Effects of advancing treelines and melting glaciers on alpine lake ecosystems: A mesocosm experiment

Climate warming promotes the upward advance of mountain treelines, thereby increasing allochthonous inputs of terrestrial organic matter into alpine lakes. Higher temperatures also accelerate glacial ablation, altering inputs of finely eroded rock particles, termed “glacial flour”. Organic matter (OM) and glacial flour (GF) both affect aquatic ecosystems; however, a knowledge gap exists concerning their combined impact. To test for the direct and interactive effects of OM and GF, we conducted a crossed two-factor outdoor mesocosm experiment. We hypothesized that GF sequesters OM through adsorption, thus reducing its effects on the abiotic environment and phytoplankton community. Addition of GF decreased underwater attenuation of ultraviolet radiation by the OM amendment (i.e., a GF-OM interaction), but not the pronounced positive effects of OM on nutrients and phytoplankton chlorophyll. GF did mediate the effect of OM on phytoplankton community composition by suppressing diatoms. These findings highlight the potential for future shifts in allochthonous inputs away from GF and towards OM to stimulate high-elevation lake ecosystems as glaciers ablate and treeline vegetation migrates to higher elevations under a warming climate.

气候变暖推动山地林线向上扩张，由此增加了陆生有机质向高山湖泊的外源输入。高温同时加速冰川消融，改变了细磨蚀岩粒的输入量，这类岩粒被称为“冰川粉（glacial flour）”。有机质（organic matter, OM）与冰川粉（glacial flour, GF）均会对水生生态系统造成影响，但目前学界关于二者的联合作用效应仍存在研究空白。为探究OM与GF的直接作用及交互效应，本研究开展了室外交叉双因子中型实验生态系统（mesocosm）实验。我们提出假说：GF可通过吸附作用固持OM，进而削弱其对非生物环境及浮游植物群落的影响。添加GF可降低OM添加处理引发的紫外辐射水下衰减（即GF与OM的交互作用），但无法抵消OM对营养盐及浮游植物叶绿素的显著正向影响。GF确实通过抑制硅藻（diatoms）类群，介导了OM对浮游植物群落组成的调控作用。本研究结果揭示了潜在的变化趋势：在气候变暖背景下，随着冰川消融、林线植被向更高海拔迁移，外源输入将从以GF为主转向以OM为主，这可能会促进高海拔湖泊生态系统的动态变化。