Development of stable-isotope studies in production ecology of bivalvia.

双壳类是封闭海湾的主要渔业资源，但在生产生态学上却有许多不为人知的地方。利用自然丰度C、N稳定同位素分析了Hamana-ko湾牡蛎长牡蛎的生产机理，并对该技术的前景和存在的问题进行了讨论。滨湖湾河口区颗粒有机质（POM）以碎屑为主，13 δ c < -21。在藻华发生时，牡蛎内脏消化部分（GD）的13δC和15δN与POM的变化同步，但POM和13δC的变化与未发生任何特定藻华时的口区相同。我们明确指出，该湖的牡蛎生产从根本上依赖于碎屑菌，额外的生长是由初级生产支持的。通过对微生态和中生态藻类POM的培养实验，发现除了藻类种类外，POM的稳定同位素比率还受藻类的营养类型、光合活性或生长阶段的影响。另一项对牡蛎代谢状态影响的培养实验表明，无论是成熟、温度升高还是生长，都会使同位素的转换变得迟钝。综上所述，对碎屑的定量调查将比对初级产品的定量调查更为重要，初级产品被认为是牡蛎的主要食物来源。在藻类POM或牡蛎的培养实验中指出波动因素后，稳定的天然13δC和15δN同位素比值将成为“有效工具”。

Bivalves have many obscurity In their production ecology, though they are majour fishiery resorces of a closed bay. We analysed production mechanism of the oyster Crassostrea gigas in Hamana-ko bay using natural abundance ratios of C and N stable isotopes, in addition to the discussion on the prospects and problems of the technic.In a mouth area of Hamana-ko bay, particulated organic matter (POM) was mainly composed of detritus, and l3δC was lower than -21. In inner areas, both 13δC and 15δN of a ganadal-digestional part (GD) of the oyster changed synchronously with those of POM when algal bloom was observed, but the satates of both POM and 13δC were the same as in a mouth area when any particular blooming didn't occur. We made clear that oyster production in the lake depends on detrirus fundamentally and additional growth are supported by primary production.From culturing experiments on algal POM both in microcosms and mesocosms, stable isotope ratios of POM were revealed to be affected either with nutritiona1 type, photo-synthesis activity or growth phase of algae, in addition to algal species.Another culturing experiments on the effects of metabolic states of the oyster showed that either maturation, increasing temperature or growth makes the turnover of isotopes dull.In conclusion, quantitative investigations on detritus would be more inportant than on primary production which was thought to be main food source of the oyster. And, stable isotope ratios of natural 13δC and 15δN will be "effective tools" after fluctuation factors pointed out in culturing experiments on algal POM or the oyster.