Daily changes in the spatial distribution of the dinoflagellate Peridinium cunningtonii and the euglenoid Trachelomonas hispida in a shallow pond
Yamamoto, Yoshimasa; Nakahara, Hiroyuki
A field experiment on the daily changes in the spatial distribution of two freshwater red tide-forming species, the dinoflagellate Peridinium cunningtonii Lemmermann and the euglenoid Trachelomonas hispida (Perty) Stein et Deflandre, was carried out in a shallow artificial pond in the summer of 2003. These flagellates showed distinctive features of cell density and distributional behaviour. The pattern of horizontal distribution of P. cunningtonii in the surface was not consistent among the observation days. On a cloudy day, the surface cell density of P. cunningtonii remained low and was distributed uniformly throughout the day. When the daily integrated irradiance increased, the surface density increased either before the dawn or daytime, whereas the most heterogeneous horizontal distribution always occurred during the daytime (the maximum coefficient of variation values of cell density among stations occurred between 9:00-15:00). In late June, P. cunningtonii tended to accumulate near the surface at night. T. hispida showed a clearer pattern of daily changes in the surface cell density independent of the weather conditions. The surface cell density reached a maximum between 9:00-12:00, followed by a rapid decline, whereas daily vertical migration of this species was not clear because most of cells remained near the bottom throughout the day in June.