1982 Volume 30 Issue 1 Pages 1-9
Succeeding to the previous report, the diel vertical migration of main plankters was studied at the two fixed stations respectively 500 and 3, 500 meters apart from the outlet of warm water effluent in Uchiura Bay for a period of 24 hours from 12: 00, October 11, 1979 to the following day. Seven times sampling were carried out by vertical haul with a plankton-net (30 cm in diameter, 0.09 mm in mesh size and set with a filtration-meter) at the two fixed stations at every three meters from the surface to 15 meters depth. Simultaneously hydrographic observations were also made. The results obtained are summarized as follows:
1) Main zooplankters collected at the two stations were Paracalanus parvus, Euterpina actifrons, Oncaea venusta and nauplii of Balanidae. No definite difference in the diel migration of the former three species were observed between the natural condition and the area affected by the warm water effluent, while the other dominant planktonic animals as nauplii of Balanus sp. showed the utterly different diet migration between the two stations. Most of the nauplii population under the natural condition nearl the mouth of the bay showed “so-called” the typical twilight migration.
On the contrary, a larger part of the nauplii collected at 500 meters apart from the outlet did not show the typical migration, mainly suspending in the layer above 3 meters depth, probably more than approximately 25°C.
2) These phenomena mentioned above may indicate that the warm water effluent seems to restrict the sinking activity of certain planktonic animals toward the middle or bottom layers and to affect their early growth, development and metamorphosis.
