Seasonal changes of zooplankton community at five stations in the enclosed Honjyo area, which may be reclaimed in the future, and its neighboring waters of Lake Naka-umi, Shimane Prefecture, western Japan, were investigated monthly from July 1997 to June 1998. The number of taxa found in this survey (excluding larval plankton) was approximately 40 (tintinnids 2 spp.; hydrozoans over 2 spp.; rotifers over 3 spp.; cladocerans 4spp.; copepods over 23 spp.; mysids 2 spp.; chaetognaths 1 sp.; appendicularians 1 sp.). These were composed of brackish and eutrophic neritic zooplankters.
No freshwater zooplankters occurred. The composition and abundance of zooplankters greatly changed with seasons.
The Honjyo area and a channel connected to it were characterized by high abundances of copepods, Oithona davisae (November and December) and Acartia sinjiensis (August and September), and rotifers, Brachionus spp. (August), compared with those at other stations. Since the latter two taxa are typical indicators of brackish waters, these areas may assume more brackish waters. The high abundance (up to 1.7 × 10^6inds./m^3) of O. davisae in these stations may have been caused by stagnant water in the area, high abundance of prey phytoplankton, low predation pressure, and decreasing of competitors. Adults of O. davisae were more abundant constantly through the year (over 80%) than juveniles, which may have be related to rapid growths of the population in November/December (and June at a station of the Honjyo area).
Resting eggs of cladocerans Penilia and Podon/Evadne a copepod Eurytemora pacifica were found in sediments in the Honjyo area in summer. The density of cladoceran resting eggs corresponded nearly to that in the Seto Inland Sea, western Japan (3～5×10^3 eggs/m^2). That of E.pacifica was about 1.1×10^5 eggs/m^2. The presence of these resting eggs suggests that their life cycles are completed inside the area.