Developmental process and material flow in tropical seagrass beds

1991年和1992年春季，我们派出科考团前往斐济德拉武尼岛，调查了热带海草床生态系统的生物结构和生物元素的联系。1991年，参加者大多是生物学家，实地观察海草生物量、生长、生产的有机物质向次生生产者、底栖生物和附生植物的转移，以及这些群落的精细生物结构。1992年，开展了优势动物代谢率的环境和实验研究。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约为1个月，地下部分的平均周转时间为500天以上。我们在巴布亚新几内亚的Thalassia也得到了类似的结果，海草地下部分的常积量大，周转时间慢是热带海草的共同特征，反映了外部环境的营养贫乏。(2)在氮供应方面，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上，固氮贡献约占海草种群氮需求量的10%。但是，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)对于海草的分解，直接食草动物并不多，异养动物主要是滤食性动物，包括双壳类和复合海鞘类。，表明悬浮有机物质作为食品的重要性。另一方面，沉积物-水柱界面的细菌活性较高，表明它们的过程对死海草的分解具有重要意义。

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.