Antagonistic effects of temperature and dissolved organic carbon on fish growth in California mountain lakes

Resources and temperature play major roles in determining biological production in lake ecosystems. Lakes have been warming and 'browning' over recent decades due to climate change and increased loading of terrestrial organic matter. Conflicting hypotheses and evidence have been presented about whether these changes will increase or decrease fish growth within lakes. Most studies have been conducted in low-elevation lakes where terrestrially derived carbon tends to dominate over carbon produced within lakes. Understanding how fish in high-elevation mountain lakes will respond to warming and browning is particularly needed as warming effects are magnified for mountain lakes and treeline is advancing to higher elevations. We sampled 21 trout populations in the Sierra Nevada Mountains of California to examine how body condition and individual growth rates, measured by otolith analysis, varied across independent elevational gradients in temperature and dissolved organic carbon (DOC). We found that fish grew faster at warmer temperatures and higher nitrogen (TN), but slower in high DOC lakes. Additionally, fish showed better body condition in lakes with higher TN, higher elevation and when they exhibited a more terrestrial δ13C isotopic signature. The future warming and browning of lakes will likely have antagonistic impacts on fish growth, reducing the predicted independent impact of warming and browning alone.

生源要素与水温是决定湖泊生态系统生物生产力的核心驱动因子。近数十年来，受气候变化与陆地有机质输入量增加的影响，全球湖泊正经历暖化与“褐变”过程。针对此类变化会促进还是抑制湖泊鱼类生长这一问题，学界已提出诸多相互矛盾的假说与实证结果。现有多数相关研究均聚焦于低海拔湖泊，这类水体中陆地源碳往往占据主导地位，远超内源碳。鉴于山地湖泊的暖化效应被放大，且林线正向高海拔区域推进，我们亟需明确高海拔山地湖泊中的鱼类对暖化与褐变的响应机制。本研究对加利福尼亚州内华达山脉的21个鳟鱼种群开展了采样工作，通过耳石（otolith）分析法定量测定鱼类的体况与个体生长速率，以此探究其在温度与溶解性有机碳（dissolved organic carbon, DOC）各自独立的海拔梯度下的变化规律。研究结果显示：在水温更高、总氮（total nitrogen, TN）浓度更高的湖泊中，鱼类生长速率更快；而在溶解性有机碳浓度较高的湖泊中，鱼类生长则更为缓慢。此外，在总氮浓度更高、海拔更高的湖泊中，以及当鱼类的δ13C同位素信号更偏向陆地来源时，其体况更佳。未来湖泊的暖化与褐变过程大概率会对鱼类生长产生拮抗效应，抵消单独由暖化或褐变所带来的预期影响。