Hydrologic and seasonal controls on organic matter pools and transformation in a forested headwater stream network_data

Streams are dynamic reactors, where flow often controls carbon (C) dynamics. Riverine C exists largely as dissolved and fine particulate organic matter (DOM<0.7 µm and 0.7 µm<FPOM<1 mm, respectively). Although FPOM contributes significantly to C stocks and stream metabolism, FPOM processing is often unaccounted for in stream C cycling measurements and budgets. To address this knowledge gap, we investigated OM fate across flow conditions at two sites in a forested stream network in Alabama, USA: the upstream headwaters and the downstream outlet of Pendergrass Creek. At each site, we quantified DOM and FPOM in surface water and the streambed as benthic organic matter (BOM) for one year. We found that FPOM and BOM diluted with flow and increased during leaf on, while DOM (as dissolved organic C) did not change with flow and increased during leaf off. To assess the microbial metabolic activity (MMA) of different OM pools, we conducted resazurin-resorufin incubation assays as a proxy for respiration. Rates of MMA across all three OM pools decreased while FPOM C:[N]itrogen ratios increased at higher flows, indicating more reactive OM is likely stored and respired at low flows. We also observed that FPOM often accounted for the majority of water column respiration potential. Our findings, corroborated by a hydrologic model of flowing network length, suggest that as stream flows decrease with climate change, the quantity of OM stored and respired in headwater streams may increase, influencing C emissions and downstream resources.

溪流是动态反应器，水流通常调控碳（C）动态。河流碳主要以溶解态和细颗粒有机物质形式存在（分别为DOM<0.7 µm和0.7 µm<FPOM<1 mm）。尽管FPOM对碳储量和溪流代谢贡献显著，但在溪流碳循环测量与预算中，其周转过程常被忽略。为填补这一知识空白，我们在美国阿拉巴马州森林溪流网络的两个位点（彭德格拉斯溪的上游源头和下游出口），研究了不同水流条件下有机物质（OM）的归宿。在每个位点，我们对地表水及河床底栖有机物质（BOM）中的DOM和FPOM进行了为期一年的定量分析。结果发现，FPOM和BOM随水流稀释而减少，在叶生期增加；而DOM（以溶解有机碳计）不随水流变化，在叶落期增加。为评估不同有机物质库的微生物代谢活性（MMA），我们采用刃天青-试卤灵孵育实验作为呼吸作用的替代指标。所有三种有机物质库的MMA速率随水流增大而降低，FPOM的碳氮比（C:N）则升高，表明低水流条件下更易反应的有机物质可能被储存并进行呼吸作用。我们还观察到，FPOM常占水柱呼吸潜力的大部分。经溪流网络长度水文模型验证，我们的研究结果表明：随着气候变化导致溪流流量减少，源头溪流中储存和呼吸的有机物质数量可能增加，进而影响碳排放和下游资源。