Temporal changes of zooplankton in the detention pond closed off in 1997 from Isahaya Bay, Kyushu, in relation to desalination

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Zooplankton surveys were made from May, 1997, to July, 2000, in the detention pond closed off from Isahaya Bay, an inlet of Ariake Sound, to follow changes in the zooplankton community concomitant with desalination after the closing of the dike in April, 1997. Mean salinity at the surface decreased from 27 to 3.9 psu in the four months after closing. Mean density of zooplankton (excluding protozoans) increased from 11.71-1 in May, 1997, to 225.71-1 in August, 1997, mainly due to increases in copepod nauplii and the rotifers Brachionus plicatilis and probably B. rotundiforms. Copepods consisted mostly of Oithona davisae (86%) and Acartia pacifica (10%) in May, 1997, but these had been completely replaced by brackish-water species, Sinocalanus tenellus (44%), Pseudo-diaptomus inopinus (35%), and Paracyclopina nana (19%) by August, 1997. With further desalination, the proportion of freshwater copepods, consisting mostly of Thermocyclops spp., increased. Zooplankton densities in the pond were comparable to those in other brackish waters until one year after closing. However, densities in the summers of 1998 and 1999 were significantly lower (<101-1) than in 1997 and 2000; food-limitation and the extremely low salinities (<1.0 psu) are discussed as possible causes of the low densities in 1998 and 1999. Occurences of marine copepods and the appendicularian Oikopleura sp. in 2000 suggest inflow of seawater through the gates of the dike.