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.

中文翻译：

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有害赤潮甲藻Karenia mikimotoi在水华衰退期的垂直分布

摘要 赤潮甲藻Karenia mikimotoi 一天可垂直迁移~20 m。这种特性使 K. mikimotoi 能够在广泛的深度范围内获取营养并获得最佳的辐照度。我们对水华下降阶段的昼夜垂直迁移 (DVM) 进行了日夜观测，并在日本西部的四个不同海湾对环境因素与 K. mikimotoi 水华之间的关系进行了每日每周调查。此外，我们使用来自本研究和先前研究的数据对 DVM 参数（例如迁移距离和环境条件）进行了相关分析，这些 DVM 在 K. mikimotoi 绽放的发育或静止阶段被观察到。对 K. mikimotoi 明显 DVM 的观察通常不存在，尽管 K. 一天中积累的 mikimotoi 因观察而异。洄游距离与花期下降的天数呈正相关，与垂直分层呈负相关。在 DVM 观察前 1-2 周，降水量低，辐照度高，溶解的无机氮 (DIN) 和磷 (DIP) 浓度不足以支持细胞生长。此外，水柱中最高 K. mikimotoi 细胞浓度深度处的平均 DIP 浓度以及在 DVM 观察期间根据 DIN 和 DIP 浓度估计的增长率显示与迁移距离呈正相关。在 DVM 观察期间，当 K. mikimotoi 细胞留在深水中时，接收到的光量不足以生长，当它们留在地表水中时会发生光抑制。结果表明，营养缺乏和强烈分层等不利条件可能会阻止 K. mikimotoi 获得 DVM 行为的好处，从而促进水华的崩溃。