Stable Isotope Feasibility Study for Testing the Role of Algal Carbon in a Tropical Floodplain

奥里诺科盆地是少数几个没有管制的大河与变化最小的泛滥平原毗邻的地方之一。因此，奥里诺科河为研究天然泛滥平原现象提供了特殊的机会。过去对沿奥里诺科干流的质量通量以及在小段河滩上的物理、化学和生物事件的阶段性的研究已经产生了几条证据，表明藻类有机质(浮游植物、附生植物)在维持奥里诺科河滩生态系统的水生食物网方面具有出人意料的高度重要性。该系统的其他重要有机物质来源包括来自上游陆地环境的颗粒物、河岸凋落物和水生维管植物。然而，目前的证据表明，这些来源的大部分生物质都是微生物降解的，没有有效地进入消费者食物链。稳定的同位素似乎为这一假说的直接测试提供了最好的前景。然而，稳定同位素的用处在某种程度上是特定地点的，因为同位素特征和必须通过使用这些特征来解决的隔室的数量不同。为此，本提案概述了一项测试使用稳定同位素解决关于奥里诺科河泛滥平原藻类碳的广泛假设的可行性的计划。将建立奥里诺科泛滥平原代表性地点主要有机质隔层中C、N和S稳定同位素特征的均值和方差，并将测试碎屑和悬浮藻类等不同功能隔层的物理分离方法。这些信息将用于量化奥里诺科河泛滥平原中稳定同位素的分辨率。

The Orinoco basin is one of the few locations where a large unregulated river adjoins a minimally-altered floodplain. The Orinoco therefore presents special opportunities for the study of natural floodplain phenomena. Past studies of mass flux along the Orinoco main stream and of the phasing in physical, chemical, and biological events on small segments of floodplain have produced several lines of evidence that indicate an unexpectedly high importance of algal organic matter (phytoplankton, periphyton) in sustaining the aquatic food webs of the Orinoco floodplain ecosystem. Other prominent sources of organic matter for the system include particulate material from upstream terrestrial environments, riparian litterfall, and aquatic vascular plants. However, current evidence indicates that the majority of biomass from these sources is degraded microbially without passing efficiently into consumer food chains. Stable isotopes appear to offer the best prospect for a direct test of this hypothesis. However, the usefulness of stable isotopes is to some degree site-specific because of variations in isotope signatures and in the number of compartments that must be resolved through the use of these signatures. For this reason, the present proposal outlines a plan for testing the feasibility of using stable isotopes in resolving the broad hypothesis concerning algal carbon in the Orinoco floodplain. Means and variances will be established for stable isotope signatures of C, N, and S in major organic-matter compartments at a representative site on the Orinoco floodplain, and methods for physical separation of functionally different compartments such as detritus and suspended algae will be tested. This information will be used in quantifying the resolving power of stable isotopes in the Orinoco floodplain.