Relationship between the bloom of Noctiluca scintillans and environmental factors in the coastal waters of Sagami Bay, Japan

In order to study the mechanism of the bloom formation of Noctiluca scintillans, the relationship between the variation in the abundance of N. scintillans and environmental factors was examined in the coastal waters of Sagami Bay, Japan. Hydrographic (temperature, salinity, water stability), biological (chlorophyll a concentration, zooplankton biomass) and meteorological (rainfall, wind velocity, wind direction) factors were investigated from 1997 to 2004. For all years, the abundance of N. scintillans started to increase from March and reached a maximum in spring between April and May. The abundance in 1997 and 2000 was relatively high compared to the other years while the abundance in 1998 and 2004 was relatively low. A stepwise multiple linear regression analysis showed that the wind direction and rainfall were significantly correlated with the variation in the abundance of N. scintillans. Our results suggest that bloom formation can be separated into a three-step process: (i) initial increase in the abundance of N. scintillans attributed to an increase in optimum hydrographic and biological factors, (ii) N. scintillans is then accumulated by convergence of seawater by the factors of low rainfall and wind and (iii) swarmer-effects suggested enhanced bloom formation. Accumulation is considered to be a key trigger in this process of the formation of large-scale blooms.