[A-OS17] Ocean circulation and material cycle in coastal seas

[AOS17-P09] Standing stock and species composition of planktonic copepods occurred in Harima Nada in the eastern Seto Inland Sea, Japan

*Hamazaki Yuya¹, Masatoshi Nakakuni², Hitomi Yamaguchi², Kuninao Tada^2,3, Kazuhiko Ichimi^2,3 (1.The United Graduate school of Agricultural Science, Ehime University, 2.Faculty of Agriculture, Kagawa University, 3.Seto Inland Sea Regional Research Center, Kagawa University)

Keywords:Seto Inland Sea, Harima-Nada, 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 decreasing nutrient, 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 2022. Ten liters of the seawater samples were collected using a water sampler at 0, 10, 20, 30 m depth. Planktonic copepods were concentrated with a 100 µm mesh size of plankton net. Adult and copepodite individual were identified to the genus level under a microscope and the total number (abundance: inds. m^-3) was calculated. The ploseme length was also measured and converted into biomass (mgC m^-3).
The genus Paracalanus spp., Oithona spp., and Microsetella sp. occurred as predominate species of copepods. The total abundance and the biomass were in the range of 0.02–6.28 x 10⁴ inds. m^-3 and 0.047–61.6 mgC m^-3, respectively. Paracalanus spp. appeared though the years. Oithona spp. appeared in the spring and winter. Microsetella sp. appeared in the summer and fall. 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 total abundance of them was 0.83–1.4 x 10⁴ inds. m^-3 in the early 1990s, while, it was 0.20–1.8 x 10⁴ inds. m^-3 in this study, which shows the abundance was similar or slightly decreasing trend. On the other hand, the total biomass of them was 2.4–25 mgC m^-3 from the late 1970s to early 1980s and in early 1990s, while it was 1.5–12 mgC m^-3 in this study, which shows the biomass was clearly decreasing trend. In addition, Calanus spp., 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.

Reference
Uye, S., Kuwata H. and Endo, T.: Journal of Oceanography, 42, 421–434 (1987)
Uye, S. and Shimazu, T.: Journal of Oceanography, 53, 529–538 (1997)