The reason why red tides do not occur during winter in Dokai Bay under hyper-eutrophicated condition is investigated. Phytoplankton at the upper layer is advected out of Dokai Bay within 2.5 days by a strong estuarine circulation throughout a year but phytoplankton can reach a density of red tides due to high growth rate during summer because of high water temperature and strong solar radiation. On the other hand, phytoplankton at the upper layers is flushed out of the bay before reaching a density of red tides during winter due to low growth rate because of low water temperature and weak solar radiation.

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On the basis of the field observations of water quality characteristics of Southern Iyo-Nada and Osaka Bai in the Seto Inland Sea by Shikoku Electric Power Co., Inc. and Osaka Prefectural Fisheries Experimental Station from 1980's to 1990's, the seasonal and year-to-year variations of DIN, DIP, and DIN/DIP and nitrogen and phosphorus cycling are discussed. The situation of Southern Iyo-Nada is oligotrophic (DIN concentration is less than 4.1 μM, DIP concentration is less than 0.22 μM on annual average) throughout the year from 1982 to 1998. There was no significant seasonal variation in N/P ratio. Nutrient concentrations in the bottom layer in August were higher than those in the surface layer by 1.7 μM for DIN and 0.14 μM for DIP. It is suggested that nutrients in Southern Iyo-Nada mainly come from the continental shelf. Osaka Bay was eutrophic through the year from 1980's to 1990's (DIN concentration was more than 11.5 μM, and DIP concentration was more than 0.48 μM on annual average), and there was no year-to-year variation in DIN concentration and DIN/DIP. The decreasing tendency of DIP concentration since 1973 stopped in 1985, and since then it has been stable. DIN concentration in the surface layer was higher than that in the bottom layer by 5.6 μM on annual average, because DIN was supplied from rivers. DIP supplied by DIP release from the bottom sediments in summer. Not only nutrients supply from the river but also DIP release from sediments in summer affect nutrients cycling in Osaka Bay. Therefore primary Production in Osaka Bay in February and May is limited by phosphorus, but a shortage of phosphorus is canceled in November due to high DIP release from sediments.

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Silicon is an essential element in the ocean ecosystems, because it is required for the growth of diatoms which represent a major portion of primary producers. Measuring the biogenic silica(diatom silica) content in marine sediments and suspended matter reveals plenty of important information regarding to the ocean ecosystems and the biogeochemical cycle of silicon in the ocean. Several methods have been proposed to estimate biogenic silica in marine samples. Many of these techniques still have serious limitations depending on the amount of biogenic silica and the nature of the coexisting aluminosilicate compounds in samples. This paper summarized the common methods existing in literature, and finally a few comments on these advantages and limitations are described in brief.

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