Dissolved organic carbon dynamics in an urban desert stream ecosystem in central Arizona-Phoenix

Variation in stream chemistry is a function of the strength of terrestrial-aquatic linkages, the extent to which surface and groundwater exchanges, and the rate of instream biotic processes. The importance of these variables may fluctuate as a function of climate regime, catchment geomorphology, or level of human impact. A mechanistic understanding of the influence of each of these variables on ecosystem functioning will increase understanding of the role of streams in global nutrient and carbon cycles. Of particular interest is dissolved organic carbon (DOC), an important source of carbon and energy for microbial processes. Respiration by heterotrophic bacterial communities has recently been linked to the quality (ability of microbes to utilize C source) of the DOC pool in streams. DOC not only influences stream nutrient supply, but also the transport of contaminants and the attenuation of UV radiation. This dissertation focused on DOC delivery to two arid-land stream ecosystems, one native desert (Sycamore Creek, AZ), and one urban (Phoenix, AZ). The overall objectives of this work were to (1) document patterns in DOC quantity and chemical composition in response to flooding and groundwater exchange, (2) generate and test hypotheses explaining variation in DOC quantity and quality and (3) relate this variation to microbial activity. In the native desert stream ecosystem, the climate regime influenced seasonal variation in the quantity and quality of DOC inputs, with higher complexity and higher concentrations of DOC in summer monsoonal runoff. In contrast, human alteration of geomorphology and hydrologic flowpaths in the Phoenix metropolitan area significantly influenced streamwater chemistry in comparison to low-impact streams in the Sonoran Desert. In the city, mechanisms of nutrient retention and transformation were often shifted from dominance by biotic to abiotic ones, severely dampening the influence of climate regime and substituting instead the maintenance of food production and waste management as the dominant large-scale controlling factors. Patterns of microbial respiration and extracellular enzyme production indicate that the community's ability to react to changes in DOM composition may create shifts in extracellular enzyme production while maintaining relatively consistent levels of community respiration.河流化学的变化是陆地-水生联系强度、地表水与地下水交换程度以及河流内生物过程速率的函数。这些变量的重要性会随气候格局、流域地貌或人类活动强度发生波动。对上述各变量如何影响生态系统功能的机制性认知，将有助于深化我们对河流在全球营养与碳循环中所扮演角色的理解。 其中溶解态有机碳（dissolved organic carbon, DOC）尤为值得关注，它是微生物代谢过程重要的碳源与能量来源。近期研究发现，异养细菌群落的呼吸作用与河流中DOC库的质量（即微生物利用碳源的能力）密切相关。DOC不仅会影响河流的营养供给，还会对污染物迁移以及紫外线辐射的衰减产生作用。 本学位论文聚焦于两类旱地河流生态系统的DOC输运过程，分别为原生荒漠生态系统（亚利桑那州梧桐溪）与城市河流生态系统（亚利桑那州凤凰城河段）。本研究的总体目标包括：（1）记录洪水事件与地下水交换驱动下的DOC数量与化学组成变化模式；（2）提出并检验用于解释DOC数量与质量变化的假说；（3）将上述变化与微生物活动建立关联。 在原生荒漠河流生态系统中，气候格局主导了DOC输入的数量与质量的季节变化，夏季季风径流中的DOC复杂度与浓度均更高。与之形成鲜明对比的是，相较于索诺兰荒漠中受人类影响较低的河流，凤凰城都会区的人类活动对地貌与水文路径的改造显著改变了河流水化学特征。在城市河段，营养保留与转化的机制往往从以生物作用为主转变为以非生物作用为主，极大地削弱了气候格局的调控作用，并代之以粮食生产与废物管理作为主导的大型控制因子。 微生物呼吸与胞外酶产生的模式表明，微生物群落对溶解态有机物（dissolved organic matter, DOM）组成变化的响应能力，可能会在维持群落呼吸水平相对稳定的同时，引发胞外酶产生模式的改变。