Lake ecosystems, as integrators of watershed and climate stressors, are sentinels of change. However, there is an inherent time-lag between stressors and whole-lake response. Aquatic metabolism, including gross primary production (GPP) and respiration (R), of stream–lake transitional zones may bridge the time-lag of lake response to allochthonous inputs. In this study, we used high-frequency dissolved oxygen data and inverse modeling to estimate daily rates of summer epilimnetic GPP and R in a nutrient-limited oligotrophic lake at two littoral sites located near different major inflows and at a pelagic site. We examined the relative importance of stream variables in comparison to meteorological and in-lake predictors of GPP and R. One of the inflow streams was substantially warmer than the other and primarily entered the lake’s epilimnion, whereas the colder stream primarily mixed into the metalimnion or hypolimnion. Maximum GPP and R rates were 0.2–2.5 mg O₂ L⁻¹ day⁻¹ (9–670%) higher at littoral sites than the pelagic site. Ensemble machine learning analyses revealed that > 30% of variability in daily littoral zone GPP and R was attributable to stream depth and stream–lake transitional zone mixing metrics. The warm-stream inflow likely stimulated littoral GPP and R, while the cold-stream inflow only stimulated littoral zone GPP and R when mixing with the epilimnion. The higher GPP and R observed near inflows in our study may provide a sentinel-of-the-sentinel signal, bridging the time-lag between stream inputs and in-lake processing, enabling an earlier indication of whole-lake response to upstream stressors.

湖泊生态系统作为流域和气候压力源的整合者，是变化的哨兵。然而，压力源和全湖反应之间存在固有的时间滞后。河流-湖泊过渡带的水生代谢，包括初级生产总值 (GPP) 和呼吸作用 (R)，可以弥合湖泊对外来输入的反应时滞。在这项研究中，我们使用高频溶解氧数据和反演模型来估计位于不同主要流入量附近的两个沿海地点和一个远洋地点的营养有限的贫营养湖中夏季外浸 GPP 和 R 的每日速率。与 GPP 和 R 的气象和湖内预测因子相比，我们检查了河流变量的相对重要性。其中一条流入河流比另一条更温暖，主要进入湖泊的表层，而较冷的流主要混入金属离子或低温离子。最大 GPP 和 R 率为 0.2–2.5 mg O₂ L ^-1  day ^-1 (9–670%) 在沿海站点比远洋站点高。集成机器学习分析表明，沿岸带 GPP 和 R 的每日变异性 > 30% 可归因于河流深度和河流-湖泊过渡带混合指标。暖流流入可能刺激了沿岸的 GPP 和 R，而冷流的流入在与表层混合时仅刺激了沿岸带的 GPP 和 R。在我们的研究中观察到的较高的 GPP 和 R 在流入附近可能会提供一个哨兵信号，弥合河流输入和湖内处理之间的时间滞后，从而能够更早地指示全湖对上游压力源的反应。