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

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 communityÂ’s ability to react to changes in DOM composition may create shifts in extracellular enzyme production while maintaining relatively consistent levels of community respiration.河流化学的变异取决于陆地-水生联系的强度、地表水与地下水交换的程度，以及河道内生物过程的速率。上述变量的重要性会随气候格局、流域地貌或人类活动影响程度发生波动。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溪流在全球营养物与碳循环中所扮演角色的理解。 其中尤为值得关注的是溶解态有机碳（dissolved organic carbon, DOC）——它是微生物代谢过程中重要的碳源与能量来源。近期研究表明，异养细菌群落的呼吸作用与溪流中DOC库的质量（即微生物利用碳源的能力）密切相关。DOC不仅会影响溪流的营养物供给，还会对污染物迁移及紫外线辐射衰减产生作用。 本研究聚焦于向两处旱地溪流生态系统输送的DOC：一处为原生荒漠溪流（亚利桑那州梧桐溪），另一处为城市溪流（亚利桑那州凤凰城）。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）提出并检验可解释DOC数量与质量变异的假说；（3）将上述变异与微生物活动关联起来。 在原生荒漠溪流生态系统中，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复杂度更高、浓度也更大。 与之形成对比的是，与索诺兰沙漠中受人类影响较低的溪流相比，凤凰城都会区对地貌与水文路径的人为改造显著改变了溪流水化学特征。在城市区域，营养物保留与转化的机制往往从以生物主导转变为以非生物主导，极大地削弱了气候格局的影响，转而将粮食生产与废物管理作为主导性的大规模控制因素。 微生物呼吸与胞外酶生产的模式显示，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，引发胞外酶生产的转变。