Effects of diffusion and upwelling on the formation of red tides |
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Authors: | Yamamoto Tamiji; Okai Mitsuru |
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Institution: | Faculty of Applied Biological Science, Hiroshima University, Higashi-Hiroshima 739-8528, Japan |
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Abstract: | In this paper, records on the timing and location of specificred tides monitored once or twice a week in Mikawa Bay, Japan,are related to horizontal and vertical mixing rates determinedfrom a numerical model. Horizontal (Kh) and vertical (Kz) diffusioncoefficients, and upwelling velocities, were estimated usinga box model analysis. In the wind-mixed period and in the upperlayer during the stratified period, Kh was estimated to be ofthe order of 102 m2 s1. During the stratified period,Kz was estimated to be of the order of 105 m2 s1.The upwelling velocity was calculated to be in the range 0.355.1m day1 with an average of 1.5 m day1. Comparisonbetween the literature values of the specific growth rate (µ)of the red tide-forming diatoms and calculated Kh values duringthe red tides show that diatoms which have a low µ cannotform red tides in a strongly diffusive environment, while specieshaving a high µ can form red tides even in a strong diffusiveenvironment. On the other hand, no clear relationship was foundbetween µ of the flagellate group and Kh, although theflagellate group formed red tides even in severe diffusive conditions.From the comparison between the literature values of sinkingrate and swimming speed and the physical parameters associatedwith vertical processes, it was concluded that flagellates willform red tides, even in severe diffusive conditions, by usingtheir swimming ability, while diatoms form red tides by theirhigh growth rates with the aid of vertical diffusion and theupwelling movement of water. |
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