Spatial and temporal variations in primary production and community structure of phytoplankton in relation to water column structure in the East/Japan Sea
- Spatial and temporal variations in primary production and community structure of phytoplankton in relation to water column structure in the East/Japan Sea
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- This dissertation examined the phytoplankton productivity and community composition on the variable ocean environment in the East/Japan Sea (hereafter, the EJS). Although, the EJS located the northwestern Pacific Ocean, is a small sea of mediterranean type and that is one of the most productive region. The EJS has dynamic physical processes such as thermohaline circulation, subpolar fronts, mesoscale eddies, and deep water formation. The physical processes determine water column properties and thereby control phytoplankton productivity and community composition. Therefore, the EJS is believed to be an ideal region for studying of oceanic processes as a nature laboratory. However, we have only several studies on phytoplankton dynamics in relation to physical forcing in the EJS. Accordingly, intense integrated field observations are necessary to improve our understanding of the current marine ecosystem in the EJS.
This dissertation contains three subjects as follows:
i) to better understand the phytoplankton productivity and community structure in the Ulleung Basin (UB) (chapter II).
ii) to figure out the mechanisms of high summer primary productivity in the Ulleung Basin (chapter III).
iii) to examine variations of phytoplankton productivity and community composition in relation to physical forcing (chapters IV and V).
The annual primary production in the Ulleung Basin were 273.0 g C m–2 y–1. New carbon production by phytoplankton is estimated as 145.6 g C m–2 y–1 which indicates that a large portion (53.9%) of total annual primary production might potentially be exported from the diatom-dominated euphotic zone to a deeper zone in the UB.
A subsurface maximum in primary productivity at or above the pycnocline and a high f-ratio further support the importance of this source of nitrate for maintaining the high summer primary productivity in the UB. In comparison, the high PP layer was observed at the surface in the following fall and spring in the UB. Our results demonstrate the importance of hydrographic features in enhancing PP in this oligotrophic Tsushima Warm Current system.
The CHEMTAX results using the marker pigments showed that diverse phytoplankton groups occupy in the EJS. The cluster and the canonical correspondence analyses well explained spatial variation of phytoplankton assemblage according to the water mass properties. Finally, these results suggest that dynamic structure of water column determines biomass and productivity of phytoplankton as well as their diverse size- and taxonomic groups in the EJS.
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