Conditions necessary for Chattonella antiqua red tide outbreaks

Amano, Kunihiko, Masataka Watanabe, Kunio Kohata, Shigeki Harada

Limnol. Oceanogr., 43(1), 1998, 117-128 | DOI: 10.4319/lo.1998.43.1.0117

ABSTRACT: Chattonella antiqua (Hada) Ono (Raphidophyceae) red tides have often been observed in the Seto Inland Sea, Japan, since the 1960s causing tremendous damage to fisheries. To estimate the conditions necessary for C. antiqua red tide outbreaks we developed an ecological model for diel vertical migration and nocturnal nutrient uptake. The model was expressed as vertically one-dimensional differential equations for PO3-, NO3-, NO4+, C. antiqua cell concentration, P cell quota, and N cell quota. To deal with diel vertical migration, the growth function was given by the Droop equation and nutrient uptake kinetics were given by the Michaelis-Menten equation. The model was calibrated with several sets of experimental data. Model calculations showed that the formation of a shallow nutricline (at ~6 m) was crucial for rapid growth of C. antiqua. Given that a shallow nutricline was formed, the calculation also suggested that other environmental factors have to be within certain ranges for C. antiqua red tides to occur. If the retention time was <6 d or the grazing pressure was more intensive than that caused by 40 ind. liter-1 of Paracalanus parvus, which feeds on C. antiqua, 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 during summer, seem to enhance the chances of a C. antiqua red tide outbreak by strengthening water column stratification and making the water stagnant. If this occurs after a shallow nutricline has formed and active 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 ~20-22°C), often due to intrusion of warm, saline Kuroshio Current water. The conditions predicted by our model as necessary for C. antiqua red tides are consistent with this hypothesis. Our model quantitatively supports the hypothesis that C. antiqua can grow to the level of red tides by exploiting the unique environmental conditions found in the Seto Inland Sea.

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