Collaborative Research: An exploration of the direct and indirect effects of climatic warming on arctic lake ecosystems, Fog Lakes, Alaska, 2014-2021

We attempted to experimentally warm an arctic whole lake to mimic the effects of on-going climate warming on food web and whole lake ecosystem process and function. The goals were to increase water temperature by up to 4°C (celsius), deepen epilimnion (warmer surface water layer) by up 2 meters (m), and delay ice-on by up to 2 weeks. Logistically this proved very challenging, and took a year or so to figure out the mechanics. By 2018, we had successfully warmed lake Fog 1, with 44 days of heating, a 3°C warmer, deeper epilimnion that mixed 15 days later. We were able to use our lake temperature and warming data to improve upon our ability to simulate lake mixing processes, and were able to predict the lake thermal response to the lake warming manipulation. Those results provide a more complete understanding of lake thermal processes in arctic freshwater lake systems and how they will respond to predicted future warming. For fish growth, one of our primary response variables, we detected an interaction in that growth decreased with increasing temperatures, but only when food availability was low. In addition, our bioenergetic simulations supported our experimental results and suggested that the benefits related to thermoregulation will be highest when food availability is low. In terms of other measured lake factors (e.g., zooplankton, macroinvertebrates) which may have been affected by lake warming, we observed that inter-annual differences among lakes in food web composition and temperature regime tended to overwhelm any potential lake warming responses. In addition, the long, cold winters appear to reset the lakes after warming, with no carryover effects. However, by 2020, the project was largely shut down by the Covid pandemic, and in 2021-2022, by the Covid-based regulations put in place at the field camp. In sum, based on our limited time series of experimental lake warming, these Arctic lakes appear to be somewhat robust to the direct effects of small levels of climatic warming; however, the indirect effects of climate warming and variability (e.g., thermokarst failure [tundra melt and slough]) may be more profound and are under exploration.本研究尝试通过实验手段对北极全湖开展加温操作，以模拟持续气候变化对湖泊食物网及全湖生态系统过程与功能的影响。研究目标为将湖水温度最高提升4℃（摄氏度），将湖上层（epilimnion，即温暖的表层水层）的厚度最大增加2米，并将湖泊封冻时间最多推迟2周。但从实施层面来看，该实验极具挑战性，前后耗时约一年才摸清其技术机理。至2018年，我们已成功对福格1号湖（Lake Fog 1）完成加温处理：累计加温44天，使湖水温度升高3℃，湖上层厚度增加且混合过程延迟了15天。我们利用获取的湖水温度与加温数据，优化了湖泊混合过程的模拟能力，并能够精准预测湖泊加温操控后的热响应特征。上述结果让我们更全面地理解了北极淡水湖泊系统的热过程，以及其对未来预测升温的响应模式。作为核心响应变量之一的鱼类生长，我们检测到了交互效应：即鱼类生长随水温升高而下降，但这一现象仅在食物可获得性较低时出现。此外，我们的生物能学模拟结果佐证了实验发现，并表明当食物可获得性较低时，体温调节带来的收益将达到最高。针对其他受湖泊加温可能影响的监测指标（如浮游动物（zooplankton）、大型无脊椎动物（macroinvertebrates）等），我们观察到：不同湖泊间食物网组成与温度格局的年际差异，往往掩盖了湖泊加温可能带来的潜在响应。此外，漫长寒冷的冬季似乎会重置湖泊的加温后状态，未产生遗留效应。然而到2020年，该项目因新冠疫情基本中止；2021-2022年，野外营地实施的新冠相关防控措施进一步阻碍了实验推进。综上，基于我们有限的实验湖泊加温时间序列数据，这些北极湖泊对小幅气候变暖的直接效应表现出一定的耐受性；但气候变暖的间接效应与变异性（如热喀斯特塌陷（thermokarst failure，即苔原融化与滑坡））可能影响更为深远，目前相关研究仍在进行中。