Developmental process and material flow in tropical seagrass beds

1991年和1992年春季，我们派出科考队前往斐济的德拉武尼岛，调查热带海草床生态系统中生物元素的生物结构和附着。1991年，参与者大多是生物学家，对海草的生物量、生长、产生的有机物质向次生生产者的转移、底栖植物和附生植物以及这些群落的精细生物结构进行了实地观察。1992年对优势动物的代谢率进行了环境和实验研究。1991-1992年田间研究的主要结果如下：(1)优势海草黑麦草地上部分的平均周转时间为1个月左右，地下部分的周转时间在500天以上。我们在巴布亚新几内亚的海拉西亚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普遍特征，反映了营养不良的外部环境。(2)对于氮的供应，底栖群落和附生植物的固氮似乎对系统总氮收支的贡献很大。在靠近巴布亚大陆的巴布亚海草床上，固氮对海草种群氮素需求的贡献约为10%。但是，斐济作为一个营养贫乏的海洋岛屿，固氮占群落氮素需求的10%左右。(3)对于海草的分解，没有那么多的直接放牧动物，异养动物的主要种群是滤食性动物，包括双壳类和复合海鞘动物，这表明悬浮有机物质作为食物的重要性。另一方面，沉积物-水柱界面的细菌活性很高，表明它们的过程对死亡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.