Low‐level dissolved organic carbon subsidies drive a trophic upsurge in a boreal stream
Low‐level dissolved organic carbon subsidies drive a trophic upsurge in a boreal stream
复制标题
低水平的溶解有机碳补贴推动了北方溪流的营养激增
作者：
Caleb J. Robbins;Alyse D. Yeager;S. Cook;R. Doyle;J. Maurer;C. Walker;J. Back;D. Whigham;R. King
溪流食物网中的能量路径通常由外源基础资源驱动。然而，异源溶解有机碳（DOC）通常被视为次要甚至微不足道的基础资源，因为大部分 DOC 库包含高分子量、难降解的化合物，并且无法有效地融入生物质中。然而，越来越多的证据表明，DOC 相对较小且不稳定的部分确实可以将微生物活性提高到刺激多个营养级生产力的水平，从而导致营养激增。在这里，我们通过补贴阿拉斯加北方溪流的不稳定 DOC 池来测试营养激增假说，该溪流具有相对较高的养分利用率，但天然存在的 DOC 水平较低。我们连续 62 天不断添加生态相关（0.250 mg C/L，比环境散装 DOC 增加约 10%）浓度的不稳定 DOC（乙酸盐-C），处理河段在统计上与上游参考河段的河道形式和化学成分无法区分。我们测量了附生生物产量和生物量、全河段代谢和营养吸收、底栖无脊椎动物丰度以及幼鲑鱼（Dolly Varden、Salvelinus malma）丰度和生长的响应。对基础生态系统反应的测量总体表明，随着不稳定 DOC 的添加，食物网底部的能量动员有所增加。处理范围内的附生生物细菌产量通常>1.5×参考范围值，并且到研究结束时，处理范围内的附生生物无灰干质量、叶绿素-a和叶绿素-a：无灰干质量都更大。在整个给药过程中，九个测量中有八个测量范围内的生态系统呼吸量增加了 1.3 倍，处理范围内的溶解无机氮吸收量增加了 1.3 倍。底栖无脊椎动物数量较多，以 Baetis spp 为主。和摇蚊科，是c。 28 个给药天后增加 4 倍，c． 56 天后，处理河段上游部分的水量增加了 8 倍。丰度通常随着距加药站距离的增加而下降。在处理河段的上游部分，Dolly Varden 鱼苗和 parr 鱼苗的数量比参考河段的任何部分都多出近 2 倍，并且随着距投放站距离的增加而减少。此外，在实验前用无源集成应答器标记的多莉·瓦登（Dolly Varden）在处理范围内的瞬时增长率明显高于在参考范围内重新捕获的增长率。消费者对少量不稳定 DOC 的强烈反应反映了基础生态系统功能的显着增加，因此表明了与营养激增一致的反应。历史上，陆地 DOC 被认为对后生动物消费者贡献不大，而是调节营养物质的影响，并通过不连贯的微生物循环进行呼吸。由于我们根据附近泥炭地排水溪流的测量结果，在处理区域中添加了相应浓度的不稳定 DOC，因此我们认为，陆地 DOC 作为整个食物网的基础资源值得更多关注，类似于为绿色（本土）路径提供燃料的营养物质。
Energy pathways in stream food webs are often driven by allochthonous basal resources. However, allochthonous dissolved organic carbon (DOC) is generally viewed as a minor if not insignificant basal resource because much of the DOC pool comprises high molecular weight, recalcitrant compounds and is inefficiently incorporated into biomass. Nevertheless, there is increasing evidence that the relatively small, labile fraction of DOC may indeed fuel microbial activity to a level that stimulates productivity across multiple trophic levels, resulting in a trophic upsurge. Here, we tested the trophic upsurge hypothesis by subsidising the labile DOC pool of an Alaskan boreal stream that had relatively high nutrient availability but low levels of naturally occurring DOC. We continuously added ecologically relevant (0.250 mg C/L, c. 10% increase above ambient bulk DOC) concentrations of labile DOC (acetate‐C) for 62 days to a treatment reach that was statistically indistinguishable in its channel form and chemistry from an upstream reference reach. We measured responses of periphyton production and biomass, whole reach metabolism and nutrient uptake, benthic invertebrate abundances, and juvenile salmonid (Dolly Varden, Salvelinus malma) abundance and growth. Measurements of basal ecosystem responses collectively indicated increased energy mobilization at the base of the food web in response to labile DOC addition. Periphyton bacterial production in the treatment reach was generally >1.5× reference reach values, and periphyton ash‐free dry mass, chlorophyll‐a, and chlorophyll‐a:ash‐free dry mass were all greater in the treatment reach by the end of the study. Throughout dosing, ecosystem respiration was 1.3× greater in the treatment reach and dissolved inorganic nitrogen uptake was greater in the treatment reach on eight out of nine measurements. Benthic invertebrate counts, dominated by Baetis spp. and Chironomidae, were c. 4× greater after 28 dosing days and c. 8× greater after 56 days in the upstream portion of the treatment reach. Abundance generally declined with increasing distance from the dosing station. Dolly Varden fry and parr age classes were nearly 2× more abundant in the upstream portion of the treatment reach than in any section of the reference reach and also declined with increasing distance from the dosing station. Further, Dolly Varden tagged with passive integrated transponders prior to the experiment had significantly higher instantaneous growth rates in the treatment reach than those recaptured in the reference reach. The strong consumer responses to small quantities of labile DOC mirrored significant treatment reach increases in basal ecosystem function and therefore demonstrated a response consistent with a trophic upsurge. Terrestrial DOC has historically been viewed as contributing little to metazoan consumers, instead modulating the influence of nutrients and being respired out of a disconnected microbial loop. Because we dosed the treatment reach with a relevant concentration of labile DOC, based on measurements in nearby peatland‐draining streams, we suggest that terrestrial DOC deserves more attention as a basal resource for whole food webs, akin to nutrients fuelling green (autochthonous) pathways.
影响因子：
3.7
作者：
Bernhardt, Emily S.;Blaszczak, Joanna R.;Seybold, Erin C.
通讯作者：
Seybold, Erin C.