Title Analysis of the community structure of meso-zooplankton in the Oyashio and Transition Zone using Video Plankton Recorder (VPRII)
Authers Tadafumi ICHIKAWA
Keywords Video Plankton Recorder(VPR), zooplankton, community structure, Hydrozoa, Ctenophora
Citation Bull. Fish. Res. Agen. No.24, 23-104, 2008
Accurate estimation of the biomass, community structure and relative distribution in the water column of meso-zooplankton that play various important roles ranging from the cycling of matter, biological production, and as prey of fish resources is essential for understanding the dynamics of the marine ecosystem. However, conventional methods such as using plankton nets have various biases in relation to estimating the biomass and the distribution structure, especially for fragile plankton groups. Apart from these potential biases of conventional methods, sample sorting is time consuming and laborious. To overcome such problems, the aim of this study was to establish the observational and analytical methods using a new Video Plankton Recorder (VPRII), which has previously not been used in Japan, and with the results, to allow a more accurate estimation of the biomass of Cnidaria and Ctenophore, which are important predators. Furthermore the detailed vertical distributed structure of meso-zooplankton such as Copepoda, and then to discuss the community structure and biological interrelationship of meso-zooplankton.

1. To determine the conditions for obtaining clear images using VPRII and to review the methods needed for obtaining quantitative data, laboratory experiments using copepods and jellyfish were performed. The suitable setting range of parameters was found to be different between copepods and jellyfish, and the depth of field was different for the two plankton taxa when the best setting range of parameters for each taxon was used. Since our ROI (region of interest)extraction procedure could select the best setting range of parameters and could sort out the recorded images of plankton in a given depth of field determined by the brightness gradient of the image, VPRII can acquire in-focus and clear plankton images and can provide a better estimate of the plankton abundance including gelatinous taxa by setting up the parameters to record the out-focus objects in the field of view. Development of the method within the constant image volume objectively allowed the acquisition of high precision quantitative VPRII data.

2. To assess the utility of the VPRII for determining the zooplankton population structure, the images collected from VPRII were compared with the traditional zooplankton sampling method of MOCNESS in the Oyashio and Transition Zone off the Sanriku coast, eastern part of Japan. The correlations between the abundance measured by VPRII images and MOCNESS samples of Copepoda that were undamaged by MOCNESS sampling were statistically significant (p<0.5) when the probability of non-detection was less than 0.05 with VPRII. However for fragile members of the community, the Hydrozoa and Ctenophora abundances estimated from VPRII images were almost all higher than the MOCNESS samples. Chaetognatha, Siphonophora, Thaliacea-Doliolida, and Appendicularia showed more variability where the density was higher from VPR than MOCNESS and vice versa. This variability in results was considered to be affected by the patchy distribution pattern and escape behavior of plankton, also by technical problems such as visual field and illumination of the VPRII.

3. To examine and compare the biomass and detail the vertical distribution of Hydrozoa and Ctenophora, VPRII and plankton net were towed from 25 to 500m depth during July 2003 and June to July 2004 at 5 stations in the Oyashio and Transition zone off the Joban, Sanriku and Eastern Hokkaido coast, Japan. In comparison of Hydrozoa and Ctenophora biomass estimated from VPRII images and plankton net samples averaged over 25 to 500m, biomass estimates from VPRII were from 0.002 to 0.106 mgC / m3 (mean ± standard deviation = 0.035 ± 0.031, n = 40)and these biomasses were 14 times higher (0.3 to 260, standard deviation = 43.8, n = 40)than estimates using the plankton net samples, showing that the results by conventional net collection underestimated the biomass by ca. 1 order of magnitude. The ratio of Hydrozoa and Ctenophora to Copepoda biomass was from 0.01 to 5.13%. The depth over 1% corresponded to the distribution maximum layer of Hydrozoa or Ctenophore. The proportion of food requirement of Hydrozoa and Ctenophore for the production of Copepoda was as low as 1.5% to 5.4%; however, the role of patchiness of jellyfish in relation to the estimate of biomass requires further study.

4. We obtained the detailed vertical distribution of zooplankton community such as Hydrozoa, Ctenophore, and Copepoda in the Oyashio and Transition Zone, and discussed the effects of water mass structure on the zooplankton community and the biological interrelationship. The vertical distribution of Copepoda was continuous, and multiple distribution maximum layers occurred within the North Pacific Intermediate Water (NPIW)in the deeper points from the surface and discontinuity layer, while the vertical distributions of Hydrozoa and Ctenophore were intermittent and showed a more patchy distribution. The distribution maximum layer of Copepoda within NPIW occurred deeper than the salinity and water temperature minimum (deeper than 26.7σθ), and tended to appear in the near 26.8σθ and directly under the water mass where water mass was considered to be horizontally advected. Radiolaria showed the same distribution as Copepoda within NPIW. The results of the vertical distribution of suspended particles and review of past study cases demonstrated that the distribution maximum of Copepoda in the intermediate layer was probably affected by the water mass structure as well as Radiolaria and suspended particles, and might be related with the trophic environment. Observations using VPRII showed that the zooplankton community structure can be analyzed in new view point.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull24/ichikawa.pdf