Developmental process and material flow in tropical seagrass beds

1991年和1992年春季，我们派科学考察队到斐济德拉武尼岛考察热带海草床生态系统的生物学结构和生物元素的分布，1991年主要是生物学家，对海草的生物量、生长、所产生的有机物质向次级生产者、底栖和附生植物的转移进行了实地观察，以及这些群落的精细生物结构。1992年，对优势动物的代谢率进行了环境和实验研究。以下是1991-1992年实地研究的主要结果。（1）优势海草地上部分的平均周转时间，即，据估计，注射器是ca。1个月，而地下部分根茎的贮藏期在500天以上。在巴布亚新几内亚的Thalassia也得到了类似的结果，海草地下部分现存量大，周转时间慢是热带海草的共同特征，反映了营养贫乏的外部环境。（2）在氮的供应方面，底栖生物和附生植物的固氮作用对系统的总氮收支有重要贡献。在靠近大陆的巴布亚海草床中，固氮作用的贡献约为100%。海草种群氮需求的10%。但是，在斐济作为一个营养贫乏的海洋岛屿，固氮占社区氮需求的10%。（3）海草分解过程中，直接食草动物较少，异养动物以滤食性为主，包括双壳类和复合海鞘类。表明悬浮有机物作为食物的重要性。另一方面，沉积物-水柱界面处的细菌活动很高，表明它们的过程对死亡海草的分解很重要。

During the spring season in 1991 and 1992, we sent scientific party to Dravuni Island, Fiji, to investigate the biological structure and clcling of bioelements in tropical seagrass bed ecosystem.In 1991, the participants were mostly biologists doing the field observation of seagrass biomass, their growth, transfer of produced organic materials to secondary producers, benthic as well as epiphytes, and fine biological structures of those communities. In 1992, environmental and experimental studies on metabolic rates of dominated animals were carried out. Following are the main results of 1991-1992 field studies.(1)Averaged turnover time of above the ground part of dominated seagrass, i.c., Syringodium, was estimated to be ca. 1 month, while that of underground part, rhizome, was more than 500 days. We also obtained similar result from Thalassia in Papua New Guinea, and large standing stock of underground part of seagrass and slow turnover time is rather common characters of tropical seagrass, reflecting nutrient poor outer environments.(2)For the supply of nitrogen, nitrogen fixation by benthic communities as well as epiphytes seems to be significant contribution to the total nitrogen budget of the system. In Papuan seagrass bed, which is near the continent, contribution of nitrogen fixations ca. 10 % of nitrogen demand by seagrass population. But, in Fiji as an nutrient poor ocean island, nitrogen fixation accounted upto several 10% of community nitrogen demand.(3)For the decomposition of seagrass, there was not so many direct grazers and major population of heterotrophic animals were filter feeders, including bivalves and compound ascidians., indicating the importance of suspended organic materials as the food. On the other hand, bacterial activities at the interface of sediment-water column were high, suggesting the importance of their processes on decomposition of the dead seagrass.