Vertical distribution of a harmful red-tide dinoflagellate, Karenia mikimotoi, at the decline stage of blooms

Abstract

The red-tide dinoflagellate Karenia mikimotoi can vertically migrate ~20 m in a day. This property enables K. mikimotoi to acquire nutrients over a wide range of depths and to receive optimal irradiance. We conducted day and night observations of diurnal vertical migrations (DVMs) during the declining stage of blooms and daily-weekly investigations of the relationship between environmental factors and a K. mikimotoi bloom in four different bays of western Japan. Moreover, we performed correlation analysis between DVM parameters, such as migratory distance and environmental conditions, using data from the present study and previous studies, which DVMs were observed at the developmental or stationary stage of K. mikimotoi bloom. Observations of evident DVMs of K. mikimotoi were commonly absent, although the depths where K. mikimotoi accumulated throughout the day differed among observations. Migratory distance showed a positive correlation with the number of days it took for the bloom to decline and a negative correlation with the vertical stratification. Precipitation was low, irradiance was high, and dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations were inadequate for cell growth 1–2 weeks before the DVM observations. Moreover, average DIP concentration at the depth of the highest K. mikimotoi cell concentration in the water column and the growth rate estimated from DIN and DIP concentrations during the DVM observations showed positive correlations with migratory distance. During DVM observation, the amount of light received was inadequate for growth when K. mikimotoi cells remained in deep waters, and photoinhibition occurred when they remained in surface waters. The results suggest that adverse conditions, such as nutrient deficiency and strong stratification, may prevent K. mikimotoi from achieving the benefits of DVM behavior and thereby promote the collapse of a bloom.