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.

营养资源与水温是决定湖泊生态系统生物生产的关键调控因素。近数十年来，受气候变化与陆源有机质输入量增加的影响，全球湖泊正呈现水温上升与“褐变”的双重变化趋势。针对此类变化对湖泊内鱼类生长的影响究竟是促进还是抑制，学界已提出诸多相互矛盾的假说与实证证据。现有相关研究大多聚焦于陆源碳占主导的低海拔湖泊。鉴于山地湖泊的增温效应被放大且林线正向高海拔区域迁移，明确高海拔山地湖泊鱼类对水温上升与湖泊褐变的响应机制尤为迫切。
我们于美国加利福尼亚州内华达山脉采集了21个鳟鱼种群样本，通过耳石分析测定鱼类的躯体状况与个体生长速率，以此探究水温与溶解性有机碳（DOC）各自独立的海拔梯度对上述指标的影响。研究结果显示，在水温更高、总氮（TN）浓度更高的湖泊中，鳟鱼生长速率更快；而在溶解性有机碳浓度较高的湖泊中，其生长速率则更慢。此外，在总氮浓度更高、海拔更高的湖泊中，且当鱼类的δ¹³C同位素信号更偏向陆源特征时，其躯体状况更佳。
未来湖泊的水温上升与褐变过程，可能会对鱼类生长产生拮抗效应，削弱二者单独作用时的预测影响强度。