Path Analysis Data and Code

Biogeochemical rates within streams vary with ecosystem properties including the distribution of fishes. While many studies investigate the singular effect of fishes on ecosystem components, there is a limited understanding of how fish presence interacts with other ecosystem properties to affect ecosystem structure and function. Here, we used path analyses to elucidate direct and indirect effects of fish presence, and other ecosystem properties on ecosystem respiration (ER), gross primary production (GPP), and ammonium uptake. Experimental responses of fish removal on patch scale (300 cm2) benthic rates of ER, GPP, and ammonium uptake were measured at two sites in a prairie stream. Biogeochemical rates associated with benthic substrata were quantified by monitoring fluxes of dissolved oxygen and ammonium inside sealed acrylic chambers with internal circulation systems. The effect of fish was determined by comparing substrata from field exclosures with fish absent to substrata exposed to fish. Total path model-explained variance was greatest for ER (R2=0.55) and least for ammonium uptake (R2=0.36) and GPP (R2=0.34). Fish decreased algal biomass and directly increased all biogeochemical rates. The relative importance of the different abiotic ecosystem properties varied by process; however, FBOM and substrata size were important for most rates. This study provides evidence for primarily direct effects of fish on both stream structure and function in a prairie stream. Our results emphasize that interactions between biotic and abiotic factors should be considered when determining drivers of biogeochemical activity .

溪流内的生物地球化学速率会随各类生态系统属性发生变化，其中涵盖鱼类的空间分布特征。尽管已有诸多研究探讨了鱼类对生态系统组分的单一影响，但学界对于鱼类存在如何与其他生态系统属性交互作用，进而调控生态系统结构与功能的认知仍较为匮乏。本研究通过路径分析（path analysis），阐明了鱼类存在及其他生态系统属性对生态系统呼吸（ecosystem respiration, ER）、总初级生产力（gross primary production, GPP）以及铵盐摄取速率的直接与间接效应。本研究在一条草原溪流的两个采样点，测定了斑块尺度（300 cm²）下底栖生态系统呼吸、总初级生产力及铵盐摄取速率对鱼类移除的实验响应特征。通过在搭载内部循环系统的密封亚克力培养舱内监测溶解氧与铵盐的通量，可量化底栖基质相关的生物地球化学速率。鱼类的影响效应可通过对比鱼类排除型野外围栏的底栖基质（无鱼）与有鱼栖息的底栖基质得以确定。路径模型的总解释方差以生态系统呼吸最高（决定系数R²=0.55），铵盐摄取速率（R²=0.36）与总初级生产力（R²=0.34）的解释方差相对最低。鱼类可降低藻类生物量，并直接提升所有生物地球化学速率。不同非生物生态系统属性的相对重要性因过程而异；不过细粒底栖有机质（fine benthic organic matter, FBOM）与基质粒径对多数速率指标均具有显著影响。本研究为草原溪流中鱼类对溪流结构与功能主要产生直接效应提供了实证依据。本研究结果强调，在解析生物地球化学活动的驱动因子时，应纳入生物与非生物因子间的交互作用予以考量。