Abstract
A field investigation was carried out to examine the seasonal changes and vertical distribution of picophytoplankton in 15 water resource reservoirs in Japan. From the results obtained, the maximum abundance of picophytoplankton was above 105 cells·ml-1 in all reservoirs where their trophic states were varied from oligotrophic types to eutrophic ones. Picophytoplankton abundance reached maximum in the summer season, and the dominant types were found to be two types of cyanobacterial plankton which contained phycoerthrin (PE) or phycocyanin (PC) as a main photosynthetic pigment. Cyanobacterial picophytoplankton dominancy was related to the T-P concentration of reservoirs water. PE-rich type was dominant in the reservoirs where T-P concentration was relatively low, whereas PC-rich type was dominant in the reservoirs where T-P was relatively high.
Vertical profile of picophytoplankton in reservoirs showed that almost cells were distributed in the surface layers. This observation was different from natural lakes which commonly show abundance peak in more deep layers.
As the abundance of picophytoplankton slightly increase with the increase of T-P concentration, the proportion of picophytoplankton to the total phytoplankton by Chlorophyll-a was varied widely, and showed large values (30-45%) in the oligotrophic reservoirs as comparing with those in eutrophic ones.
