11:45 AM - 12:00 PM
[AOS15-11] Standing stock and species composition of planktonic copepods occurred in Harima Nada in the eastern Seto Inland Sea, Japan
Keywords:the Seto Inland Sea, zooplankton, copepod, seasonal variation
In the Seto Inland Sea, Japan, fish catches have been declining since the latter 1980s. One of the causative factors may be a decrease in primary production due to oligotrophic water, followed by a decrease in zooplankton community, which is a prey for fish. Detailed surveys for zooplankton have not been carried out in the Seto Inland Sea since the 2000s. As there is a definite lack of data set about zooplankton, we cannot clarify the cause of the decrease in catch. In this study, we carried out annual observations for the species composition and biomass of planktonic copepods, which play an important role as secondary producers in the ocean, in Harima Nada in the eastern Seto Inland Sea.
Monthly samplings were conducted at Stn. NH from April 2019 to December 2020. Ten liters of the seawater samples were collected using a water sampler at 0, 10, 20 m depth. Planktonic copepods were concentrated with a 100 µm mesh size of plankton net. Adult individual was identified to the genus level under a microscope and the total number (abundance: inds./m3) was calculated. The head-chest length was also measured and converted into biomass (mgC/m3). Chlorophyll a concentration of the seawater was also measured by fluorometry.
The genus Paracalanus, Oithona, and Microsetella occurred as predominate species of copepods. The total abundance and the biomass was in the range of 0.02 - 6.28 × 104 inds./m3 and 0.05 - 30.9 mgC/m3, respectively. Both of them were 2 - 10 times higher in summer-autumn than in winter-spring. The seasonal variation of abundance and biomass of copepods was similar to that of Chl a concentration (0.36 - 9.1 μg/L), indicating that the dynamics of copepods depend on the amount of prey organisms. Comparing the abundance and biomass of copepods in 1980 (Uye et al., 1987) and in 1994 (Uye and Shimazu, 1997) with the survey of this study, the abundance was similar, but the biomass tended to decrease. In addition, Paracalanus spp. still occurred as a dominant species throughout the year, but Calanus spp., relatively large species, was significantly reduced. It was considered that the abundance of planktonic copepods has not decreased, however, the biomass has been decreasing due to the decrease in large size of species.
Uye S, Kuwata H and Endo T (1987) J Oceanogr Soc Japan, 42, 421-434
Uye S and Shimazu T (1997) JO, 53, 529-538
Monthly samplings were conducted at Stn. NH from April 2019 to December 2020. Ten liters of the seawater samples were collected using a water sampler at 0, 10, 20 m depth. Planktonic copepods were concentrated with a 100 µm mesh size of plankton net. Adult individual was identified to the genus level under a microscope and the total number (abundance: inds./m3) was calculated. The head-chest length was also measured and converted into biomass (mgC/m3). Chlorophyll a concentration of the seawater was also measured by fluorometry.
The genus Paracalanus, Oithona, and Microsetella occurred as predominate species of copepods. The total abundance and the biomass was in the range of 0.02 - 6.28 × 104 inds./m3 and 0.05 - 30.9 mgC/m3, respectively. Both of them were 2 - 10 times higher in summer-autumn than in winter-spring. The seasonal variation of abundance and biomass of copepods was similar to that of Chl a concentration (0.36 - 9.1 μg/L), indicating that the dynamics of copepods depend on the amount of prey organisms. Comparing the abundance and biomass of copepods in 1980 (Uye et al., 1987) and in 1994 (Uye and Shimazu, 1997) with the survey of this study, the abundance was similar, but the biomass tended to decrease. In addition, Paracalanus spp. still occurred as a dominant species throughout the year, but Calanus spp., relatively large species, was significantly reduced. It was considered that the abundance of planktonic copepods has not decreased, however, the biomass has been decreasing due to the decrease in large size of species.
Uye S, Kuwata H and Endo T (1987) J Oceanogr Soc Japan, 42, 421-434
Uye S and Shimazu T (1997) JO, 53, 529-538