Aquatic Physics Group, Department F.-A. Forel for Environmental and Aquatic Sciences (DEFSE), Section of Earth and Environmental Sciences, Faculty of Sciences, University of Geneva, Uni Carl Vogt, 66 boulevard Carl-Vogt, CH-1211 Geneva, Switzerland.
Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Mueggelseedamm 310, 12587 Berlin, Germany.
Sci Rep. 2017 Mar 14;7:44478. doi: 10.1038/srep44478.
Diurnally-migrating Chaoborus spp. reach populations of up to 130,000 individuals m in lakes up to 70 meters deep on all continents except Antarctica. Linked to eutrophication, migrating Chaoborus spp. dwell in the anoxic sediment during daytime and feed in the oxic surface layer at night. Our experiments show that by burrowing into the sediment, Chaoborus spp. utilize the high dissolved gas partial pressure of sediment methane to inflate their tracheal sacs. This mechanism provides a significant energetic advantage that allows the larvae to migrate via passive buoyancy rather than more energy-costly swimming. The Chaoborus spp. larvae, in addition to potentially releasing sediment methane bubbles twice a day by entering and leaving the sediment, also transport porewater methane within their gas vesicles into the water column, resulting in a flux of 0.01-2 mol m yr depending on population density and water depth. Chaoborus spp. emerging annually as flies also result in 0.1-6 mol m yr of carbon export from the system. Finding the tipping point in lake eutrophication enabling this methane-powered migration mechanism is crucial for ultimately reconstructing the geographical expansion of Chaoborus spp., and the corresponding shifts in the lake's biogeochemistry, carbon cycling and food web structure.
除南极洲外，所有大陆上的湖泊中，日行性的摇蚊科（Chaoborus）物种可达到每立方米多达 13 万只的种群密度，其分布范围可深至 70 米。与富营养化有关的是，日行性的摇蚊科物种白天生活在缺氧沉积物中，晚上在含氧的表层水层中觅食。我们的实验表明，通过钻入沉积物中，摇蚊科物种利用沉积物甲烷的高气态分压来使气管囊充气。这种机制提供了一个显著的能量优势，使幼虫能够通过被动浮力而不是更耗能的游泳进行迁移。摇蚊科幼虫除了每天通过进出沉积物两次可能释放沉积甲烷气泡外，还通过气泡将孔隙水甲烷运输到水柱中，从而导致根据种群密度和水深的不同，通量在 0.01-2 mol m yr 之间。每年以蝇类形式出现的摇蚊科物种也会导致系统中 0.1-6 mol m yr 的碳输出。找到使这种甲烷驱动的迁移机制发挥作用的湖泊富营养化临界点，对于最终重建摇蚊科物种的地理扩张以及相应的湖泊生物地球化学、碳循环和食物网结构的转变至关重要。
