5:15 PM - 6:45 PM
[MIS22-P14] Zooplankton communities offshore Joetsu, Japan: horizontal and vertical characteristics of structure, trophic levels, and diet
Surface-type methane hydrates exist on the seafloor and in mud at depths exceeding 500 m offshore of Joetsu, Niigata province, Japan. Studies have focused on their potential reserves, production technology, and the environmental impact of their extraction. This study delves into the horizontal and vertical characteristics of the structure, trophic levels, and diet of zooplankton communities—an important food source for fisheries— to obtain baseline data for environmental impact assessments in the offshore area of Joetsu.
Zooplankton samples were collected at four stations, including a methane-hydrate-bearing area, using an Indian Ocean Standard Net in June 2022. The sampling depth comprised six layers. Plankton net samples were filtered through a 1-mm mesh net, and organisms retained on the net were classified as large zooplankton (LZ), whereas those passing through the mesh net were classified as small zooplankton (SZ). For community structural analysis, the zooplankton samples were preserved in 10% formalin buffered with seawater, identified according to species, and counted using a microscope. Zooplankton samples taken for stable isotope (δ15N and δ13C) and fatty acid analyses were concentrated from the SZ and LZ samples on a 10-µm filter, stored at −80 ℃, and analyzed via isotope ratio mass spectrometry and gas chromatography. The vertical profiles of water temperature and salinity were measured using a conductivity, temperature, and depth sensor. We performed a series of multivariate analyses to identify differences in zooplankton communities between the samples and generated separate data using similarity matrix and silhouette value for some groups. To identify the species indicative of each cluster group, indicator species analysis was performed.
The composition of biomass-based zooplankton communities was classified into five groups (S1–3; L1–2) by silhouette values. S1–3 featured only SZ samples. S1, S2, and S3 comprised samples collected at depths of 0–100, 100–200, and 200–1500 m, respectively. L1 and L2 contained only LZ samples, comprising samples collected from 0–200 and 200–1500 m, respectively. S1, S2, and L1 prevailed in the Tsushima Warm Current Water, and S3 and L2 prevailed in the Japan Sea Proper Water. Physonectae was the most common indicator species in S1, Oithona spp. in S2, Pseudocalanus minutus in S3, Sagitta nagae in L1, and Paraeuchaeta elongata in L2.
The δ15N and δ13C values were high in groups distributed at deeper depths. While the δ15N values were comparable between zooplankton sizes at the same depth, greater values of δ13C were observed in the LZ groups (L1–2) than in the SZ groups (S1–3). This result implies that the trophic levels of zooplankton communities at the same depth do not differ between sizes, but the organisms sustaining zooplankton communities may differ.
The groups appearing at deeper depths had lower saturated fatty acid levels but higher monounsaturated fatty acid and polyunsaturated fatty acid levels. The content of unsaturated fatty acids in the LZ groups was higher than in the SZ groups. The carnivorous index calculated from the fatty acid composition was higher in the LZ groups than in the SZ groups at identical distribution depths. The dinoflagellate/diatom index was higher in the SZ groups than in the LZ groups in the 0–200 m layer but lower in the 200–1500 m layer. These differences in trophic indices between zooplankton sizes at the same depth may reflect differences in prey organisms.
In this study, there were no clear differences in the structure, trophic level, or diet of zooplankton communities in the methane-hydrate-bearing area compared with those in other coastal and open water areas in early summer. Given that environmental conditions and zooplankton communities vary seasonally, future surveys should be conducted across seasons.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry, Japan.
Zooplankton samples were collected at four stations, including a methane-hydrate-bearing area, using an Indian Ocean Standard Net in June 2022. The sampling depth comprised six layers. Plankton net samples were filtered through a 1-mm mesh net, and organisms retained on the net were classified as large zooplankton (LZ), whereas those passing through the mesh net were classified as small zooplankton (SZ). For community structural analysis, the zooplankton samples were preserved in 10% formalin buffered with seawater, identified according to species, and counted using a microscope. Zooplankton samples taken for stable isotope (δ15N and δ13C) and fatty acid analyses were concentrated from the SZ and LZ samples on a 10-µm filter, stored at −80 ℃, and analyzed via isotope ratio mass spectrometry and gas chromatography. The vertical profiles of water temperature and salinity were measured using a conductivity, temperature, and depth sensor. We performed a series of multivariate analyses to identify differences in zooplankton communities between the samples and generated separate data using similarity matrix and silhouette value for some groups. To identify the species indicative of each cluster group, indicator species analysis was performed.
The composition of biomass-based zooplankton communities was classified into five groups (S1–3; L1–2) by silhouette values. S1–3 featured only SZ samples. S1, S2, and S3 comprised samples collected at depths of 0–100, 100–200, and 200–1500 m, respectively. L1 and L2 contained only LZ samples, comprising samples collected from 0–200 and 200–1500 m, respectively. S1, S2, and L1 prevailed in the Tsushima Warm Current Water, and S3 and L2 prevailed in the Japan Sea Proper Water. Physonectae was the most common indicator species in S1, Oithona spp. in S2, Pseudocalanus minutus in S3, Sagitta nagae in L1, and Paraeuchaeta elongata in L2.
The δ15N and δ13C values were high in groups distributed at deeper depths. While the δ15N values were comparable between zooplankton sizes at the same depth, greater values of δ13C were observed in the LZ groups (L1–2) than in the SZ groups (S1–3). This result implies that the trophic levels of zooplankton communities at the same depth do not differ between sizes, but the organisms sustaining zooplankton communities may differ.
The groups appearing at deeper depths had lower saturated fatty acid levels but higher monounsaturated fatty acid and polyunsaturated fatty acid levels. The content of unsaturated fatty acids in the LZ groups was higher than in the SZ groups. The carnivorous index calculated from the fatty acid composition was higher in the LZ groups than in the SZ groups at identical distribution depths. The dinoflagellate/diatom index was higher in the SZ groups than in the LZ groups in the 0–200 m layer but lower in the 200–1500 m layer. These differences in trophic indices between zooplankton sizes at the same depth may reflect differences in prey organisms.
In this study, there were no clear differences in the structure, trophic level, or diet of zooplankton communities in the methane-hydrate-bearing area compared with those in other coastal and open water areas in early summer. Given that environmental conditions and zooplankton communities vary seasonally, future surveys should be conducted across seasons.
This study was conducted as part of the methane hydrate research project funded by the Ministry of Economy, Trade and Industry, Japan.