2016 Volume 63 Issue 1 Pages 1-7
The biomass (wet weight) of the mesozooplankton in Mikawa Bay, a eutrophic bay of central Japan, was investigated monthly at four stations in the bay from 1981 to 2010, using 0.33-mm mesh plankton nets. The copepod communities and their abundance in these samples were also examined at the four stations during 1981–1987 and at a single station during 2000–2006. The annual mesozooplankton biomass was significantly greater in the 2000s than it was in the 1980s–1990s. Temora, which was the dominant copepod, increased markedly from the 1980s to the 2000s.
The annual means of mesozooplankton biomass and copepod abundance, and the means calculated during the summer months (July–September) when the hypoxic area generally developed, were not significantly correlated with the surface water temperature or levels of chlorophyll a. However, the means of the mesozooplankton biomass and copepod abundance calculated during the summer months were significantly negatively correlated with the hypoxic area of the bay. In June and July 1984, when the hypoxic area was well developed, the combined production rates of the three dominant copepod families—Acartiidae, Paracalanidae, and Corycaeidae—which composed more than 95％ of the copepod community, were estimated to be 5.33 (June) and 0.95 (July) mg C m-3 day-1, respectively. The rates corresponded to only 20.9％ and 5.5％, respectively, of the daily feeding rates of Japanese sardines in these months. This indicated that copepods were not adequate prey for the pelagic fish. The present results provide evidence that the development of the hypoxic area in Mikawa Bay results in a reduction in zooplankton biomass and causes shortage of food for zooplanktivorous pelagic fish.
