literature values of the specific growth rate (µ) of the red tide-forming diatoms and calculated Kh values during the red tides show that diatoms which have a low µ cannot form red tides in a strongly diffusive environment, while species having a high µ can form red tides even in a strong diffusive environment. On the other hand, no clear relationship was found between µ of the flagellate group and Kh, although the flagellate group formed red tides even in severe diffusive conditions. From the comparison between the literature values of sinking rate and swimming speed and the physical parameters associ- ated with vertical processes, it was concluded that flagellates will form red tides, even in severe diffusive conditions, by using their swimming ability, while diatoms form red tides by their high growth rates with the aid of vertical diffusion and the upwelling movement of water.