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We investigated the occurrences of Noctiluca scintillans in the coastal waters of Harima-Nada, the Seto Inland Sea, using the red tide data set. These data concerned red tide occurrences that were reported in an annual report entitled “Red tides in the Seto Inland Sea” which was published by the Seto Inland Sea Fisheries Coordination Office of the Fisheries Agency, Ministry of Agriculture, Forestry and Fisheries of Japan. These data were imported into a geographic information system, and red tide areas were calculated. During the 26 years from 1979 to 2004, in the Seto Inland Sea, eutrophication improved, nutrient concentrations in seawater decreased dramatically, and the number of red tide occurrences excluding those concerning Noctiluca scintillans, decreased. However, the number of occurrences, area, and durations of Noctiluca scintillans red tides did not decrease. The ratio of Noctiluca scintillans red tides to total red tides for an average red tide index, which permits the assessment of spatial and temporal scales, was 54%. This was higher than that of the number of occurrences (36%). This indicates the importance of N. scintillans as a red tide organism. [ABSTRACT FROM AUTHOR]

We investigated the seasonal variations and horizontal distributions of the zooplankton community of Shinkawa-Kasugagawa, a well-studied estuarine tidal flat in the eastern part of the Seto Inland Sea, to clarify planktonic productivity during a flooding tide. We estimated carbon flux, including benthic and planktonic primary producers, copepods, nauplii, tintinnids and naked ciliates, considering each trophic level. We also evaluated the trophic transfer efficiency of the tidal flat in this area. The annual averages of total zooplankton biomass were 17.6, 11.7 and 9.0 mg C m-3 in the upper, lower and subtidal zones, respectively; these zones were defined based on environmental factors. The horizontal distribution and community compositions differed among the three tidal areas, especially for tintinnids and naked ciliates in the summer. The annual averages of total zooplankton production were estimated as 18.3, 19.6 and 11.0 mg C m-3 d-1 in the upper, lower and subtidal zones, respectively. Ciliate biomass contributed greatly to total zooplankton biomass, especially on the tidal flat during the summer. Our results suggest that transfer efficiency is 17% for planktonic primary to secondary production and 2.1% for planktonic secondary to tertiary production. The former is lower than the values of the entire Seto Inland Sea, which were estimated in a previous study. Our results also suggest that only 1.4% of benthic and planktonic primary production is transferred to planktonic secondary production on this tidal flat. These facts imply that the zooplankton production mechanism is different between the tidal flat and offshore water, and the apparent lower efficiency of the former may be caused by the existence of strong competitive grazers, such as bivalves. [ABSTRACT FROM AUTHOR]