Taipale Sami, Kankaala Paula, Tiirola Marja, Jones Roger I
Department of Biological and Environmental Science, University of Jyväskylä, Jyväskylä FIN-40014, Finland.
Ecology. 2008 Feb;89(2):463-74. doi: 10.1890/07-0702.1.
Sustained whole-lake additions of 13C-enriched dissolved inorganic carbon (DIC), intended to increase experimentally the delta13C of DIC in the epilimnion of a small lake with high dissolved organic carbon (DOC), were made during three seasonal periods (spring, summer, and autumn). Coupled with carbon and nitrogen stable isotope analysis of zooplankton and several of their putative food sources, these additions were used to investigate seasonal changes in the relative contributions of different food sources to zooplankton diet in the lake. Four main potential food sources were considered: phytoplankton, heterotrophic bacteria (HB), methanotrophic bacteria (MOB), and green sulfur bacteria (GSB). Because the number of potential food sources exceeded the number of isotopes analyzed, a computer program (IsoSource) was used to estimate the range of possible contributions of the various food sources. During all three periods the added inorganic 13C quickly increased the epilimnetic DIC delta13C by between 18 per thousand and 21 per thousand above the initial value of approximately -21 per thousand. This 13C enrichment of DIC was rapidly transmitted to the particulate organic matter (POM), which included photosynthetic phytoplankton. In spring and summer, delta13C of both adult and juvenile Daphnia increased by approximately 10 per thousand, indicating that Daphnia utilized autochthonous carbon. However, this 13C labeling of Daphnia was not so obvious during the autumn period, when their delta13C generally decreased. According to the IsoSource model outputs based on both delta13C and delta15N values, Daphnia utilized all four potential food source types during spring, summer, and autumn, but in different proportions. The possible contribution of phytoplankton to Daphnia diet was substantial (25-71%) in all seasons. The possible contributions of the bacterial food sources were more variable. The possible contribution of GSB was minor (0-20%) at all times and negligible in autumn. The possible contribution of HB was higher but very variable. Methanotrophic bacteria always made a significant contribution to Daphnia diet and were likely the single most important food source in autumn. Since both HB and MOB in this high-DOC lake probably depend largely on allochthonous organic carbon, our results highlight the seasonal variability in the potential importance of ecosystem subsidies in lake food webs.
在三个季节（春季、夏季和秋季）对一个溶解有机碳（DOC）含量高的小湖泊进行了持续的全湖添加富含13C的溶解无机碳（DIC）实验，旨在通过实验提高湖上层DIC的δ13C值。结合浮游动物及其几种假定食物来源的碳和氮稳定同位素分析，这些添加物被用于研究湖泊中不同食物来源对浮游动物食物组成的相对贡献的季节性变化。研究考虑了四种主要潜在食物来源：浮游植物、异养细菌（HB）、甲烷氧化细菌（MOB）和绿硫细菌（GSB）。由于潜在食物来源的数量超过了所分析的同位素数量，因此使用一个计算机程序（IsoSource）来估计各种食物来源可能的贡献范围。在所有三个季节中，添加的无机13C迅速使湖上层DIC的δ13C比初始值约-21‰增加了18‰至21‰。DIC的这种13C富集迅速传递到颗粒有机物质（POM），其中包括光合浮游植物。在春季和夏季，成年和幼年水蚤的δ13C均增加了约10‰，这表明水蚤利用了本地碳。然而，在秋季，水蚤的这种13C标记并不那么明显，此时它们的δ13C通常下降。根据基于δ13C和δ15N值的IsoSource模型输出，水蚤在春季、夏季和秋季利用了所有四种潜在食物来源类型，但比例不同。浮游植物对水蚤食物组成的可能贡献在所有季节都很大（25%-71%）。细菌食物来源的可能贡献变化更大。GSB的可能贡献在任何时候都很小（0%-20%），在秋季可忽略不计。HB的可能贡献较高但变化很大。甲烷氧化细菌对水蚤食物组成总是有显著贡献，并且可能是秋季唯一最重要的食物来源。由于这个高DOC湖泊中的HB和MOB可能在很大程度上都依赖于外源有机碳，我们的结果突出了湖泊食物网中生态系统补贴潜在重要性的季节性变化。
