School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.
Sci Total Environ. 2021 Jul 10;777:146067. doi: 10.1016/j.scitotenv.2021.146067. Epub 2021 Feb 26.
Climate change is expected to intensify the effect of environmental stressors on riverine ecosystems. Extreme events, such as low flow and heatwaves, could have profound consequences for stream ecosystem functioning, but research on the impact of these stressors and their interaction across multiple processes, remains scarce. Here, we report the results of a two-month stream mesocosm experiment testing the effect of low flow (66% water level reduction, without gravel exposure) and heatwaves (three 8-d episodes of +5 °C above ambient with 10-15 days recovery between each episode) on a suite of ecosystem processes (i.e. detrital decomposition, biofilm accrual, ecosystem metabolism and DOC quantity and quality). Low flow reduced whole system metabolism, suppressing the rates of gross primary production (GPP) and ecosystem respiration (ER), but elevated DOC concentration. Overall, habitat contraction was the main driver of reduced ecosystem functioning in the low flow treatment. By contrast, heatwaves increased decomposition, algal accrual, and humic-like DOC, but reduced leaf decomposition efficiency. Net ecosystem production (NEP) generally decreased across the experiment but was most pronounced for low flow and heatwaves when occurring independently. Assessment of NEP responses to the three successive heatwave events revealed that responses later in the sequence were more reduced (i.e. more similar to controls), suggesting biofilm communities may acclimate to autumn heatwaves. However, when heatwaves co-occurred with low flow, a strong reduction in both ER and GPP was observed, suggesting increased microbial mortality and reduced acclimation. Our study reveals autumn heatwaves potentially elongate the growth season for primary producers and stimulate decomposers. With climate change, river ecosystems may become more heterotrophic, with faster processing of recalcitrant carbon. Further research is required to identify the impacts on higher trophic levels, meta-community dynamics and the potential for legacy effects generated by successive low flows and heatwaves.
预计气候变化将加剧环境胁迫因子对河流生态系统的影响。极端事件，如低流量和热浪，可能对溪流生态系统功能产生深远影响，但有关这些胁迫因子及其对多个过程相互作用的影响的研究仍然很少。在这里，我们报告了一项为期两个月的河流中观实验的结果，该实验测试了低流量（水位降低 66%，无砾石暴露）和热浪（三次持续 8 天的 +5°C 以上温度，每次间隔 10-15 天恢复）对一系列生态系统过程（即碎屑分解、生物膜积累、生态系统代谢以及 DOC 数量和质量）的影响。低流量减少了整个系统的新陈代谢，抑制了总初级生产力（GPP）和生态系统呼吸（ER）的速率，但提高了 DOC 浓度。总体而言，栖息地收缩是低流量处理中生态系统功能降低的主要驱动因素。相比之下，热浪增加了分解、藻类积累和腐殖质样 DOC，但降低了叶片分解效率。净生态系统生产力（NEP）在整个实验中普遍下降，但在低流量和热浪单独发生时最为明显。对 NEP 对三个连续热浪事件的响应评估表明，序列后期的响应更为减少（即与对照更相似），这表明生物膜群落可能适应秋季热浪。然而，当热浪与低流量同时发生时，观察到 ER 和 GPP 的强烈减少，这表明微生物死亡率增加和适应能力降低。我们的研究表明，秋季热浪可能会延长初级生产者的生长季节，并刺激分解者。随着气候变化，河流生态系统可能变得更加异养，对难降解碳的处理速度更快。需要进一步研究以确定对更高营养级、元群落动态以及由连续低流量和热浪产生的遗留效应的影响。
