Fisheries Technology Institute, Japan Fisheries Research and Education Agency, Yokohama Field Station, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa, 236-8648, Japan.
Shizuoka Prefectural Research Institute of Fishery and Ocean, Hamanako Branch, Hamamatsu, Japan.
Oecologia. 2024 Oct;206(1-2):151-162. doi: 10.1007/s00442-024-05618-8. Epub 2024 Sep 19.
Estuaries exhibit high physicochemical variability and the properties of estuaries and the constituent segments are not yet systematically understood. This study aims to reveal the spatial heterogeneity of predominant organic sources using carbon and nitrogen stable isotope ratios (δC and δN, respectively) of Japanese eels (Anguilla japonica), one of the ideal natural samplers. In the Miyakoda River, Japan, our study site, the effectiveness of eel δC and δN values as standard indices of predominant organic sources was tested by employing the River Continuum Concept. This study then extended the application of these indices to the estuary, i.e., the Hamana Lake system, into which the Miyakoda River flows. Once in the upper estuary, eel δC and δN values became high, with the latter peaking in this river-estuary continuum, indicating that artificial labile subsidies (nutrients and organic matter) with high δN values were rapidly assimilated into the food web. Eel δN values decreased again in the middle estuary. Nevertheless, the influence of terrestrial organic subsidies extended into this segment, as evidenced by the low δC values of eels. These results suggest that refractory organic matter with low δN values, such as plant-derived ones, is slowly assimilated into the food web in the downstream estuarine segments. The higher δC values in the lower estuary suggested that the contribution of eelgrass or macroalgae occurred in addition to benthic microalgae. Thus, our results emphasize the need to consider the multiple energy flows to understand the estuary as a continuum.
河口具有高度的物理化学变异性，而河口及其组成部分的性质尚未得到系统的理解。本研究旨在利用日本鳗鲡（Anguilla japonica）的碳氮稳定同位素比值（分别为δC 和 δN）来揭示主要有机来源的空间异质性，日本鳗鲡是理想的天然采样器之一。在日本宫田川，也就是我们的研究地点，通过采用河流连续体概念，测试了鳗鱼的 δC 和 δN 值作为主要有机来源标准指标的有效性。然后，本研究将这些指标的应用扩展到河口，即宫田川流入的滨名湖系统。一旦进入上游河口，鳗鱼的 δC 和 δN 值就会升高，后者在这条河-河口连续体中达到峰值，表明具有高 δN 值的人工易分解补贴（营养物质和有机物）迅速被纳入食物网。在中游河口，鳗鱼的 δN 值再次下降。然而，陆地有机补贴的影响延伸到了这一河段，这可以从鳗鱼的低 δC 值中得到证明。这些结果表明，在下游河口的河段中，缓慢被纳入食物网的是低 δN 值的难分解有机物，如植物源有机物。下游河口更高的 δC 值表明，除了底栖微藻外，鳗草或大型藻类的贡献也发生了。因此，我们的结果强调需要考虑多种能量流来将河口视为一个连续体。
