CONDITIONS NECESSARY FOR CHATTONELLA-ANTIQUA RED TIDE OUTBREAKS

Chattonella antiqua (Hada) Ono (Raphidophyceae) red tides have often b een observed in the Seto Inland Sea, Japan, since the 1960s, causing t remendous damage to fisheries. To estimate the conditions necessary fo r C. antiqua red tide outbreaks we developed an ecological model for d iel vertical migration and nocturnal nutrient uptake. The model was ex pressed as vertically one-dimensional differential equations for PO43- , NO3-, NO4+, C. antiqua cell concentration, P cell quota, and N cell quota. To deal with diel vertical migration, the growth function was g iven by the Droop equation and nutrient uptake kinetics were given by the Michaelis-Menten equation. The model was calibrated with several s ets of experimental data. Model calculations showed that the formation of a shallow nutricline (at similar to 6 m) was crucial for rapid gro wth of C. antiqua. Given that a shallow nutricline was formed, the cal culation also suggested that;other environmental factors have to be wi thin certain ranges for C. antiqua red tides to occur. If the retentio n time was <6 d or the grazing pressure was more intensive than that c aused by 40 ind. liter(-1) of Paracalanus parvus, which feeds on C. an tiqua, there was no chance for an outbreak in a bay of the Seto Inland Sea. Weak wind periods, which often occur in the Seto Inland Sea duri ng summer, seem to enhance the chances of a C. antiqua red tide outbre ak by strengthening water column stratification and making the water s tagnant. If this occurs after a shallow nutricline has formed and acti ve C. antiqua cells are present, which can be attributed to excystment at the bottom, then the conditions necessary for C. antiqua red tide outbreaks will be satisfied. Excystment occurs when temperatures rise (to similar to 20-22 degrees C), often due to intrusion of warm, salin e Kuroshio Current water The conditions predicted by our model as nece ssary for C. antiqua red tides are consistent with this hypothesis. Ou r model quantitatively supports the hypothesis that C. antiqua can gro w to the level of red tides by exploiting the unique environmental con ditions found in the Seto Inland Sea.